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Agriculture, Ecosystems and Environment 179 (2013) 151– 162

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment

jo ur nal ho me page: www.elsev ier .com/ locate /agee

eeds – Friend or foe? Increasing forage yield and decreasing nitrateeaching on a corn forage farm infested by redroot pigweed

ajid Gholamhoseinia, Majid AghaAlikhania,∗, Seyed Majid Mirlatifib,eyed Ali Mohammad Modarres Sanavya

Agronomy Department, Faculty of Agriculture, Tarbiat Modares University, Tehran, IranIrrigation and Drainage Engineering Department, Tarbiat Modares University, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 9 April 2013eceived in revised form 14 August 2013ccepted 18 August 2013vailable online 11 September 2013

eywords:conomic evaluationorage qualityrrigation regimes

use efficiencyeed management

a b s t r a c t

Various weed management methods have been tested without complete success and still represent amajor nuisance often negatively effecting yields. Therefore, it may be time to change attitudes aboutweeds and view them as friends of the agroecosystem rather than as foes. For the first time, field experi-ments were conducted to introduce and evaluate the yield and quality of corn–redroot pigweed mixtureforage in a semi-arid region of Iran during 2010 and 2011. A randomized complete block design witha split factorial arrangement of treatments in four replications was subjected to low irrigation andfull irrigation regimes. Subplots consisted of a factorial combination of four N levels (0, 150, 300 and450 kg N ha−1) and two forage mixtures (corn monoculture and corn–redroot pigweed mixture). Whenaveraged over both years, N addition (from 0 to 450 kg N ha−1) increased corn forage yield by 74 and42% under full and low irrigation regimes, respectively. The forage yield increased by 121 and 69% inthe corn–pigweed mixture for comparable treatments. In corn monoculture, the minimum required for-age protein (90 g kg−1) occurred only where forage yields were lower than 10 t ha−1, whereas in thecorn–pigweed mixture, all the treatments with 90 g kg−1 protein produced yield more than 11 t ha−1.

−1 −1

N enhancement (0–450 kg ha ) increased nitrate leaching loss (NLL) by 158 and 107 kg ha in cornmonoculture and 100 and 55 kg ha−1 in the corn–pigweed mixture under full and low irrigation regimes,respectively. However, an alteration in the NLL trend in response to N application grew in both foragetypes, but the NLL severity was reduced in the corn pigweed mixture. The integration of redroot pigweed(a major weed species on summer crop farms) with corn, rather than its removal, could be recommendedto ensure an acceptable forage yield/quality in a poor sandy soil while also reducing N leaching.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Corn forage is an important feed for many dairy and beef oper-tions. The value of forage corn is a function of both its yield anduality. Corn forage is a high-yielding, palatable forage with highnergy density (Armstrong and Albrecht, 2008). Among the manygronomic factors that may affect corn forage yield and quality,he application of water and N are considered to be the most

mportant. Forage or grain corn reportedly has a high irrigationequirement (Payero et al., 2006; Farre and Faci, 2009). Addition-lly, water availability can affect not only crop forage yields but also

∗ Corresponding author at: Agronomy Department, Faculty of Agriculture, Tarbiatodares University, Jalal-Al-Ahmad Highway, Nasr Bridge, Zip Code: 1411713116,

.O. Box 14115-336, Tehran, Iran. Tel.: +98 21 48292099; fax: +98 21 48292200.E-mail address: [email protected] (M. AghaAlikhani).

167-8809/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.agee.2013.08.016

forage quality. Islam et al. (2012) stated that water availability hasprofound effects on the growth and chemical composition of cornforage as a consequence of effects on plant maturity, leaf to stemratios and senescence rate.

While it follows the importance of water, N has a significant rolein realizing the maximum potential of forage crops. Nitrogen fer-tilization increases corn dry matter yield by influencing leaf areadevelopment, leaf area duration and leaf photosynthesis efficiency(Cox and Charney, 2005). Additionally, many investigators havereported that N fertilization increases corn forage quality, includingcrude protein and nutritive value (Lawrence et al., 2008; Ferri et al.,2004). Because N is a mobile nutrient in soil and when it is combinedwith water during excessive application (which often occurs, espe-

cially in sandy soils), high levels of ground water N are predictable.Several studies have investigated the effects of water and N on corngrain and forage yield (Sexton et al., 1996; Al-Kaisi and Yin, 2003;Islam et al., 2012). In general, evaluating the response of corn to

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52 M. Gholamhoseini et al. / Agriculture, Ecos

ombination of irrigation and N may help to identify an appropriatepplication of water and N to maximize profit and reduce groundater pollution.

In addition to water and N, weeds are a major limitation in cornroduction. Weeds can reduce corn dry matter and grain yieldsy 35–70% in different soil and climatic conditions (Mohammadi,007). One of the most aggressive weed species in corn fields isedroot pigweed (Amaranthus retroflexus L.). This plant is a small-eeded, broadleaf weed distributed throughout Iran and other areasf the world. Redroot pigweed is annual and can be difficult to man-ge in agronomic crops because of high seed production, long seediability, extended germination times and relatively fast growthSellers et al., 2003). Another reason that it is successful weeds its history of developing herbicide-resistance biotypes to com-

only used herbicides in row crops. Biotypes of redroot pigweedave developed resistance to different herbicide modes of actionhat once effectively controlled these weeds in row crops (Heap,006). For example, pigweed biotypes with resistance to triaziner acetolactate synthase-inhibiting herbicides have been reportedor redroot pigweed in the USA (Bensch et al., 2003). Additionally,n many developing countries such as Iran, farmer access to effec-ive herbicides for controlling these weeds is limited and othereed control methods including mechanical or biological controlave been used to little effect, so weeds are present throughouthe crop growth period. Furthermore, in developed countries suchs the USA where attention is being given to organic and lownput agriculture systems (Zoschke and Quadranti, 2002), herbicidepplications are limited, which results in the presence of weedsn corn and other crops. Therefore, in recent years redroot pig-

eed frequency and severity have increased and corn producersre often confronted with infestation levels of this weed species.or the first time, this study has assessed the possibility of inte-rating redroot pigweed (a common and dormant weed species inummer crop farms) with corn, rather than weed removal, to pro-uce forage. According to our literature review, there is no actual

nformation on the effects of water and N on corn and corn–redrootigweed forage yield and quality, N and water use efficiency, N

eaching loss or economic evaluations of these practices. Becausehese crucial traits have never been measured in a single experi-

ent, especially in sandy soils, these experiments were conductedo evaluate the yield and quality of corn–redroot pigweed mix-ure forage and to compare it with the yield and quality of forageorn.

