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Research ArticleAllelopathic Potential of Green Manure Cover Crops onGermination and Early Seedling Development of Goose Grass[Eleusine indica (L) Gaertn] and Blackjack (Bidens pilosa L)
Joyful Tatenda Rugare 12 Petrus Jacobus Pieterse 2 and Stanford Mabasa 2
1Department of Agronomy Stellenbosch University Private Bag X1 Matieland 7602 South Africa2Department of Plant Production Sciences and Technologies University of Zimbabwe PO Box MP 167 Mount PleasantHarare Zimbabwe
Correspondence should be addressed to Joyful Tatenda Rugare rugarejoyyahoocouk
Received 22 June 2021 Revised 24 September 2021 Accepted 13 October 2021 Published 1 November 2021
Academic Editor Yong In Kuk
Copyright copy 2021 Joyful Tatenda Rugare et al is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited
Green manure cover crops (GMCCs) which are recommended for improving soil fertility also have the potential of reducingweed populations in cropping systems through allelopathy e objective of this study was to evaluate the effect of eight GMCCson the germination and seedling development of two weeds of divergent morphology namely goose grass [Eleusine indica (L)Gaertn] and blackjack (Bidens pilosa L) Aqueous leaf stem and root extracts of hyacinth bean (Lablab purpureus L) redsunnhemp [Crotalaria ochroleuca (G) Don] showy rattlebox (Crotalaria grahamiana Wight amp Arn) common bean (Phaseolusvulgaris L) common rattlepod (Crotalaria spectabilis Roth) radish (Raphanus sativus L) tephrosia (Tephrosia vogelii L) andblack sunnhemp (Crotalaria juncea L) at 0 125 25 375 and 5wvminus1 were applied to weed seeds in Petri dishes to determinetheir effect on germination radicle and plumule growth and germination vigor index e experimental design was 3(tissuetypes) lowast 5(concentrations) treatment combinations replicated four times in a completely randomized design In the pot study 25seeds of either goose grass or blackjack were planted separately in approximately 400 g of soil mixed with cover crop tissue powderat 1 concentration per pot e experimental design was cover crop residues + control replicated four times in randomizedcomplete blocks ere was a significant (plt 005) extract lowast concentration interaction on all germination parameters across allGMCCs e different cover crop aqueous extracts differentially reduced all germination parameters of both weeds in the orderleafgt stemgt root extract except for radish root extracts being most inhibitory to all germination parameters of goose grass eleaf stem and root soil-incorporated residues of GMCCs significantly (plt 005) affected seedling emergence dry weight andvigor indices of both weeds Based on the results of this study it was concluded that the different GMCC tissues containallelochemicals that inhibit the emergence of both monocotyledonous and dicotyledonous weeds
1 Introduction
e widespread and improper use of synthetic herbicidesleads to environmental damage and a surge in the devel-opment of herbicide-resistant weed biotypes [1] is hascreated the need for alternative sustainable weed controlmethods Crop rotations with GMCCs mainly practiced inconservation agriculture (CA) and organic farming arearguably the most efficacious environmentally friendly andeconomically feasible alternative or complement to herbi-cides [2] Cover crops play an important role in integrated
weed management (IWM) either as smother crops mulchesor allelopathic crops [3]
Most of the cover crops that are being promoted in thesmallholder sector of Zimbabwe are fast growing andproduce a lot of biomass resulting in rapid canopy closurewhich smothers weeds when they are used as live mulches[4] Alternatively their dead residues may be used as surfacemulches or be incorporated into the soil Weed suppressionwhen cover crop residues are incorporated into the soil mayoccur via altered nutrient dynamics depending on severalother factors including type of tillage used carbon to
HindawiInternational Journal of AgronomyVolume 2021 Article ID 6552928 13 pageshttpsdoiorg10115520216552928
nitrogen ratio (C N) of the decomposing cover crop ma-terial soil type and the environment [5] In addition covercrops can suppress weeds by competing for resources [6]disruption of life cycles of crop bound and crop associatedweeds [7] through resource and light competition [8 9]creation of soil conditions that promote seed decay andpredation [10] and prevention of weed seed set and dispersal[5] Furthermore some cover crops such as stooling rye(Secale cereal L) barley (Hordeum vulgare L) and blacksunnhemp (Crotalaria juncea L) have demonstrated alle-lopathic activity [11ndash13] Ferreira and Reinhardt [14] re-ported the possibility of using cover crops to manageherbicide-resistant weeds
Allelopathy a phenomenon occurring in natural oragricultural communities of plants is defined as the in-hibitory or stimulatory effects of one plant species onanother through the release of chemicals called alle-lochemicals [15] Allelochemicals are released by liveplants directly into the environment as volatiles leachatesandor root exudates [16] e allelochemicals are alsoreleased when the residues of the plants decompose [17]According to Ayeni [18] allelopathy can be exploited toachieve reduced application of synthetic herbicides sincemost allelochemicals do not have residual effects and cantherefore be exploited for early season weed control inarable fields without affecting successive crops in croprotations Barberi and Lo Cascio [19] reported that themaize-cover crop rotations reduced weed density underfield conditions demonstrating the possibility ofexploiting allelopathic cover crops for effective weedcontrol in IWM
e fact that allelopathic effects from decomposingcover crop residues were reported to be more pronouncedon small seeds and early emerging species compared tolarge-seeded crops which offers an opportunity forexploiting them for selective weed control in arable cropproduction [20] Aqueous extracts of velvet bean (Mucunapruriens L) and Jack bean (Canavalia ensiformis L) re-duced weed germination and early seedling development[21 22]
Most of the cover crops being proposed for adoption insmallholder CA are legumes with a low carbon to nitrogen(C N) ratio resulting in a faster decomposition than cerealresidues that have higher C N ratios [23] As such whenallelopathic materials of leguminous cover crops are appliedin the field a considerable amount of nutrients are addedwhich may cause stimulation of weed growth due to min-eralisation [23 24] and hormesis e need for finding al-lelopathic cover crops whose phytotoxicity outweighs theirweed growth stimulation activity is therefore indispensable[25] is work aimed at the assessment of the allelopathicpotential of leaf stem and root aqueous extracts and in-corporated biomass of eight cover crops that are beingpromoted for the adoption in smallholder CA in Zimbabwee hypothesis tested in the present study was that aqueousextracts and soil-incorporated cover crop biomass suppressblackjack (Bidens pilosa L) and goose grass [Eleusine indica(L) Gaertn] germination emergence and early seedlinggrowth
2 Materials and Methods
21 Experimental Site e study was carried out at theUniversity of Zimbabwe (UZ) situated at 1778degS 3105degEwith an altitude of 1523m above sea level Laboratorybioassays were carried out in Petri dishes in the WeedScience laboratory and pot experiments were carried out ina greenhouse at the Department of Plant Production Sci-ences and Technologies between January 2015 and June2016 Average day and night temperatures in the glasshousewere 281degC and 152degC No artificial light was used in theglasshouse
22 Biomass Preparation e GMCCs used in the studywere grown under irrigation in a field at UZrsquos Department ofPlant Production Sciences and Technologies in September2014 e GMCCs were grown in soils with 18 clay 16silt and 66 sand with a pH (CaCl2) of 52 Exchangeablecations in milliequivalents percent (me ) of soil were 761417 and 032 for calcium magnesium and potassiumrespectively Green manure cover crops were grown in thefield at the University of Zimbabwe as described in Rugareet al [26] e preparation of dry biomass powder was donefollowing the method described by Rugare et al [21] withoutany amendments