. Materials and methods

.1. Experimental location and general methodology

The experiment was conducted in the 2010 and 2011 grow-ng seasons at the research farm of Tarbiat Modares University,ehran, Iran (35◦41′ N, 51◦19′ E and 1215 masl). The regions characterized as semi-arid with a mean annual precipitationf 298 mm, which mostly falls during the autumn and winteronths. Daily meteorological data on precipitation and air tem-

erature (see supplementary file, Table S1) were obtained fromhe nearest weather station (500 m from the experimental site).everal soil samples were taken before planting at depths of–30 and 30–60 cm, and composite samples were collected, air-

ried, crushed, and tested for physical and chemical propertiessee supplementary file, Table S2). The soil texture was sandyoam based on the texture triangle classification (Gee and Bauder,986).

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.agee.2013.08.016.

s and Environment 179 (2013) 151– 162

2.2. Land preparation and treatment

Corn was planted in different sections of the field each yearfollowing canola (Brassica napus L.) in 2010 and wheat (Triticumaestivum L.) in 2011. The field, with 1–2% slope, was prepared byshallow plowing followed by disking in late May. Each experimen-tal unit was 8 m long and consisted of 7 rows spaced 0.75 m apart.There were 2.5 m gaps between the blocks, and a 1.5 m alley wasestablished between each plot to prevent lateral water movementand other interference. A polyethylene pipeline and a flowmeterwere installed to control irrigation. The experiment was conductedusing a randomized complete block design with a split factorialarrangement of treatments in four replications (see supplementaryfile, Figure S1). The main plots were subjected to irrigation regimes,which were defined with respect to water shortages as follows: L,irrigation was initiated after using 80% of the available water (lowirrigation); and F, irrigation was initiated after using 40% of theavailable water (full irrigation). The subplots consisted of a facto-rial combination of four N levels (0, 150, 300, and 450 kg N ha−1) andtwo forage mixtures, namely a corn monoculture and corn–redrootpigweed mixture. These N rates reflect feasible inputs (below aver-age, average or conventional and high average) currently used inIran. The conventional N treatment (300 kg N ha−1) represents atypical farmer’s practice for similar soils in the region.

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.agee.2013.08.016.

The corn cultivar (hybrid SC 604) was sown by hand at depthsof 4–5 cm on 28 May 2010 and 26 May 2011. To ensure good emer-gence, the experimental plots were overseeded and then thinned(to 17 cm spacing in row) to achieve the recommended plant den-sity of 78,000 plants ha−1 at the two-leaf stage (V2, corn growthstage identified according to Ritchie et al. (1997)). At the same timethe corn was seeded, all weedy plot rows were seeded with red-root pigweed at depths of 1 cm in a 14 cm band over the corn rows.Redroot pigweed seeds were collected locally, and their viabilitywas verified in germination tests each year before planting. Redrootpigweed seeds were planted in excess and thinned to populationdensities of 12 plants m−2 at the two-leaf stage. The weed popula-tion for the experiment had a widespread density similar to whatwas observed at infested corn farms (Knezevic et al., 1994; Aguyohand Masiunas, 2003). The soil was irrigated immediately after sow-ing and the irrigation cycle of each plot was closed to avoid runoff.Irrigation was applied by furrow method and irrigation schedulingwas determined according to daily changes in the soil water content(�SW) at the depth of root development. A deficit approach wasused to estimate irrigation requirements, and the soil water con-tent at field capacity (FC) was defined as no water deficit. Availablewater was determined by taking the difference between the watercontent at field capacity and permanent wilting point (PWP). Untilthe corn two-leaf stage (V2), all plots were irrigated in a similarmanner in which 40% of available water was consumed at the depthof root development. N fertilizer (from urea [(NH2)2CO] source) wasapplied by top dressing at the three- to four-leaf stage (1/2 of Ntreatment) and seven- to eight-leaf stage (1/2 of the remaining N).Potassium and P were not applied because the soil had adequateamounts of these minerals (see supplementary file, Table S2). Allweeds other than redroot pigweed were removed throughout thegrowing season with hand hoes.

Time-domain reflectometry (TDR) probes with tube access(TRIME-FM, England) were used to measure soil water content (�v)in the experimental plots (4 points in each plot) at a soil depth of0–80 cm (at 20 cm intervals). Data on soil volumetric water content

were collected daily during the growing season. Prior to seed sow-ing and at the same time of TDR tube access probe installation, soilwater sampler tubes (Model 1900, Soil Moisture Equipment Co.)were inserted into vertical holes (with a diameter of 5–6 cm and

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M. Gholamhoseini et al. / Agriculture, Ecos

epth of 80 cm) created with a hand auger in the middle of eachlot (1 point in each plot). To avoid possible contamination, theeramic cups of the soil water sampler tubes were washed beforensertion, and to facilitate good contact between the ceramic cupnd the soil, the gaps were filled with soft soil.

.3. Soil water sampling and analysis

Determining solute-leaching losses required two sets of infor-ation, namely the drainage flux quantity and the solute

oncentration of the drainage solution. A portable vacuum pumpModel 2005 G2, Soil Moisture Equipment Co.) was used to apply30 kPa of tension to collect water samples every 4–6 days orhen drainage was expected to have occurred, such as after rain

r irrigation (when soil water content was likely to exceed fieldapacity). Water samples were taken from the soil water samplerubes using a thin collection vessel, a vacuum Erlenmeyer flask and

vacuum hand pump. The samples were acidified with sulfuriccid (1 ml per liter) and stored in a refrigerator. Water samplesere analyzed via spectrophotometry (Model dr/2500, Hach Co.)

or NO3− concentrations using the cadmium method. For daily mea-

urements of deep percolation, the water balance equation (Errebhit al., 1998) was used (Eq. (1)) as follows:

aily deep percolation = P + I − �SW − ETC − R (1)

here P is precipitation (mm), I is applied irrigation water (mm),SW is the daily change in soil water content (mm) at the depth of

oot development (as measured by TDR), ETC is crop evapotrans-iration (mm) and R is runoff (mm). There was no runoff becausehe irrigation cycles in each plot were closed. Percolation occurshenever the sum (P + I) is higher than (�SW + ETC) (Vazquez et al.,

005). Water input from irrigation and rainfall were measured athe experiment site. Crop evapotranspiration was calculated dailysing Eqs. (2) and (3) as follows:

TC (in monoculture plots) = ET0 × KC (2)

TC (in mixture plots) = ET0 × 2KC (3)

here ET0 refers to evapotranspiration as calculated by the FAO-enman-Monteith method (Allen et al., 1998), which depends onaily weather conditions at the site, and KC is the crop coeffi-ient. Values of KC calculated by the FAO method (Doorenbos andruitt, 1977; Doorenbos and Kassam, 1979) were used for each cornrowth stage. The initial water storage was equal to the soil waterolding capacity to 80 cm deep (before sowing, when the soil was

ully saturated), and subsequent changes in water storage (�SW)ere determined on a daily basis. In the low irrigation regime (L),

he ETC was adjusted and calculated by Equation 4 (Chow et al.,988; Allen et al., 1998) as follows:

TC-adj = KS × KC × ET0 (4)

here KC and ET0 are the same as in Eq. (2) and (3), and KS is aorrection coefficient (with no dimension) for calculating ETC underater deficient conditions. KS was calculated by Eq. (5) as follows:

S = TAW − DrTAW − RAW

, KS = 1 if Dr < RAW (5)

here TAW is the total available water in the root zone (in mm, dif-erence between the water content at FC and PWP), Dr is the amountf water depletion from the root zone (in mm, monitored on a

aily basis by TDR) and RAW is readily available water in the rootone (in mm, and calculated by multiplying TAW by MAD (manage-ent allowed depletion), which was defined as an 80% depletion

f available soil water in the low irrigation regime).

s and Environment 179 (2013) 151– 162 153

The mass of NO3− in leachate was determined as follows:

NO−3 mass (kg ha−1) = NC × DWP × 0.01 (6)

where NC is the concentrations of nitrate in leachate (mg L−1), DWPis the amount of deep water percolation in each plot (mm) and 0.01is the conversion factor from mg m−2 to kg ha−1.

2.4. Forage measurements

To determine the forage yields, 12 m2 of each plot was handharvested at the corn kernel 50% milk stage (Roth and Lauer, 2008).When corn was at 50% kernel milk, redroot pigweed was often inthe seed filling stage (green-brown seeds). In mixture plots, theforage yield of corn and redroot pigweed was separately measuredand then the sum of the corn and redroot pigweed yield was usedto find the total forage yield in mixed plots. In mixture plots and indifferent irrigation and N fertilization treatments, the percentageof participation by each plant (corn or pigweed) in the total for-age yield was determined. Sub-samples (2 kg plant biomass) weretaken from each plot and then the samples were oven-dried at72 ◦C for 48 h until the weight was constant. Dry samples (wholeplant) were ground to 2 mm using an electrical mill and stored atroom temperature for further nutritional value analysis. It shouldbe stated that sub-samples for forage quality analysis of mixtureplots were taken on the basis of each plant’s participation per-centage from the total forage yield (for example, if the total forageyield in the A mixture plot was 200 units and the corn and redrootpigweed portion of the total yield were 60 and 40%, respectively,the sub-sample for quality analysis was 60% corn biomass and 40%redroot pigweed biomass). The total forage N concentration wasdetermined through a titration method using a Kjeltec instrument(Auto 1030 Analyzer, Tecator). The crude protein (CP) levels werecalculated by multiplying the percentage of N by 6.25 as suggestedby AOAC (1990). In addition, the ash and organic matter weredetermined by using an AOAC (1990) method. Neutral detergentfiber (NDF), acid detergent fiber (ADF) and lignin were estimatedin accordance with Van Soest et al. (1991). Additionally, the totaldigestible nutrients (TDN), concentration of forage NO3

− (Singh,1988) and oxalic acid (Savage et al., 2000) were also determined.

2.5. N use efficiency, irrigation water productivity and economicevaluation

The nitrogen use efficiency (NUE) was calculated for each treat-ment according to the ratio of forage yield to applied N fertilizer(Lopez-Bellido et al., 2005). Furthermore, the irrigation water pro-ductivity (IWP) was estimated by taking the forage yield (kg ha−1)divided by the total seasonal applied irrigation water (mm ha−1).The economic value of the forage was calculated on the basis ofthe TDN% and CP%, which are significant factors in determiningnutritional value (Rostamza et al., 2011). Forage yield revenue iscalculated by multiplying the price of forage based on the CP, TDN% and dry matter yield production for each treatment. The price foreach treatment was estimated as follows (Rostamza et al., 2011):

P =(

(0.55 × TDN%) + (0.45 × CP%)35.9

)× 120 (7)

where P is the price (in US$), TDN% and CP% are the percentageof total digestible nutrients and crude protein for each treatment,respectively, 0.55, 0.45 and 35.9 are equation constants and 120 isthe lowest local price for 1000 kg of forage. The total costs were cal-

culated by adding the water use × water price and N use × N price.The price of 50 kg of urea fertilizer and 1000 m3 of water were con-sidered to be 8 US$ and 12.5 US$, respectively (Mokhtassi-Bidgoliet al., 2012). It should be stated that other costs were the same for

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ig. 1. Trends of forage yield in different treatments in response to applied N. In thhe same letter are not significantly different (p ≤ 0.05). Abbreviations: C-L ir, cornegime; CP-L ir, corn–pigweed mixture under low irrigation regime; CP-F ir, corn–p

ll treatments. The profit was calculated by subtracting the costsrom the revenue.

.6. Data statistical analysis

All data were subjected to an analysis of variance (ANOVA) withAS software (SAS Institute, 2002). Nitrate concentration data wereog-transformed before analysis to achieve normal distribution oromogeneity of variance. Because Bartlett’s test did not show vari-nce homogeneity among the traits, the data were subjected to annalysis of variance year by year. When an F-test indicated statisti-al significance at p < 0.01 or p < 0.05, the protected least significantifference (LSD) was used to separate the means of the main effect,nd the significant interaction effects were separated by the regres-ion and slicing methods.

. Results and discussion

.1. Forage yield

In both years, the forage yield of corn–pigweed mixture reactedore to enhanced N application than corn monoculture (Fig. 1).hen averaged over both years, the N application enhancement

0–450 kg N ha−1) resulted in an increase of 74 and 42% in cornorage yield under full and low irrigation regimes, respectively.n contrast, the forage yield increased by 121 and 69% in theorn–pigweed mixture treatment when the N application wasnhanced (0–450 kg N ha−1) in full and low irrigation regimes,espectively (Fig. 1).

When averaged over the years and irrigation regimes, the differ-nces between the corn–pigweed mixture and corn monocultureorage yields were 206 and 3241 kg ha−1 at N0 and N450, respec-ively (Fig. 1). It seems that the corn–pigweed mixture yield isuperior to corn monoculture, especially at high N applications (300nd 450 kg N ha−1), which is directly related to the presence of pig-eed. It has been reported that some weed species such as redroot

igweed are considered to be luxury consumers of N (Blackshawnd Brandt, 2008), and this might contribute to their ability to takep greater amounts of N when high amounts of N fertilizer werepplied. In this experiment, the enhancement of N at each level

r section of the figures, the N treatment in each row (0–450 kg N ha−1) followed byculture under low irrigation regime; C-F ir, corn monoculture under full irrigationd mixture under full irrigation regime. F.Y: Forage yield.