23 Aqueous Extract Preparation Fifty grams of GMCCpowder were soaked in 1000ml distilled water to produce a5wvminus1 solution on a dry weight basis e solution wasthen stirred for 24 hours at room temperature (25degC) on anorbital shaker at 100 rpm e extracts were then strainedthrough four layers of cheese cloth before being centrifugedat 4000 rpm for 15 minutes e clear solution was pipettedto separate it from the supernatant is stock solution wasthen stored in parafilm sealed bottles at 4degC for 24 hoursbefore being used e stock solution was then diluted withdistilled water to produce 125 25 and 375 plantextracts prior to use Conductivity of the 5wvminus1 aqueousextract concentrations of the ten GMCCs was measuredusing a conductivity meter (Model SX713 version 20 2013-7-30) and the values obtained were used to calculate theosmotic potential using the following formula [27]
Osmotic potential (in MPa) conductivity (in mS)lowast minus0036Osmotic potential values for the different stock solu-tions (5wvminus1 concentrations) of the different GMCCtissue aqueous extract treatments ranging from minus026to minus009
In order to ascertain whether the inhibitory activityexhibited by different extract concentrations of the covercrop tissues was not due to differences in the osmoticpressure of the solutions germination bioassays wereconducted using polyethylene glycol (PEG) 6000 solutions at000 005 010 015 020 025 and 030MPa (ie the rangeof osmotic potentials of the 5wvminus1 of the cover croptissues)
2 International Journal of Agronomy
24 Effect of Cover Crop Aqueous Extracts on Weed SeedGermination and Early SeedlingGrowth e weed bioassayswere carried out in the Weed Science laboratory betweenJanuary and March 2015 e weed laboratory bioassays forindividual GMCCs were laid out as 3 lowast 5 factorial experi-ments with two factors in a completely randomized design(CRD) with four replications e factors were extract tissuetype at three levels (leaf stem and root) and extract con-centration at five levels (0 125 250 375 and 5) In all thelaboratory experiments distilled water was used as thecontrol Each weed species was considered a separate ex-periment and the experiments were repeated once Twenty-five seeds of the respective weeds were counted sterilised in1 sodium hypochlorite for 10minutes rinsed four timeswith distilled water and placed in 90mm diameter Petridishes lined with Whatman No 2 filter paper e seeds inthe Petri dishes were treated with 10ml of the respectiveextract concentration sealed with parafilm and placedrandomly on a table in the laboratory at room temperature(approximately 25degC day temperature) Data on germina-tion plumule and radicle length were collected from fiverandomly selected plants on day 10 and day 14 in blackjackand goose grass respectively Germination was consideredto have occurred when the radicle was 2mm long Ger-mination percentage (G) and germination vigor indices(VI) were calculated using the formulae given below
G a
b1113874 1113875lowast 100 (1)
where a is the number of germinated seeds and b is the totalnumber of seeds in each Petri dish
25 Greenhouse Experiment e glasshouse study wascarried out using the method described by Rugare et al [21]
Each pot experiment was laid out as a completely ran-domized design (CRD)e effect of the different cover croptissues on the emergence and seedling growth of goose grassand blackjack were compared separately (ie leaves of thedifferent cover crops and the stems and roots were evaluatedseparately) No cover crop residues were put in the controlpots e pot experiments were replicated four times andonly goose grass experiments were repeated once e soilused in the goose grass bioassays was granite-derived sandsfrom Domboshava (17deg 37prime S 31deg 10prime E and 1560 metresabove sea level) whereas UZ red soils were used in theblackjack bioassay in order to mimic edaphic factors underwhich the two weeds exert their dominance [22] echemical and physical properties of the soils used are shownin Table 1
Pots measuring 90mm bottom diameter 105mm topdiameter and 65mm height were filled with 400 g of soil inwhich one Gram of compound D (7 N 14 P2O5 7K2O) was added e respective cover crop tissue powderwas thoroughly mixed with soil at a concentration of 1[cover crop biomass concentration adopted from Caamal-Maldonado et al [29] and Fujii [30]] aiming to provide aconcentration of the cover crop residues similar to whatwould be obtained by the cover crops in nutrient depletedsoils under dry land conditions ereafter 25 seeds of therespective weed species were counted and placed on the soilsurface in the pots after which they were covered with a thinlayer of soil A uniform amount of tap water of 150ml wasapplied to the pots daily using a perforated cup e numberof emerged goosegrass and blackjack seedlings in the potswas recorded daily until no further emergence was notede final emergence percentage was calculated using thefollowing formula
emergence percentage number of emerged seeds
total number of seeds in each pottimes 100 (2)
On day 21 after planting of the weed seeds the seedlingswere harvested and washed gently with tap water to removeany soil from the rootse uprooted weeds were oven-driedfor 72 hours at 70degC to obtain the dry weight of the weedse seedling vigor index (SVII) was calculated using theformula adopted from Abdul-Baki and Anderson [28] asfollows
SVII seedling emergence times seedling dry weight (g)
(3)
e inhibition of different cover crop plant parts wasevaluated using the method of Hong et al [24] where in-hibition magnitudes were ranked based on the mean inhi-bition of leaf stem and root soil incorporated biomass in
descending order e average inhibition percentages werethen grouped into three categories where more than 8050 and 20 were classified as the first second and thirdstrongest inhibitory degree respectively e inhibitionpercentage was calculated as follows
inhibition percentage 1 minustreatmentcontrol
1113876 1113877 lowast 100 (4)
26 Data Analysis e data were tested for normality usingthe ShapirondashWilk test and subjected to analysis of variance(ANOVA) using Genstat 18th edition e seedling vigorindex (SVII) data did not meet the assumptions of ANOVAand were radic (x+ 05) transformed before being analysed
International Journal of Agronomy 3
using Genstat version 18 Significantly different means wereseparated using Fischerrsquos protected least significance dif-ference (LSD) at 5 significance level
3 Results
31 Germination and Early Seedling Growth of Blackjack andGoosegrass in PEG 6000 Solutions e seedling vigor indexof goose grass seeds was significantly (plt 005) affected bythe different PEG solutions (Table 2) On the other handthere were no significant differences in the seedling vigorindex of blackjack in PEG 6000 solutions of different os-motic potentials (Table 2)
32 Effect of GMCC Aqueous Extracts on Goose Grass andBlackjack Germination Parameters e interaction of ex-tract tissue and concentration was significant (plt 005) onthe germination percentage of goose grass and blackjackacross all the GMCCs (Table 3) Leaf extracts reduced thegermination of both weeds significantly better than the stemand root extracts Similarly the interaction between extracttissue and concentration was significant (plt 005) on radicleand plumule length of goosegrass and blackjack across allGMCCs (Tables 4 and 5 respectively) GMCC aqueousextracts exhibited phytotoxic activity on radicle growth inthe order leafgt rootgt stem Consequently leaf extractssignificantly (plt 05) reduced the germination vigor indexbetter than the other tissue extracts across all the GMCCsexcept radish whose root extracts showed greater phytotoxicactivity than the other extract tissues (Table 6)