(0–450 kg N ha−1) significantly increased pigweed dry matter yieldin both irrigation regimes. In contrast, higher amounts of N above300 kg N ha−1 did not significantly increase the corn dry matteryield (Fig. 1). Therefore, it is expected that a significant increasein the corn–pigweed mixture forage yield will appear in relationto corn monoculture, especially at high levels of N application (300and 450 kg N ha−1), which is due to the intense response of pigweeddry matter yields to N application.

In corn monoculture, a decrease in water (from full irrigationto low irrigation) and N (from 450 to 150 kg N ha−1) availabilitydecreased the forage yield by 3331 and 2586 kg ha−1, respec-tively. In the corn–pigweed mixture, a reduction in the water andN availability decreased forage yields by 4138 and 4581 kg ha−1,respectively (Fig. 1). It seems that the higher soil root density inthe corn–pigweed mixture is effective in reducing the sensitiv-ity of the corn–pigweed mixture to water deficiency. Studies oncorn root distribution showed that root growth occurred in a seriesof stages associated with the corn developmental stage (Massingaet al., 2003). In contrast, the redroot pigweed was observed to havedeep and rapid root expansion (Aldrich and Kremer, 1997), a char-acteristic that enabled it to successfully extract water. Therefore,the root system of redroot pigweed allowed it to extract water fromdeeper in the soil profile, leaving more water available near the soilsurface for use by corn in the corn–pigweed mixture.

Additionally, results showed that an application of 450 kg N ha−1

to the corn–pigweed mixture in the low irrigation regime resultedin similar forage yield production with corn monoculture, whichreceived full irrigation accompanied by 300 kg N ha−1 (Fig. 1). Theseresults demonstrate that if optimum amounts of N are available,the corn–pigweed mixture will have the capacity to produce highforage yields, even at low water conditions.

According to Figure S2 (see supplementary file, Figure S2) forboth years and irrigation regimes, the corn portion from the totalyield of the corn–pigweed mixture was decreased with enhancedN application, while the pigweed portion increased. However,more than 50% of the forage yield in the corn–pigweed mixture

was attributed to corn at all irrigation regimes and N applica-tion treatments during both years, but the proportion of pigweedin the mixture tended to increase as the N availability increased.In addition, a water shortage increased the pigweed participation

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Table 1Mean comparisons of irrigation regimes, forage types and N rates main effects on forage yield and quality.

Treatments Traits

Forage yield (kg ha−1) Crude protein (g kg−1) #NDF (g kg−1) ADF (g kg−1) Lignin (g kg−1)

2010 2011 2010 2011 2010 2010 2010 2011 2010 2011

Irrigation regimesLow Irrigation (L) 8710 a 10,060 b 80.6 a 97.0 a 451 a 467 a 297 a 323 a 35.5 b 30.1 bFull Irrigation (F) 12,270 a 13,890 a 73.1 b 78.5 b 463 a 470 a 311 a 327 a 40.6 a 38.0 aF-test ns * ** ** ns ns ns ns ** *

Forage typeCorn 9840 b 10,900 b 72.9 b 76.3 b 447 b 450 b 279 b 309 b 31.6 b 25.9 aCorn + Pigweed mixture 11,140 a 13,050 a 80.9 a 99.2 a 467 a 486 a 330 a 341 a 44.5 a 42.2 aF-test ** ** * ** * ** ** ** ** **

Nitrogen rates (kg ha−1)N0 7340 d 8480 d 41.7 c 58.6 c 460 a 474 a 309 a 323 a 36.2 b 33.7 aN150 9880 c 11,190 c 75.0 b 87.5 b 455 a 465 a 315 a 330 a 35.2 b 31.9 aN300 11,480 b 13,250 b 94.0 a 103.2 a 456 a 471 a 310 a 320 a 40.0 a 33.8 aN450 13,260 a 14,980 a 96.8 a 101.6 a 459 a 460 a 283 a 328 a 40.8 a 36.9 aF-test ** ** ** ** ns ns ns ns ** ns

I × W × N (F-test) * * * * ns ns ns ns ns nsCV (%) 15 14 9 7 13 4 7 10 9 12Year average 10,490 b 11,970 a 76.9 b 87.7 a 457 a 468 a 304 a 325 a 38.0 a 34.1 b

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DF, neutral detergent fiber; ADF, acid detergent fiber. Means within each column

* Statistically significant effect in 0.05 of probability levels** Statistically significant effect in 0.01 of probability levels.

ercentage in forage mixture yields during 2010, but there was nouch trend in 2011 (see supplementary file, Figure S2).

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.agee.2013.08.016.

.2. Forage quality

.2.1. Crude protein contentCrude protein is one of the most important types of nutritious

ompounds in livestock feed, and its deficiency in forage can reduceivestock production yields (Peyraud and Astiggaraga, 1998). Inoth forage types (corn and corn–pigweed mixture), N availabilitynhancement and water availability reduction increased the foragerotein content (Table 1). The increased forage protein concen-ration under water shortage conditions did not necessarily resultrom a stimulation of protein synthesis (Wang and Frei, 2011), butather from a concentration effect due to reduced biomass produc-ion under a low irrigation regime. Other investigators also reportedhat N application enhancement increased corn forage protein con-ent (Islam et al., 2012; Cox and Charney, 2005). However, the Npplication enhancement increased the forage protein contents oforn and the corn–pigweed mixture, but a greater response wasbserved in the corn–pigweed mixture than the corn monocultureFig. 2). The greater increase of forage protein in the corn–pigweed

ixture relative to the corn monoculture is linked to pigweed’sbility to absorb and retain high amounts of N in its biomass, espe-ially when N availability is optimal (300–450 kg N ha−1). The high

uptake of pigweed may be a consequence of its greater transpi-ation because soil N is transported primarily by the mass flow ofoil solution (Sleugh et al., 2001). Greater transpiration could resultf pigweed had a smaller root diameter, greater leaf area index, or

ore open stomata during daylight hours.It has been reported that the minimum optimum forage pro-

ein content for livestock rations is 90 g kg−1 (NRC, 2001). By thistandard, the protein content did not reach its optimum level inorn monoculture, even when 450 kg N ha−1 was applied under

full irrigation regime (Fig. 2). Other researchers have shownhat a low concentration of crude protein in corn forage is the

ost important weakness (Simsek et al., 2011; Armstrong et al.,008). Although an N application of more than 300 kg N ha−1 under

h section followed by the same letter are not significantly different (p ≤ 0.05).

low irrigation regime increased the corn forage protein contentto 90 g kg−1, the corn forage yield was low under this treatment.In contrast, when more than 150 kg N ha−1 was applied to thecorn–pigweed mixture in both irrigation regimes, more forage pro-tein was obtained (≥90 g kg−1). In fact, the corn–pigweed mixturesupplied the standard amount of forage protein for most irrigationregimes and fertilizer treatments because pigweed has an optimalability to take up and use N. A simultaneous evaluation of yieldas a quantitative trait and protein content as a qualitative traitof forage showed that the minimum required amount of forageprotein (90 g kg−1) was obtained in corn monoculture, but only intreatments with forage yields lower than 10 t ha−1, whereas in thecorn–pigweed mixture, all the treatments with 90 g of protein kg−1

produced yields higher than 11 t ha−1. The improved protein con-tent of the corn–pigweed mixture could reduce the amount ofsupplements fed from off-farm sources by livestock producers. Inaddition, the increased yield of corn–pigweed mixture per unitarea could supply higher quality forage for livestock and reducethe amount of land needed to produce the required forage.