33 Effect of Cover Crop Soil-Incorporated Residues onEmergenceDryWeight andSeedlingVigor IndexofBlackjacke emergence of blackjack was significantly (plt 005) af-fected by different cover crop leaf stem and root soil-in-corporated residues (Table 7) Leaf stem and root soil-incorporated biomass inhibited blackjack seedling emer-gence by 42ndash96 49ndash94 and 46ndash93 respectivelyGenerally the leaf tissues were more inhibitory than thestem and root tissues Overall tephrosia and hyacinth beanresidues inhibited blackjack emergence significantly(plt 005) better than the other cover crops with a percentagemean inhibition of above 80 Leaf stem and root soil-incorporated residues of all cover crops significantly(plt 005) suppressed the dry weight of blackjack (Table 7)Results showed that among the eight cover crop speciescommon bean hyacinth bean and tephrosia exhibited thegreatest average suppression on dry weight of blackjack witha mean inhibition above 80 over the control A showy
rattlebox showed the least mean inhibition of 46 eseedling vigor index of the blackjack was significantly(plt 005) reduced by different tissue residues of the covercrops (Table 7) Percentage inhibition ranged from 82ndash9976ndash100 and 73ndash97 over control where leaf stem androot residues were used respectively Overall all the covercrops were ranked to be in the first level of allelopathicpotential with mean inhibition above 80
34 Effect of Cover Crop Soil-Incorporated Biomass onEmergence Dry Weight and Seedling Vigor Index of GooseGrass Leaf biomass of all the cover crops except showyrattlebox exhibited significant (plt 005) phytotoxic activityon the emergence of goose grass with inhibition percentagesthat ranged from 24ndash56 (Table 8) Most of the cover cropscaused a weak inhibition of goose grass emergence exceptjack bean hyacinth bean red sunnhemp and commonrattlepod that reduced emergence by more than 50 overthe control Similarly stem residues apart from the showyrattlebox exhibited significant (plt 005) allelopathic activityon the emergence of goose grass with the highest percentagereduction of 70 being observed where black sunnhempstem residues were mixed with the soil Except for showyrattlebox and common bean soil incorporated root residuesof cover crops significantly (plt 005) reduced the emergenceof goose grass Radish and hyacinth bean caused the highestinhibition percentage over the control compared to the othercover crops Results in Table 8 show that the effect of thecover crop leaf residue type on the dry weight potminus1 of goosegrass was significant (plt 005) e soil-incorporated stembiomass of the cover crops significantly (plt 005) reducedgoose grass dry weight by 57ndash97 except showy rattleboxresidues that stimulated weed growth by 12 above the
Table 2 Effect of different osmotic potentials of polyethyleneglycol (PEG) solutions on the seedling vigor index of blackjack andgoose grass
PEG solution osmotic potential (MPa) Goose grass Blackjack000 16342b 58072005 24797a 60883010 24333a 56240015 17726b 60466020 15429b 59266025 24710a 52018030 20888ab 60051p value 0013 0202LSD 65263 NSMeans followed by different letters in the same column are significantlydifferent at plt 005
Table 1 Physical and chemical properties of soil that was used in the study
Soilproperties Clay Silt Sand Ca me Mg me K me Na me CEC me H2O
control Red sunnhemp common bean common rattlepodand black sunnhemp showed strong allelopathic potentialwith mean inhibition percentages that were above 80 Twocover crops (red sunnhemp and common rattlepod) sig-nificantly (plt 005) suppressed seedling vigor of goose grassby more than 80 over the control
4 Discussion
Results obtained demonstrated that all the GMCC extractsused in this study inhibited the germination and earlyseedling growth of goosegrass and blackjack e degree ofinhibition varied greatly amongst the different cover cropsand tissues within each cover crop with leaf extracts showing
the greatest phytotoxic activity followed by the stems exceptradish root extracts that were more efficient on goosegrassthan the other tissue extracts e inhibition of germinationof both weeds especially by leaf extracts of all the cover cropsexcept radish on goosegrass suggests the presence of alle-lochemicals in these GMCCs e reduced seedling growthcaused by the cover crop extracts could also imply reducedearly seedling growth which would give the crops a headstart and competitive advantage over the weeds resulting inreduced crop-weed competition during the early stages ofcrop development when crop seedlings would be sensitive toweed pressure [21]e seedling vigor index gives the overalleffect of the treatments on seedling germination and growth[31] In this study leaf extracts of all the GMCCs and root
Table 3 Effect of aqueous extracts of eight green manure cover crops on germination percentage of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 10582 9834Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 5
extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds
Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of
plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]
e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination
Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack
Extracttissue
Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0997 4851Means followed by the same letter for each GMCC are not significantly different at plt 005
6 International Journal of Agronomy
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
nitrogen ratio (C N) of the decomposing cover crop ma-terial soil type and the environment [5] In addition covercrops can suppress weeds by competing for resources [6]disruption of life cycles of crop bound and crop associatedweeds [7] through resource and light competition [8 9]creation of soil conditions that promote seed decay andpredation [10] and prevention of weed seed set and dispersal[5] Furthermore some cover crops such as stooling rye(Secale cereal L) barley (Hordeum vulgare L) and blacksunnhemp (Crotalaria juncea L) have demonstrated alle-lopathic activity [11ndash13] Ferreira and Reinhardt [14] re-ported the possibility of using cover crops to manageherbicide-resistant weeds
Allelopathy a phenomenon occurring in natural oragricultural communities of plants is defined as the in-hibitory or stimulatory effects of one plant species onanother through the release of chemicals called alle-lochemicals [15] Allelochemicals are released by liveplants directly into the environment as volatiles leachatesandor root exudates [16] e allelochemicals are alsoreleased when the residues of the plants decompose [17]According to Ayeni [18] allelopathy can be exploited toachieve reduced application of synthetic herbicides sincemost allelochemicals do not have residual effects and cantherefore be exploited for early season weed control inarable fields without affecting successive crops in croprotations Barberi and Lo Cascio [19] reported that themaize-cover crop rotations reduced weed density underfield conditions demonstrating the possibility ofexploiting allelopathic cover crops for effective weedcontrol in IWM
e fact that allelopathic effects from decomposingcover crop residues were reported to be more pronouncedon small seeds and early emerging species compared tolarge-seeded crops which offers an opportunity forexploiting them for selective weed control in arable cropproduction [20] Aqueous extracts of velvet bean (Mucunapruriens L) and Jack bean (Canavalia ensiformis L) re-duced weed germination and early seedling development[21 22]
Most of the cover crops being proposed for adoption insmallholder CA are legumes with a low carbon to nitrogen(C N) ratio resulting in a faster decomposition than cerealresidues that have higher C N ratios [23] As such whenallelopathic materials of leguminous cover crops are appliedin the field a considerable amount of nutrients are addedwhich may cause stimulation of weed growth due to min-eralisation [23 24] and hormesis e need for finding al-lelopathic cover crops whose phytotoxicity outweighs theirweed growth stimulation activity is therefore indispensable[25] is work aimed at the assessment of the allelopathicpotential of leaf stem and root aqueous extracts and in-corporated biomass of eight cover crops that are beingpromoted for the adoption in smallholder CA in Zimbabwee hypothesis tested in the present study was that aqueousextracts and soil-incorporated cover crop biomass suppressblackjack (Bidens pilosa L) and goose grass [Eleusine indica(L) Gaertn] germination emergence and early seedlinggrowth
2 Materials and Methods
21 Experimental Site e study was carried out at theUniversity of Zimbabwe (UZ) situated at 1778degS 3105degEwith an altitude of 1523m above sea level Laboratorybioassays were carried out in Petri dishes in the WeedScience laboratory and pot experiments were carried out ina greenhouse at the Department of Plant Production Sci-ences and Technologies between January 2015 and June2016 Average day and night temperatures in the glasshousewere 281degC and 152degC No artificial light was used in theglasshouse
22 Biomass Preparation e GMCCs used in the studywere grown under irrigation in a field at UZrsquos Department ofPlant Production Sciences and Technologies in September2014 e GMCCs were grown in soils with 18 clay 16silt and 66 sand with a pH (CaCl2) of 52 Exchangeablecations in milliequivalents percent (me ) of soil were 761417 and 032 for calcium magnesium and potassiumrespectively Green manure cover crops were grown in thefield at the University of Zimbabwe as described in Rugareet al [26] e preparation of dry biomass powder was donefollowing the method described by Rugare et al [21] withoutany amendments
23 Aqueous Extract Preparation Fifty grams of GMCCpowder were soaked in 1000ml distilled water to produce a5wvminus1 solution on a dry weight basis e solution wasthen stirred for 24 hours at room temperature (25degC) on anorbital shaker at 100 rpm e extracts were then strainedthrough four layers of cheese cloth before being centrifugedat 4000 rpm for 15 minutes e clear solution was pipettedto separate it from the supernatant is stock solution wasthen stored in parafilm sealed bottles at 4degC for 24 hoursbefore being used e stock solution was then diluted withdistilled water to produce 125 25 and 375 plantextracts prior to use Conductivity of the 5wvminus1 aqueousextract concentrations of the ten GMCCs was measuredusing a conductivity meter (Model SX713 version 20 2013-7-30) and the values obtained were used to calculate theosmotic potential using the following formula [27]
Osmotic potential (in MPa) conductivity (in mS)lowast minus0036Osmotic potential values for the different stock solu-tions (5wvminus1 concentrations) of the different GMCCtissue aqueous extract treatments ranging from minus026to minus009
In order to ascertain whether the inhibitory activityexhibited by different extract concentrations of the covercrop tissues was not due to differences in the osmoticpressure of the solutions germination bioassays wereconducted using polyethylene glycol (PEG) 6000 solutions at000 005 010 015 020 025 and 030MPa (ie the rangeof osmotic potentials of the 5wvminus1 of the cover croptissues)
2 International Journal of Agronomy
24 Effect of Cover Crop Aqueous Extracts on Weed SeedGermination and Early SeedlingGrowth e weed bioassayswere carried out in the Weed Science laboratory betweenJanuary and March 2015 e weed laboratory bioassays forindividual GMCCs were laid out as 3 lowast 5 factorial experi-ments with two factors in a completely randomized design(CRD) with four replications e factors were extract tissuetype at three levels (leaf stem and root) and extract con-centration at five levels (0 125 250 375 and 5) In all thelaboratory experiments distilled water was used as thecontrol Each weed species was considered a separate ex-periment and the experiments were repeated once Twenty-five seeds of the respective weeds were counted sterilised in1 sodium hypochlorite for 10minutes rinsed four timeswith distilled water and placed in 90mm diameter Petridishes lined with Whatman No 2 filter paper e seeds inthe Petri dishes were treated with 10ml of the respectiveextract concentration sealed with parafilm and placedrandomly on a table in the laboratory at room temperature(approximately 25degC day temperature) Data on germina-tion plumule and radicle length were collected from fiverandomly selected plants on day 10 and day 14 in blackjackand goose grass respectively Germination was consideredto have occurred when the radicle was 2mm long Ger-mination percentage (G) and germination vigor indices(VI) were calculated using the formulae given below
G a
b1113874 1113875lowast 100 (1)
where a is the number of germinated seeds and b is the totalnumber of seeds in each Petri dish
25 Greenhouse Experiment e glasshouse study wascarried out using the method described by Rugare et al [21]
Each pot experiment was laid out as a completely ran-domized design (CRD)e effect of the different cover croptissues on the emergence and seedling growth of goose grassand blackjack were compared separately (ie leaves of thedifferent cover crops and the stems and roots were evaluatedseparately) No cover crop residues were put in the controlpots e pot experiments were replicated four times andonly goose grass experiments were repeated once e soilused in the goose grass bioassays was granite-derived sandsfrom Domboshava (17deg 37prime S 31deg 10prime E and 1560 metresabove sea level) whereas UZ red soils were used in theblackjack bioassay in order to mimic edaphic factors underwhich the two weeds exert their dominance [22] echemical and physical properties of the soils used are shownin Table 1
Pots measuring 90mm bottom diameter 105mm topdiameter and 65mm height were filled with 400 g of soil inwhich one Gram of compound D (7 N 14 P2O5 7K2O) was added e respective cover crop tissue powderwas thoroughly mixed with soil at a concentration of 1[cover crop biomass concentration adopted from Caamal-Maldonado et al [29] and Fujii [30]] aiming to provide aconcentration of the cover crop residues similar to whatwould be obtained by the cover crops in nutrient depletedsoils under dry land conditions ereafter 25 seeds of therespective weed species were counted and placed on the soilsurface in the pots after which they were covered with a thinlayer of soil A uniform amount of tap water of 150ml wasapplied to the pots daily using a perforated cup e numberof emerged goosegrass and blackjack seedlings in the potswas recorded daily until no further emergence was notede final emergence percentage was calculated using thefollowing formula
emergence percentage number of emerged seeds
total number of seeds in each pottimes 100 (2)
On day 21 after planting of the weed seeds the seedlingswere harvested and washed gently with tap water to removeany soil from the rootse uprooted weeds were oven-driedfor 72 hours at 70degC to obtain the dry weight of the weedse seedling vigor index (SVII) was calculated using theformula adopted from Abdul-Baki and Anderson [28] asfollows
SVII seedling emergence times seedling dry weight (g)
(3)
e inhibition of different cover crop plant parts wasevaluated using the method of Hong et al [24] where in-hibition magnitudes were ranked based on the mean inhi-bition of leaf stem and root soil incorporated biomass in
descending order e average inhibition percentages werethen grouped into three categories where more than 8050 and 20 were classified as the first second and thirdstrongest inhibitory degree respectively e inhibitionpercentage was calculated as follows
inhibition percentage 1 minustreatmentcontrol
1113876 1113877 lowast 100 (4)
26 Data Analysis e data were tested for normality usingthe ShapirondashWilk test and subjected to analysis of variance(ANOVA) using Genstat 18th edition e seedling vigorindex (SVII) data did not meet the assumptions of ANOVAand were radic (x+ 05) transformed before being analysed
International Journal of Agronomy 3
using Genstat version 18 Significantly different means wereseparated using Fischerrsquos protected least significance dif-ference (LSD) at 5 significance level
3 Results
31 Germination and Early Seedling Growth of Blackjack andGoosegrass in PEG 6000 Solutions e seedling vigor indexof goose grass seeds was significantly (plt 005) affected bythe different PEG solutions (Table 2) On the other handthere were no significant differences in the seedling vigorindex of blackjack in PEG 6000 solutions of different os-motic potentials (Table 2)