3.2.2. Neutral detergent fiber (NDF), acid detergent fiber (ADF)and lignin

During both experimental years, NDF and ADF amounts ofcorn forage were significantly lower than the corn–pigweed mix-ture (Table 1). When averaged over both years, the NDF andADF amounts were 448 and 294 g kg−1 in corn, whereas in thecorn–pigweed mixture, the NDF and ADF increased by 7 and14% and reached 476 and 336 g kg−1, respectively (Table 1). Themost important reason for NDF and ADF enhancement in thecorn–pigweed mixture is that the NDF and ADF content of pigweedis higher than in corn. Furthermore, an enhanced plant density (dueto the simultaneous presence of corn and pigweed) also has a directeffect on forage NDF and ADF increases in the corn–pigweed mix-ture. Plant density affects the amounts of forage NDF and ADF byaltering the plant dry matter allocation to different plant organs.In dense canopies such as a corn–pigweed mixture, the plants

elongate their stems to intercept more light. Forage crop stemsreportedly have a higher concentration of cell walls (NDF and ADF)than leaves (Buxton, 1996). Therefore, the NDF and ADF amountsincrease because of enhanced shoot to leaf ratios in the denser

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F e lowt monor igwee

cmtttc3qa3

tridmarsioccTca(lt

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ig. 2. Trends of crude protein in different treatments in response to applied N. In thhe same letter are not significantly different (p ≤ 0.05). Abbreviations: C-L ir, cornegime; CP-L ir, corn–pigweed mixture under low irrigation regime; CP-F ir, corn–p

anopy of the corn–pigweed mixture when compared with cornonoculture. However, the NDF and ADF amounts were higher in

he corn–pigweed mixture than in corn monoculture, but a reduc-ion in forage quality from NDF and ADF increases did not occur inhe corn–pigweed mixture. An optimal amount of forage cell wallontent ranged between 410 and 540 g kg−1 for NDF and 240 and50 g kg−1 for ADF (NRC, 2001). By this standard, the NDF and ADFuantities of the corn–pigweed mixture forage were optimal (467nd 486 g kg−1 for NDF in 2010 and 2011, respectively and 330 and41 g kg−1 for ADF in 2010 and 2011, respectively).

Lignin is one important fibrous-structural compound in plantshat is accumulated in the cell wall during plant aging, and it isesponsible for the lignification of herbaceous tissues. Lignin is anndigestible material for ruminant herbivores and it inhibits theegradation of other cell wall fractions, such as cellulose, by rumenicrobes (Van Soest, 2006). The results of this study showed that

decrease in water availability under the low irrigation regimeeduced the lignin content of both forage types (Table 1). Waterhortage decreased corn forage lignin content by 20 and 32%n 2010 and 2011, respectively. In contrast, the lignin contentf the corn–pigweed mixture was less affected by water defi-iency and low irrigation, which reduced the lignin content of theorn–pigweed mixture by 6 and 13% in 2010 and 2011, respectively.he main reason for the reduction in drought-induced forage ligninontent is the high sensitivity of lignin-producing enzymes, suchs phenylalanine ammonia lyase, to water deficiency. Vincent et al.2005) used a proteomic method to demonstrate that the level ofignin biosynthesis enzymes was reduced in maize plants exposedo water shortage conditions.

When averaged over irrigation and N treatments, the ligninn the corn–pigweed mixture was 41 and 63% higher than inhe corn monoculture in 2010 and 2011, respectively (Table 1).owever, the enhancement of forage lignin concentration broughtbout a forage quality reduction, but it was in accordance withhe standards of the U.S. National Research Council (NRC, 2001),nd the forage lignin content of the corn–pigweed mixture even at

ts maximum (50.3 g kg−1 in a 450 kg N ha−1 treatment under fullrrigation in 2011) is categorized as high quality forage. The aver-ge amount of corn–pigweed mixture lignin (44.5 and 42.2 g kg−1

n 2010 and 2011, respectively) was considerably lower than the

er section of the figures, the N treatment in each row (0–450 kg N ha−1) followed byculture under low irrigation regime; C-F ir, corn monoculture under full irrigationd mixture under full irrigation regime; C.P.: crude protein.

forage lignin content of alfalfa (90 g kg−1) (NRC, 1978) and winterforage grasses such as oats (64 g kg−1) (NRC, 2001). This result canbe considered as an advantage of the corn–pigweed mixture foragequality.

3.3. Antinutritional factors

Two important antinutritional factors in pigweed are nitrateand oxalic acid (Sleugh et al., 2001). Hence, investigating the con-centration of these two substances in corn–pigweed forage underdifferent irrigation regimes and N treatments is of critical impor-tance.

Nitrate toxicity occurs in forages when the rate of nitrate conver-sion to nitrite is higher than the conversion of nitrate to ammonia(Sleugh et al., 2001). Once absorbed into the blood, nitrate will bindto hemoglobin, forming methemoglobin. Because methemoglobinis less efficient in oxygen transport, animals can literally suffocate(Vough et al., 1991). The results showed that although an increasein N application and decrease in water availability increased thenitrate concentration of both forage types, the response of thecorn–pigweed mixture, especially in 2011, was more severe thanin corn (Table 2). Applied N enhancement (0–450 kg ha−1) caused2.15 and 2-fold increases in forage nitrate concentration in cornmonoculture under a low irrigation regime in 2010 and 2011,respectively (Fig. 3). In the corn–pigweed mixture, N enhancementresulted in 2.25 and 4.45-fold increases in forage nitrate concen-tration under low water conditions in 2010 and 2011, respectively(Fig. 3). After applying N fertilizers, the plant N uptake increased.The assimilation of absorbed N within plant protein structures isan energy-consuming process and its necessary energy is providedby plant photosynthesis. Therefore, under low water conditions inwhich photosynthesis capability declines (Andrade et al., 2002),a portion of absorbed N accumulates in plant vacuoles in theform of nitrate, and the forage nitrate concentration consequentlyincreases. This effect is more severe in pigweed, which is a luxury N

consumer. It seems that two factors have an effect on the enhancednitrate concentration in the corn–pigweed mixture relative to thecorn monoculture when different water and N treatments areapplied as follows: (1) the pigweed presence, results showed that

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Table 2Mean comparisons of irrigation regimes, forage types and N rates main effects on forage quality, efficiency and leaching traits.