32 Effect of GMCC Aqueous Extracts on Goose Grass andBlackjack Germination Parameters e interaction of ex-tract tissue and concentration was significant (plt 005) onthe germination percentage of goose grass and blackjackacross all the GMCCs (Table 3) Leaf extracts reduced thegermination of both weeds significantly better than the stemand root extracts Similarly the interaction between extracttissue and concentration was significant (plt 005) on radicleand plumule length of goosegrass and blackjack across allGMCCs (Tables 4 and 5 respectively) GMCC aqueousextracts exhibited phytotoxic activity on radicle growth inthe order leafgt rootgt stem Consequently leaf extractssignificantly (plt 05) reduced the germination vigor indexbetter than the other tissue extracts across all the GMCCsexcept radish whose root extracts showed greater phytotoxicactivity than the other extract tissues (Table 6)
33 Effect of Cover Crop Soil-Incorporated Residues onEmergenceDryWeight andSeedlingVigor IndexofBlackjacke emergence of blackjack was significantly (plt 005) af-fected by different cover crop leaf stem and root soil-in-corporated residues (Table 7) Leaf stem and root soil-incorporated biomass inhibited blackjack seedling emer-gence by 42ndash96 49ndash94 and 46ndash93 respectivelyGenerally the leaf tissues were more inhibitory than thestem and root tissues Overall tephrosia and hyacinth beanresidues inhibited blackjack emergence significantly(plt 005) better than the other cover crops with a percentagemean inhibition of above 80 Leaf stem and root soil-incorporated residues of all cover crops significantly(plt 005) suppressed the dry weight of blackjack (Table 7)Results showed that among the eight cover crop speciescommon bean hyacinth bean and tephrosia exhibited thegreatest average suppression on dry weight of blackjack witha mean inhibition above 80 over the control A showy
rattlebox showed the least mean inhibition of 46 eseedling vigor index of the blackjack was significantly(plt 005) reduced by different tissue residues of the covercrops (Table 7) Percentage inhibition ranged from 82ndash9976ndash100 and 73ndash97 over control where leaf stem androot residues were used respectively Overall all the covercrops were ranked to be in the first level of allelopathicpotential with mean inhibition above 80
34 Effect of Cover Crop Soil-Incorporated Biomass onEmergence Dry Weight and Seedling Vigor Index of GooseGrass Leaf biomass of all the cover crops except showyrattlebox exhibited significant (plt 005) phytotoxic activityon the emergence of goose grass with inhibition percentagesthat ranged from 24ndash56 (Table 8) Most of the cover cropscaused a weak inhibition of goose grass emergence exceptjack bean hyacinth bean red sunnhemp and commonrattlepod that reduced emergence by more than 50 overthe control Similarly stem residues apart from the showyrattlebox exhibited significant (plt 005) allelopathic activityon the emergence of goose grass with the highest percentagereduction of 70 being observed where black sunnhempstem residues were mixed with the soil Except for showyrattlebox and common bean soil incorporated root residuesof cover crops significantly (plt 005) reduced the emergenceof goose grass Radish and hyacinth bean caused the highestinhibition percentage over the control compared to the othercover crops Results in Table 8 show that the effect of thecover crop leaf residue type on the dry weight potminus1 of goosegrass was significant (plt 005) e soil-incorporated stembiomass of the cover crops significantly (plt 005) reducedgoose grass dry weight by 57ndash97 except showy rattleboxresidues that stimulated weed growth by 12 above the
Table 2 Effect of different osmotic potentials of polyethyleneglycol (PEG) solutions on the seedling vigor index of blackjack andgoose grass
PEG solution osmotic potential (MPa) Goose grass Blackjack000 16342b 58072005 24797a 60883010 24333a 56240015 17726b 60466020 15429b 59266025 24710a 52018030 20888ab 60051p value 0013 0202LSD 65263 NSMeans followed by different letters in the same column are significantlydifferent at plt 005
Table 1 Physical and chemical properties of soil that was used in the study
Soilproperties Clay Silt Sand Ca me Mg me K me Na me CEC me H2O
control Red sunnhemp common bean common rattlepodand black sunnhemp showed strong allelopathic potentialwith mean inhibition percentages that were above 80 Twocover crops (red sunnhemp and common rattlepod) sig-nificantly (plt 005) suppressed seedling vigor of goose grassby more than 80 over the control
4 Discussion
Results obtained demonstrated that all the GMCC extractsused in this study inhibited the germination and earlyseedling growth of goosegrass and blackjack e degree ofinhibition varied greatly amongst the different cover cropsand tissues within each cover crop with leaf extracts showing
the greatest phytotoxic activity followed by the stems exceptradish root extracts that were more efficient on goosegrassthan the other tissue extracts e inhibition of germinationof both weeds especially by leaf extracts of all the cover cropsexcept radish on goosegrass suggests the presence of alle-lochemicals in these GMCCs e reduced seedling growthcaused by the cover crop extracts could also imply reducedearly seedling growth which would give the crops a headstart and competitive advantage over the weeds resulting inreduced crop-weed competition during the early stages ofcrop development when crop seedlings would be sensitive toweed pressure [21]e seedling vigor index gives the overalleffect of the treatments on seedling germination and growth[31] In this study leaf extracts of all the GMCCs and root
Table 3 Effect of aqueous extracts of eight green manure cover crops on germination percentage of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 10582 9834Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 5
extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds
Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of
plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]
e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination
Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack
Extracttissue
Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0997 4851Means followed by the same letter for each GMCC are not significantly different at plt 005
6 International Journal of Agronomy
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
24 Effect of Cover Crop Aqueous Extracts on Weed SeedGermination and Early SeedlingGrowth e weed bioassayswere carried out in the Weed Science laboratory betweenJanuary and March 2015 e weed laboratory bioassays forindividual GMCCs were laid out as 3 lowast 5 factorial experi-ments with two factors in a completely randomized design(CRD) with four replications e factors were extract tissuetype at three levels (leaf stem and root) and extract con-centration at five levels (0 125 250 375 and 5) In all thelaboratory experiments distilled water was used as thecontrol Each weed species was considered a separate ex-periment and the experiments were repeated once Twenty-five seeds of the respective weeds were counted sterilised in1 sodium hypochlorite for 10minutes rinsed four timeswith distilled water and placed in 90mm diameter Petridishes lined with Whatman No 2 filter paper e seeds inthe Petri dishes were treated with 10ml of the respectiveextract concentration sealed with parafilm and placedrandomly on a table in the laboratory at room temperature(approximately 25degC day temperature) Data on germina-tion plumule and radicle length were collected from fiverandomly selected plants on day 10 and day 14 in blackjackand goose grass respectively Germination was consideredto have occurred when the radicle was 2mm long Ger-mination percentage (G) and germination vigor indices(VI) were calculated using the formulae given below
G a
b1113874 1113875lowast 100 (1)
where a is the number of germinated seeds and b is the totalnumber of seeds in each Petri dish
25 Greenhouse Experiment e glasshouse study wascarried out using the method described by Rugare et al [21]