Treatments Traits

Forage NitrateConcentration (g kg−1)

Forage Oxalic acidConcentration (g kg−1)

NUE# (kg kg−1) IWP (kg m−3) NO3− leaching loss

(kg ha−1)

2010 2011 2010 2011 2010 2010 2010 2011 2010 2011

Irrigation regimesLow Irrigation (L) 7.12 a 5.62 a 19.3 a 18.4 a 37 a 42 b 1.6 a 2.0 a 51 ± 5 b 27 ± 2.4 bFull Irrigation (F) 4.82 b 4.79 a 14.7 b 14.8 b 52 a 59 a 1.3 a 1.6 b 93 ± 9.2 a 54 ± 4.6 aF-test ** ns ** ** ns * ns * ** **

Forage typeCorn 4.33 b 3.40 b 5.9 b 5.4 b 42 b 46 b 1.3 b 1.6 b 91 ± 7.3 a 51 ± 4.7 aCorn + Pigweed mixture 7.61 a 7.01 a 28.0 a 27.8 a 47 a 55 a 1.5 a 2.0 a 53 ± 4.7 b 30 ± 2.7 bF-test ** ** ** ** * ** ** ** ** **

Nitrogen rates (kg ha−1)N0 4.07 d 2.60 d 15.6 b 14.6 b - - 1.0 d 1.3 d 16 ± 0.9 d 7 ± 0.3 dN150 5.18 c 4.38 c 16.3 ab 17.2 ab 66 a 75 a 1.4 c 1.7 c 39 ± 2.2 c 17 ± 1.2 cN300 6.67 b 6.21 b 17.1 ab 17.0 ab 38 b 44 b 1.6 b 2.0 b 80 ± 3.6 b 52 ± 2.7 bN450 7.95 a 7.63 a 19.0 a 17.7 a 29 c 33 c 1.8 a 2.2 a 152 ± 6.3 a 85 ± 3.9 aF-test ** ** * * ** ** ** ** ** **

I × W × N (F-test) * * * * ns ns * * * **

CV (%) 13 15 9 4 16 14 12 14 25 24Year average 5.90 a 5.20 a 17.0 a 16.6 a 45 b 51 a 1.4 b 1.8 a 72 a 40 b

N olumn

pari

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c

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UE, nitrogen use efficiency; IWP, irrigation water productivity. Means within each c* Statistically significant effect in 0.05 of probability levels

** Statistically significant effect in 0.01 of probability levels.

igweed had higher biomass nitrate concentrations than in corn atll levels of applied N, and (2) decreased light availability, whichesulted from higher plant density in corn–pigweed mixture thann corn monoculture.

It has been reported that the formation, stability and activ-ty of nitrate reductive enzymes (nitrate reductase) are directlyependent on the amount of light received by leaves (Marschner,995; Aslam and Huffaker, 1984). Therefore, when low level of light

ntensity and amount penetrates into the lower layers of a denseranopy of corn–pigweed mixture (relative to the corn monocultureanopy), the nitrate reduction ability of plant leaves (a consequencef reduced nitrate reductase enzyme activity) declines, and a high

mount of nitrate is accumulated in corn–pigweed forage.

Although enhanced N (0–450 kg N ha−1) increased forage nitrateoncentrations in both forage types, especially under a low

ig. 3. Trends of forage nitrate concentration in different treatments in response to appliedollowed by the same letter are not significantly different (p ≤ 0.05). Abbreviations: C-L ir,rrigation regime; CP-L ir, corn–pigweed mixture under low irrigation regime; CP-F ir, co

of each section followed by the same letter are not significantly different (p ≤ 0.05).

irrigation regime, this nitrate increase did not result in a severereduction in forage quality. While the actual nitrate concentrationsthat cause toxicity are not clearly defined, Adams et al. (1992),Vough et al. (1991) and Sleugh et al. (2001) noted that a foragenitrate concentration of more than 17.60 g kg−1 presents a potentiallivestock health concern. In this experiment, the maximum nitrateconcentration in corn forage was 6.35 and 4.47 g kg−1 in 2010 and2011, respectively. In the corn–pigweed mixture, the maximumnitrate concentrations were 12.62 and 12.48 g kg−1 in 2010 and2011, respectively (Fig. 3). When averaged over years and treat-ments, the corn monoculture and corn–pigweed mixture foragehad 78 and 48% reductions in nitrate content, respectively, when

compared with the forage nitrate standard value (17.60 g kg−1).Thus, the nitrate level is not critical for toxicity in both foragetypes.

N. In the lower section of the figures, the N treatment in each row (0–450 kg N ha−1) corn monoculture under low irrigation regime; C-F ir, corn monoculture under fullrn–pigweed mixture under full irrigation regime; N.C., nitrate concentration.

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F plied Nf C-L ir,i ir, co

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ig. 4. Trends of oxalic acid concentration in different treatments in response to apollowed by the same letter are not significantly different (p ≤ 0.05). Abbreviations:

rrigation regime; CP-L ir, corn–pigweed mixture under low irrigation regime; CP-F

The study of oxalic acid concentrations as an antinutritionalactor in different species of pigweed forage is of critical impor-ance. Oxalic acid exists in minute amounts in many plants, butts high quantity (>40 g kg−1) in forage could negatively affect thevailability of high value nutrients to livestock (Teutonica andnorr, 1985). Oxalic acid is bound with calcium in livestock rationsnd forms a non-absorbable complex in the digestion system ofivestock (Judparsong et al., 2006). The results showed that thexalic acid concentration was very low in corn forage (Fig. 4).n contrast, the concentration of oxalic acid in corn–pigweedorage was significantly higher than in the corn monocultureuring both experimental years (Table 2). Additionally, differ-nt irrigation and N treatments did not significantly affect thexalic acid concentration in corn forage, whereas enhanced Npplication and decreased water availability led to a significantncrease in the oxalic acid concentration of corn–pigweed mix-ure forage (Fig. 4). Oxalic acid concentration increases in differentpecies of pigweed under low water conditions were reportedy Bressani (1993). In addition, other investigators showed thatxalic acid distribution inside pigweed plants is not homogeneous,nd its highest and lowest amounts were observed in the leavesnd stems, respectively (Savage et al., 2000). The results of thistudy showed that enhanced N application to the corn–pigweedixture resulted in increased leaf area and leaf number in pig-eed (data not shown). The high density of pigweed leaves in

esponse to N application is likely accompanied by an increasen the oxalic acid concentration of the corn–pigweed mixture for-ge.