Each pot experiment was laid out as a completely ran-domized design (CRD)e effect of the different cover croptissues on the emergence and seedling growth of goose grassand blackjack were compared separately (ie leaves of thedifferent cover crops and the stems and roots were evaluatedseparately) No cover crop residues were put in the controlpots e pot experiments were replicated four times andonly goose grass experiments were repeated once e soilused in the goose grass bioassays was granite-derived sandsfrom Domboshava (17deg 37prime S 31deg 10prime E and 1560 metresabove sea level) whereas UZ red soils were used in theblackjack bioassay in order to mimic edaphic factors underwhich the two weeds exert their dominance [22] echemical and physical properties of the soils used are shownin Table 1
Pots measuring 90mm bottom diameter 105mm topdiameter and 65mm height were filled with 400 g of soil inwhich one Gram of compound D (7 N 14 P2O5 7K2O) was added e respective cover crop tissue powderwas thoroughly mixed with soil at a concentration of 1[cover crop biomass concentration adopted from Caamal-Maldonado et al [29] and Fujii [30]] aiming to provide aconcentration of the cover crop residues similar to whatwould be obtained by the cover crops in nutrient depletedsoils under dry land conditions ereafter 25 seeds of therespective weed species were counted and placed on the soilsurface in the pots after which they were covered with a thinlayer of soil A uniform amount of tap water of 150ml wasapplied to the pots daily using a perforated cup e numberof emerged goosegrass and blackjack seedlings in the potswas recorded daily until no further emergence was notede final emergence percentage was calculated using thefollowing formula
emergence percentage number of emerged seeds
total number of seeds in each pottimes 100 (2)
On day 21 after planting of the weed seeds the seedlingswere harvested and washed gently with tap water to removeany soil from the rootse uprooted weeds were oven-driedfor 72 hours at 70degC to obtain the dry weight of the weedse seedling vigor index (SVII) was calculated using theformula adopted from Abdul-Baki and Anderson [28] asfollows
SVII seedling emergence times seedling dry weight (g)
(3)
e inhibition of different cover crop plant parts wasevaluated using the method of Hong et al [24] where in-hibition magnitudes were ranked based on the mean inhi-bition of leaf stem and root soil incorporated biomass in
descending order e average inhibition percentages werethen grouped into three categories where more than 8050 and 20 were classified as the first second and thirdstrongest inhibitory degree respectively e inhibitionpercentage was calculated as follows
inhibition percentage 1 minustreatmentcontrol
1113876 1113877 lowast 100 (4)
26 Data Analysis e data were tested for normality usingthe ShapirondashWilk test and subjected to analysis of variance(ANOVA) using Genstat 18th edition e seedling vigorindex (SVII) data did not meet the assumptions of ANOVAand were radic (x+ 05) transformed before being analysed
International Journal of Agronomy 3
using Genstat version 18 Significantly different means wereseparated using Fischerrsquos protected least significance dif-ference (LSD) at 5 significance level
3 Results
31 Germination and Early Seedling Growth of Blackjack andGoosegrass in PEG 6000 Solutions e seedling vigor indexof goose grass seeds was significantly (plt 005) affected bythe different PEG solutions (Table 2) On the other handthere were no significant differences in the seedling vigorindex of blackjack in PEG 6000 solutions of different os-motic potentials (Table 2)
32 Effect of GMCC Aqueous Extracts on Goose Grass andBlackjack Germination Parameters e interaction of ex-tract tissue and concentration was significant (plt 005) onthe germination percentage of goose grass and blackjackacross all the GMCCs (Table 3) Leaf extracts reduced thegermination of both weeds significantly better than the stemand root extracts Similarly the interaction between extracttissue and concentration was significant (plt 005) on radicleand plumule length of goosegrass and blackjack across allGMCCs (Tables 4 and 5 respectively) GMCC aqueousextracts exhibited phytotoxic activity on radicle growth inthe order leafgt rootgt stem Consequently leaf extractssignificantly (plt 05) reduced the germination vigor indexbetter than the other tissue extracts across all the GMCCsexcept radish whose root extracts showed greater phytotoxicactivity than the other extract tissues (Table 6)
33 Effect of Cover Crop Soil-Incorporated Residues onEmergenceDryWeight andSeedlingVigor IndexofBlackjacke emergence of blackjack was significantly (plt 005) af-fected by different cover crop leaf stem and root soil-in-corporated residues (Table 7) Leaf stem and root soil-incorporated biomass inhibited blackjack seedling emer-gence by 42ndash96 49ndash94 and 46ndash93 respectivelyGenerally the leaf tissues were more inhibitory than thestem and root tissues Overall tephrosia and hyacinth beanresidues inhibited blackjack emergence significantly(plt 005) better than the other cover crops with a percentagemean inhibition of above 80 Leaf stem and root soil-incorporated residues of all cover crops significantly(plt 005) suppressed the dry weight of blackjack (Table 7)Results showed that among the eight cover crop speciescommon bean hyacinth bean and tephrosia exhibited thegreatest average suppression on dry weight of blackjack witha mean inhibition above 80 over the control A showy
rattlebox showed the least mean inhibition of 46 eseedling vigor index of the blackjack was significantly(plt 005) reduced by different tissue residues of the covercrops (Table 7) Percentage inhibition ranged from 82ndash9976ndash100 and 73ndash97 over control where leaf stem androot residues were used respectively Overall all the covercrops were ranked to be in the first level of allelopathicpotential with mean inhibition above 80
34 Effect of Cover Crop Soil-Incorporated Biomass onEmergence Dry Weight and Seedling Vigor Index of GooseGrass Leaf biomass of all the cover crops except showyrattlebox exhibited significant (plt 005) phytotoxic activityon the emergence of goose grass with inhibition percentagesthat ranged from 24ndash56 (Table 8) Most of the cover cropscaused a weak inhibition of goose grass emergence exceptjack bean hyacinth bean red sunnhemp and commonrattlepod that reduced emergence by more than 50 overthe control Similarly stem residues apart from the showyrattlebox exhibited significant (plt 005) allelopathic activityon the emergence of goose grass with the highest percentagereduction of 70 being observed where black sunnhempstem residues were mixed with the soil Except for showyrattlebox and common bean soil incorporated root residuesof cover crops significantly (plt 005) reduced the emergenceof goose grass Radish and hyacinth bean caused the highestinhibition percentage over the control compared to the othercover crops Results in Table 8 show that the effect of thecover crop leaf residue type on the dry weight potminus1 of goosegrass was significant (plt 005) e soil-incorporated stembiomass of the cover crops significantly (plt 005) reducedgoose grass dry weight by 57ndash97 except showy rattleboxresidues that stimulated weed growth by 12 above the
Table 2 Effect of different osmotic potentials of polyethyleneglycol (PEG) solutions on the seedling vigor index of blackjack andgoose grass
PEG solution osmotic potential (MPa) Goose grass Blackjack000 16342b 58072005 24797a 60883010 24333a 56240015 17726b 60466020 15429b 59266025 24710a 52018030 20888ab 60051p value 0013 0202LSD 65263 NSMeans followed by different letters in the same column are significantlydifferent at plt 005
Table 1 Physical and chemical properties of soil that was used in the study
Soilproperties Clay Silt Sand Ca me Mg me K me Na me CEC me H2O
control Red sunnhemp common bean common rattlepodand black sunnhemp showed strong allelopathic potentialwith mean inhibition percentages that were above 80 Twocover crops (red sunnhemp and common rattlepod) sig-nificantly (plt 005) suppressed seedling vigor of goose grassby more than 80 over the control
4 Discussion
Results obtained demonstrated that all the GMCC extractsused in this study inhibited the germination and earlyseedling growth of goosegrass and blackjack e degree ofinhibition varied greatly amongst the different cover cropsand tissues within each cover crop with leaf extracts showing