However, the concentration of oxalic acid in the corn–pigweedixture was significantly higher than in the corn monoculture,

ut the oxalic acid concentration in the corn–pigweed forage didot have any severe antinutritional effects on livestock. It waseported that when animals were provided with feed contain-ng high amounts of oxalic acid (>70 g oxalic acid kg−1 foragery weight), acute poisoning occurs (James and Panter, 1993;yub Shah, 2000). In this experiment, the maximum concen-

ration of oxalic acid in the corn–pigweed forage was 34.2 and3.8 g kg−1 in 2010 and 2011, respectively (Fig. 4). This quantityas 50% lower than the toxicity threshold (70 g kg−1) for this sub-

tance.

. In the lower section of the figures, the N treatment in each row (0–450 kg N ha−1) corn monoculture under low irrigation regime; C-F ir, corn monoculture under fullrn–pigweed mixture under full irrigation regime; O.A., oxalic acid.

3.4. Nitrogen use efficiency (NUE), irrigation water productivity(IWP) and nitrate leaching loss (NLL)

In both experimental years, the NUE was significantly higher inthe corn–pigweed mixture than in the corn monoculture (Table 2).In the corn–pigweed mixture, 47 and 55 kg of dry matter wereobtained for each kg of N applied during 2010 and 2011, respec-tively. In contrast, each kg of N produced 42 and 46 kg dry matter incorn monoculture during 2010 and 2011, respectively. The resultsalso showed that N uptake was higher in the corn–pigweed mixture(175 kg ha−1) than in corn monoculture (123 kg ha−1). Associationsbetween plants with different root systems in the corn–pigweedforage could cause greater N extraction from different depths ofsoil than would normally be observed in corn monoculture. There-fore, the superiority of the corn–pigweed mixture in terms of Nuptake relative to corn resulted in a higher NUE.

Moreover, the results demonstrated that enhanced N appli-cation increased the IWP of both forage types (Table 2). Whenaveraged over the years, the N increase (0–450 kg ha−1) in cornmonoculture led to an enhanced IWP by 73 and 43% in full and lowirrigation regimes, respectively (Fig. 5). In the corn–pigweed mix-ture, a greater IWP response to N was observed, so the enhancedN application resulted in a 120 and 69% increase in the IWPover full and low irrigation regimes, respectively (Fig. 5). Accord-ing to Fig. 6, results showed that the difference between theIWP of the corn–pigweed mixture and the corn monoculture wasinsignificant at low levels of applied N, and this difference becamesignificant when the N was increased. The IWP superiority of thecorn–pigweed mixture over corn monoculture, especially at highapplied N, could be presented as follows: (1) many reports haveshown that pigweed drought tolerance is higher than that of cornbecause of its stronger root system (Putnam, 1990) and lowertranspiration coefficient (Johnson and Henderson, 2002). Hence,pigweed is an effective drought tolerant plant that is directlyaffected by the IWP enhancement in corn–pigweed mixtures. (2)A denser canopy of corn–pigweed mixture relative to the corn

monoculture decreased soil water loss via reduced soil surfaceevaporation, which consequently increased the IWP.

The nitrate leaching loss (NLL) was more than 80% higher in2010 than in 2011 because the nitrate concentration and water

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M. Gholamhoseini et al. / Agriculture, Ecosystems and Environment 179 (2013) 151– 162 159

F ppliedf C-L ir,i r, corn

pbdtaiti2ca

Fff

ig. 5. Trends of irrigation water productivity in different treatments in response to aollowed by the same letter are not significantly different (p ≤ 0.05). Abbreviations:

rrigation regime; CP-L ir, corn–pigweed mixture under low irrigation regime; CP-F i

ercolation were higher in 2010 than in 2011. This difference maye attributed to higher irrigation in 2010, which was necessaryue to the higher temperature relative to 2011 (see supplemen-ary file, Table S1). When averaged over forage types, the seasonalverage nitrate concentration at 80 cm depths was different forrrigation regimes and N levels. The minimum nitrate concen-ration was observed for the LN0 (without N application at low

rrigation regime) treatment (8.63 and 4.87 mg l−1 in 2010 and011, respectively) and the maximum occurred for the FN450 (appli-ation of 450 kg N ha−1 at full irrigation regime) treatment (109.17nd 77.67 mg l−1 in 2010 and 2011, respectively). Other studies

ig. 6. Trends of nitrate leaching loss in different treatments in response to applied N. Iollowed by the same letter are not significantly different (p ≤ 0.05). Abbreviations: C-L iull irrigation regime; CP-L ir, corn–pigweed mixture under low irrigation regime; CP-F ir

N. In the lower section of the figures, the N treatment in each row (0–450 kg N ha−1) corn monoculture under low irrigation regime; C-F ir, corn monoculture under full–pigweed mixture under full irrigation regime; I.W.P., irrigation water productivity.

have reported that leachate nitrate concentrations range from 20to 30 mg l−1 at or below the corn root zone (Gheysari et al., 2009;Cameira et al., 2003). Comparing with other studies (Cameira et al.,2003; Mack et al., 2005), observed leachate nitrate concentrationin the present experiment was high (average over years and treat-ments, 41.57 mg l−1). This is mainly due to sandy soil and irrigationsystem (furrow irrigation method) of the experimental site. More-

over, the results showed that the highest NLL during the bothyears was concurrent with the highest irrigation and N fertilizerapplication (FN450 treatment, 186 and 112 kg ha−1 in 2010 and2011, respectively). These results indicate that as more water was

n the lower section of the figures, the N treatment in each row (0–450 kg N ha−1)r, Corn monoculture under low irrigation regime; C-F ir, Corn monoculture under, corn–pigweed mixture under full irrigation regime; N.L.L., nitrate leaching loss.

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Table 3Mean comparisons of interaction effect between irrigation regimes × forage types × N rates sliced by N on economic traits.