the greatest phytotoxic activity followed by the stems exceptradish root extracts that were more efficient on goosegrassthan the other tissue extracts e inhibition of germinationof both weeds especially by leaf extracts of all the cover cropsexcept radish on goosegrass suggests the presence of alle-lochemicals in these GMCCs e reduced seedling growthcaused by the cover crop extracts could also imply reducedearly seedling growth which would give the crops a headstart and competitive advantage over the weeds resulting inreduced crop-weed competition during the early stages ofcrop development when crop seedlings would be sensitive toweed pressure [21]e seedling vigor index gives the overalleffect of the treatments on seedling germination and growth[31] In this study leaf extracts of all the GMCCs and root
Table 3 Effect of aqueous extracts of eight green manure cover crops on germination percentage of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 10582 9834Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 5
extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds
Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of
plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]
e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination
Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack
Extracttissue
Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0997 4851Means followed by the same letter for each GMCC are not significantly different at plt 005
6 International Journal of Agronomy
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
using Genstat version 18 Significantly different means wereseparated using Fischerrsquos protected least significance dif-ference (LSD) at 5 significance level
3 Results
31 Germination and Early Seedling Growth of Blackjack andGoosegrass in PEG 6000 Solutions e seedling vigor indexof goose grass seeds was significantly (plt 005) affected bythe different PEG solutions (Table 2) On the other handthere were no significant differences in the seedling vigorindex of blackjack in PEG 6000 solutions of different os-motic potentials (Table 2)
32 Effect of GMCC Aqueous Extracts on Goose Grass andBlackjack Germination Parameters e interaction of ex-tract tissue and concentration was significant (plt 005) onthe germination percentage of goose grass and blackjackacross all the GMCCs (Table 3) Leaf extracts reduced thegermination of both weeds significantly better than the stemand root extracts Similarly the interaction between extracttissue and concentration was significant (plt 005) on radicleand plumule length of goosegrass and blackjack across allGMCCs (Tables 4 and 5 respectively) GMCC aqueousextracts exhibited phytotoxic activity on radicle growth inthe order leafgt rootgt stem Consequently leaf extractssignificantly (plt 05) reduced the germination vigor indexbetter than the other tissue extracts across all the GMCCsexcept radish whose root extracts showed greater phytotoxicactivity than the other extract tissues (Table 6)
33 Effect of Cover Crop Soil-Incorporated Residues onEmergenceDryWeight andSeedlingVigor IndexofBlackjacke emergence of blackjack was significantly (plt 005) af-fected by different cover crop leaf stem and root soil-in-corporated residues (Table 7) Leaf stem and root soil-incorporated biomass inhibited blackjack seedling emer-gence by 42ndash96 49ndash94 and 46ndash93 respectivelyGenerally the leaf tissues were more inhibitory than thestem and root tissues Overall tephrosia and hyacinth beanresidues inhibited blackjack emergence significantly(plt 005) better than the other cover crops with a percentagemean inhibition of above 80 Leaf stem and root soil-incorporated residues of all cover crops significantly(plt 005) suppressed the dry weight of blackjack (Table 7)Results showed that among the eight cover crop speciescommon bean hyacinth bean and tephrosia exhibited thegreatest average suppression on dry weight of blackjack witha mean inhibition above 80 over the control A showy
rattlebox showed the least mean inhibition of 46 eseedling vigor index of the blackjack was significantly(plt 005) reduced by different tissue residues of the covercrops (Table 7) Percentage inhibition ranged from 82ndash9976ndash100 and 73ndash97 over control where leaf stem androot residues were used respectively Overall all the covercrops were ranked to be in the first level of allelopathicpotential with mean inhibition above 80
34 Effect of Cover Crop Soil-Incorporated Biomass onEmergence Dry Weight and Seedling Vigor Index of GooseGrass Leaf biomass of all the cover crops except showyrattlebox exhibited significant (plt 005) phytotoxic activityon the emergence of goose grass with inhibition percentagesthat ranged from 24ndash56 (Table 8) Most of the cover cropscaused a weak inhibition of goose grass emergence exceptjack bean hyacinth bean red sunnhemp and commonrattlepod that reduced emergence by more than 50 overthe control Similarly stem residues apart from the showyrattlebox exhibited significant (plt 005) allelopathic activityon the emergence of goose grass with the highest percentagereduction of 70 being observed where black sunnhempstem residues were mixed with the soil Except for showyrattlebox and common bean soil incorporated root residuesof cover crops significantly (plt 005) reduced the emergenceof goose grass Radish and hyacinth bean caused the highestinhibition percentage over the control compared to the othercover crops Results in Table 8 show that the effect of thecover crop leaf residue type on the dry weight potminus1 of goosegrass was significant (plt 005) e soil-incorporated stembiomass of the cover crops significantly (plt 005) reducedgoose grass dry weight by 57ndash97 except showy rattleboxresidues that stimulated weed growth by 12 above the
Table 2 Effect of different osmotic potentials of polyethyleneglycol (PEG) solutions on the seedling vigor index of blackjack andgoose grass
PEG solution osmotic potential (MPa) Goose grass Blackjack000 16342b 58072005 24797a 60883010 24333a 56240015 17726b 60466020 15429b 59266025 24710a 52018030 20888ab 60051p value 0013 0202LSD 65263 NSMeans followed by different letters in the same column are significantlydifferent at plt 005
Table 1 Physical and chemical properties of soil that was used in the study
Soilproperties Clay Silt Sand Ca me Mg me K me Na me CEC me H2O
control Red sunnhemp common bean common rattlepodand black sunnhemp showed strong allelopathic potentialwith mean inhibition percentages that were above 80 Twocover crops (red sunnhemp and common rattlepod) sig-nificantly (plt 005) suppressed seedling vigor of goose grassby more than 80 over the control
4 Discussion
Results obtained demonstrated that all the GMCC extractsused in this study inhibited the germination and earlyseedling growth of goosegrass and blackjack e degree ofinhibition varied greatly amongst the different cover cropsand tissues within each cover crop with leaf extracts showing
the greatest phytotoxic activity followed by the stems exceptradish root extracts that were more efficient on goosegrassthan the other tissue extracts e inhibition of germinationof both weeds especially by leaf extracts of all the cover cropsexcept radish on goosegrass suggests the presence of alle-lochemicals in these GMCCs e reduced seedling growthcaused by the cover crop extracts could also imply reducedearly seedling growth which would give the crops a headstart and competitive advantage over the weeds resulting inreduced crop-weed competition during the early stages ofcrop development when crop seedlings would be sensitive toweed pressure [21]e seedling vigor index gives the overalleffect of the treatments on seedling germination and growth[31] In this study leaf extracts of all the GMCCs and root
Table 3 Effect of aqueous extracts of eight green manure cover crops on germination percentage of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 10582 9834Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 5
extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds
Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of
plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]
e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination
Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack
Extracttissue
Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0997 4851Means followed by the same letter for each GMCC are not significantly different at plt 005
6 International Journal of Agronomy
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
control Red sunnhemp common bean common rattlepodand black sunnhemp showed strong allelopathic potentialwith mean inhibition percentages that were above 80 Twocover crops (red sunnhemp and common rattlepod) sig-nificantly (plt 005) suppressed seedling vigor of goose grassby more than 80 over the control
4 Discussion
Results obtained demonstrated that all the GMCC extractsused in this study inhibited the germination and earlyseedling growth of goosegrass and blackjack e degree ofinhibition varied greatly amongst the different cover cropsand tissues within each cover crop with leaf extracts showing
the greatest phytotoxic activity followed by the stems exceptradish root extracts that were more efficient on goosegrassthan the other tissue extracts e inhibition of germinationof both weeds especially by leaf extracts of all the cover cropsexcept radish on goosegrass suggests the presence of alle-lochemicals in these GMCCs e reduced seedling growthcaused by the cover crop extracts could also imply reducedearly seedling growth which would give the crops a headstart and competitive advantage over the weeds resulting inreduced crop-weed competition during the early stages ofcrop development when crop seedlings would be sensitive toweed pressure [21]e seedling vigor index gives the overalleffect of the treatments on seedling germination and growth[31] In this study leaf extracts of all the GMCCs and root
Table 3 Effect of aqueous extracts of eight green manure cover crops on germination percentage of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 10582 9834Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 5
extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds
Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of
plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]
e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination
Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack
Extracttissue
Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0997 4851Means followed by the same letter for each GMCC are not significantly different at plt 005
6 International Journal of Agronomy
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds
Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of
plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]
e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination
Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack
Extracttissue
Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0997 4851Means followed by the same letter for each GMCC are not significantly different at plt 005
6 International Journal of Agronomy
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage
Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory
Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0472 6283Means followed by the same letter for each GMCC are not significantly different at plt 005
International Journal of Agronomy 7
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts
Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root
indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common
Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack
Extracttissue
Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5
LSD 0894 0868Means followed by the same letter for each GMCC are not significantly different at plt 005
8 International Journal of Agronomy
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time
Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]
Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could
Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
International Journal of Agronomy 9
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]
e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed
Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis
Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass
Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages
10 International Journal of Agronomy
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]
5 Conclusion
In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover
crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]
Data Availability
e data used to support the findings of this study areavailable from the corresponding author upon request
Conflicts of Interest
e authors declare that they have no conflicts of interest
Acknowledgments
is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)
References
[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016
[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000
[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015
[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015
[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015
[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011
[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a
International Journal of Agronomy 11
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010
[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015
[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998
[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014
[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012
[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014
[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012
[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010
[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011
[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012
[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984
[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016
[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001
[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000
[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a
[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b
[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011
[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003
[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003
[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019
[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010
[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973
[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001
[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003
[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013
[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002
[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015
[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015
[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013
[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010
[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016
[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014
[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006
12 International Journal of Agronomy
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012
International Journal of Agronomy 13
[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001
[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006
[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007
[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979
[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011
[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010
[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014
[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009
[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006
[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016
[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017
[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005
[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988
[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006
[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012