Forage type Irrigation regimes Revenue (US$ ha−1) Costs (US$ ha−1) Profit (US$ ha−1)N (kg ha−1) N (kg ha−1) N (kg ha−1)

0 150 300 450 0 150 300 450 0 150 300 450

2010Corn Low irrigation (L) 832 b 1163 a 1211 a 1378 a 67 119 171 223 765 b 1044 ab 1040 ab 1155 a

Full irrigation (F) 1048 c 1369 b 1732 a 2059 a 120 173 225 277 928 c 1196 bc 1507 ab 1782 aCorn + pigweed mixture Low irrigation (L) 860 c 1093 bc 1317 b 1677 a 67 119 171 223 793 c 974 bc 1141 ab 1454 a

Full irrigation (F) 1038 d 1597 c 1946 b 2284 a 120 173 225 277 918 c 1424 b 1721 ab 2007 a

2011Corn Low irrigation (L) 993 b 1193 ab 1390 a 1412 a 64 116 168 220 929 a 1077 a 1222 a 1192 a

Full irrigation (F) 1212 c 1558 b 1825 ab 2045 a 107 160 212 264 1105 c 1398 bc 1613 ab 1781 aCorn + pigweed mixture Low irrigation (L) 1059 b 1337 b 1660 a 1830 a 64 116 168 220 995 c 1221 bc 1492 ab 1610 a

7 b

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Full irrigation (F) 1110 d 1828 c 232

eans within each row of each section (0–450 kg N ha−1) followed by the same lett

pplied, it induced more drainage and eventually NLL increased. Onhe other hand, the effect of irrigation on NLL was amplified by anncrease in the amount of N applied. Additionally, the results indi-ated that enhanced N (0–450 kg ha−1) increased the NLL by 158nd 107 kg ha−1 in corn monoculture and 100 and 55 kg ha−1 inhe corn–pigweed mixture under full and low irrigation regimes,espectively (Fig. 6). In the corn monoculture, from 150, 300 and50 kg ha−1 applied N; 35, 37 and 42% at full irrigation and 13, 18nd 25% at low irrigation regime, N was wasted as NLL. These val-es for the corn–pigweed mixture were 19, 22 and 24% under full

rrigation and 8, 11 and 14% in the low irrigation regime. Accordingo Fig. 6, although the NLL alteration in response to N applicationas ascending in both forage types, the severity of NLL was reduced

n the corn–pigweed mixture. In the corn–pigweed mixture, Nbsorption by pigweed accounts for the NLL decline relative to theorn monoculture treatment. Generally, these results suggestedhat an increase in the plant density and/or use of mixed culture

ethods could be adapted as an efficacious strategy for decreasinghe NLL in sandy soils. On the other hand, the results demonstratedhat the corn–pigweed mixture did not have noticeable effects onhe NLL reduction when low levels of N (0–150 kg ha−1) and waterlow irrigation regime) were applied (Fig. 6). It seems that the majorLL potentials such as soil nitrate concentration and deep waterercolation are low under low input conditions, so any secondaryactors (in this experiment they would include the simultaneousresence of corn and pigweed) are not able to significantly reduceitrate leaching.

.5. Economic evaluation

Before explaining the results, it is necessary to note here thatrofit means the gross margin because the fixed costs have noteen excluded. However, the fixed costs were the same for allreatments. The results revealed that the maximum revenues fromorn monoculture (2059 and 2045 US$ ha−1 in 2010 and 2011,espectively) and corn–pigweed mixture (2248 and 2735 US$ ha−1

n the first and second year, respectively) were obtained when50 kg N ha−1 and full irrigation were applied (Table 3). In cornonoculture, N applications higher than 150 (under low irrigation

egime) and 300 kg ha−1 (under full irrigation regime) did not sig-ificantly increase revenue. In contrast, each greater level of applied

significantly increased the revenue in the corn–pigweed mixturender both irrigation regimes (Table 3).

When averaged over years and treatments, the enhanced Nfrom 0 to 450 kg ha−1) increased revenue and profit by 89 and

9%, respectively (Table 3). In corn monoculture and corn–pigweedorage, enhanced N application improved revenue by 702 and115 US$ ha−1, respectively. A comparison between the cornonoculture and corn–pigweed mixture showed the resulting

2732 a 107 160 212 264 1003 d 1668 c 2115 b 2468 a

not significantly different (p ≤ 0.05).

revenue from corn–pigweed forage was 13 and 39% higher thanthe corn monoculture during 2010 and 2011, respectively (Table 3).These results revealed that the revenue difference between thecorn–pigweed mixture and corn monoculture under low inputconditions (150 kg N ha−1 under low irrigation regime) was −70and 144 US$ ha−1 in 2010 and 2011, respectively. This differencereached 225 and 687 US$ ha−1 at full input conditions (i.e., an appli-cation of 450 kg N ha−1 at full irrigation regime) in the first andsecond years, respectively. On the other hand, the economic superi-ority of the corn–pigweed mixture relative to the corn monoculturewas noticeable when high amounts of water and N were applied.According to this finding, forage production costs in both foragetypes were assumed to be equal (the costs of pigweed control suchas herbicide application to corn monoculture must be added tothe total costs of this treatment), so the results confirmed that thecorn–pigweed mixture significantly increased the revenue and netprofit of the farmer because of the high forage yield, use of effec-tive input and production of forage containing higher amounts ofprotein.

4. Conclusions

In general, in both forage types (corn and corn–pigweed mix-ture), water and N availability enhancement increased the forageyield and protein content. Additionally, the results showed thatalthough an increase in N application and decrease in water avail-ability increased the nitrate concentration (as an antinutritionalfactor) of both forage types, the response of the corn–pigweed mix-ture was more severe than in corn. When compared with the foragenitrate standard value (17.60 g kg−1), the corn monoculture andcorn–pigweed mixture forage had 78 and 48% reductions in nitratecontent, respectively. Thus, the nitrate level is not critical for toxic-ity in both forage types. The results indicate that the corn–pigweedmixture forage had a higher yield and quality than did corn alone. Inaddition, the corn–pigweed mixture used inputs (water and N) withmore efficiency than that of corn. The superiority of corn–pigweedin terms of forage yield and quality resulted in an improvementin farmer economic conditions (revenue and net profit) comparedwith corn. Therefore, the results suggest that instead of remov-ing the weeds from corn farms infested by redroot pigweed, whichis mainly done by applying large amounts of herbicides, farmerscan harvest corn–pigweed as a mixed forage. This new strategy notonly increased forage yields but also enhanced input use efficiency,forage quality and net profit. Additionally, our results revealedthat the presence of pigweed in corn farm, can be beneficial for

decreasing N leaching loss and improving the sustainability ofagricultural systems. Therefore, forage producers may be able toimprove the economic and environmental sustainability of theiroperations by replacing corn monoculture with a corn–pigweed

Page 11

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ixture. However, more research (such as evaluating increase orecrease of diseases and pests load in the following crops, effect onarm machinery and effect on the following crop weed density) isecessary to more precisely determine how corn–pigweed mixtureffects corn farms and following crops.Acknowledgements

The authors gratefully acknowledge Dr. Yaghoub Fathollahiice-Chancellor for Research Affairs of Tarbiat Modares Univer-ity for his supports and assistance. Appreciation extended toamed Zakikhani, Aydin Khodaie Joghan, Aria Dolatabadian and

avad Rezaei for their technical assistance. We also thank Mr.ebreil, Sefolah, Mostafa and Davoud for their help in conductingeld experiments. The two anonymous reviewers are also grate-

ully acknowledged for their comments, which helped the authorsmprove the manuscript.

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