MECHANICAL PROPERTIES AFFECTING

MECHANICAL PROPERTIES AFFECTINGLEAF LOSS IN ALFALFA

G.S. Vijaya RaghavanStudent Member CSAE

Agricultural Engineering DepartmentColorado State University

Fort Collins, Colorado

W.K. Bilanski

Member CSAE

School of EngineeringUniversity of Guelph

Guelph, Ontario

INTRODUCTION

Alfalfa is an important hay crop withan annual production of about 100million tons in the United States and

about 3 million tons in Ontario (4). Thisamounts to about 20% of the total

harvested cropland (1).

Alfalfa has a protein content from 16to 23% and a carotene content from 190

to 274 ppm (3). Leaf plus petiole fractionconstitute 58% of the total dry weightand contain 92% of the total carotene

and most of the protein value of the crop(3). At present, there is considerable lossof leaves during harvest, resulting in alarge loss of protein and carotene.

Researchers are studying the strippingof leaves from the plants. Stripped stemsregrow leaves in approximately 2 wk.Currence and Buchele (2) reported thatstripped material from the leaf-strip harvester has higher nutrient content andlower fiber content than the whole plant.

This study was made to find theeffects of tension, bending, impact, vibration, and wind on alfalfa leaf and petiolelosses for different moisture contents at

different stages of maturity. It was feltthat the above variables have the most

effects on leaf loss. Understanding theimportance of the above would assist inminimizing leaf loss and supporting theoptimum design of present-day harvestingmachinery.

MATERIAL AND MEASUREMENTS

Alfalfa was field-cut at ground leveland brought to the laboratory. Early bud,first-flower, and full-flower stages of ma-

Paper No. 71-342 presented at the AmericanSociety of Agricultural Engineers Annual Meeting, 1971. ASAE relinquished right to firstpublication at authors' request.

RECEIVED FOR PUBLICATION MAR. 1,

1972

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turity were used in the tests. Each groupof experiments was carried out at different moisture levels from the green stateto dry state. The moisture content ofeach sample was found on a wet basis,using the following relation:

Percent moisture content =

wet wt - dry wt

wet wtX 100 (wet basis) (1)

The above weights were determined to anaccuracy of 0.0001 g (error within ±1%of the sample moisture content). Dryingwas accomplished by placing the samplein a Fischer Isotemperature oven set at280°F (137.8°C) for 2 h. The dry-weightloss of the sample was less than 0.05% ofthe weight of the samples. Leaf loss wasestablished for vibration and tumblingtests using the expression:

Percent leaf loss =

leaf loss (wet wt)sample wt

X100 (2)

APPARATUS AND METHOD

Axial Force Test

A constant-rate loading mechanism (5)(Figure 1) was used to find the force forthis test.

Petioles of the plant were grouped intothree distinct groups: (/') petioles containing large leaves; (h) petioles containingmedium-sized leaves; and (hi) petiolescontaining small-sized leaves. The plantwas held upside down by a rubber-facedclip on a movable jaw (Figure 2). Theother clip at the end of the chain wasused to hold the desired leaf. Initial

readings were noted and care was takento see that the loop of the chain formedwas a minimum. The switch was turned

on and the rider moved down, increasingthe overhang. When the leaf was brokenfrom the petiole, the clip fell off, operating the trip switch to activate the electromagnet and stop the motor. The reading on the scale gave the value of theforce needed to separate the leaf from thepetiole.

Figure 1. Apparatus for determining tensileand bending forces.

Figure 2. Close-up showing leaf clip and tripswitch.

CANADIAN AGRICULTURAL ENGINEERING, VOL. IS, NO. 1, JUNE 1973

This test was carried out for all three

stages of maturity. For each stage, 100plants were taken at different levels ofmoisture contents. From each plant,three samples were tested. Each of thesesamples of the plant was selected fromdifferent sizes of the leaves.

Bending Test

Samples for the test were selectedfrom each stage of maturity. Each plantwas cut into a segment 1-2 inches(2.54-5.08 cm) long leaving only thepetiole and its leaves at the center of thecut stem. By cutting the stem thus, it wasconsidered that the moisture distribution

throughout this sample was even. To treatthe petiole as a cantilever beam, it wasnecessary to position the petiole perpendicular to the stem. Therefore, this stemwas glued to the face of the carriage.Then a load was applied at the loadingpoint (point where the leaves are connected to the petiole) by the constant-rateloading mechanism. When the petiole wasbroken from the stem, the load wasrecorded as the ultimate bending force,and the geometric dimensions of thepetiole were noted. The diameter of thepetiole was measured near the point offracture and the span length of thepetiole was measured from the point offracture to the loading point. One hundred plants at various moisture contentswere used for this test. From each plant,four samples were taken for the test. Intotal, 400 tests were conducted for eachstage of maturity.

Vibration

A single stalk of the plant was fastenedin an inverted position. Samples werethen vibrated at various frequencies forperiods up to 1.25 h. A pan locatedbeneath the sample collected any leavesthat were shaken loose. The samples weretaken at various moisture contents from

the green state to dry state and also atthree different stages of maturity. Foreach test, leaf loss was expressed as apercentage of the sample under test.Moisture content of the stem, leaves, andplant were noted separately for eachparticular test.

Tumbling

The test material was obtained bycutting the plant into 4-inch (10.16-cm)sections. The material was tested at dif

ferent moisture-content levels from the

green state to the air-dry state. Themoisture content of the stem, leaves, andthe combination of stem and leaves was

deteimined. The samples were weighedand placed in the rotating drum for a testperiod of 5 min. After the test, the stem

portion of the plant was carefully separated from the leaves that were broken

from the stem during the treatment. Eachportion of the sample was then weighedon the Mettler balance to determine the

percentage of leaf loss. A total of 300samples was tested at three differentstages of maturity.

Wind Tunnel Test

A suitable rubber-faced wooden clampwas prepared for fixing samples in thevertical wind tunnel. To study the effectsof wind on leaf loss, individual stalksfrom alfalfa were subjected to variouswind velocities in a vertical wind tunnel.

A fairly uniform air velocity across allregions of the stems was expected as thesamples were at right angles to the direction of the wind flow. The leaves lost

during the test could not be recoveredafter the treatment; hence, the totalnumber of leaves per stem at the beginning of each test was recorded for comparison with the number remaining at theend of the test. Seventy samples weretested in the wind tunnel at an air

velocity ranging from 2,000 to 2,800ft/min (0.61 to 0.85 km/min). The testperiod per trial was 1.5 h and the testswere conducted for various moisture-

content levels at all three stages of maturity.

Critical Moisture Content

Previous tests showed that there was a

leaf-moisture content at which alfalfa

leaves became brittle and as such were

susceptible to being broken from thestem. Therefore, the critical moisturecontent was considered to be the leaf-

moisture content at the transition pointbetween the ductile and brittle condi

tions. The following tests were made toestablish the range of the critical moisturecontent. Tests were conducted to find the

highest moisture content possible in thebrittle material, and the lowest moisturecontent possible in the nonbrittle material. To do this, samples were dried atdifferent temperature treatments. As eachsample dried out it became brittle, andwhen this state was reached (assumed), itwas subjected to a crushing action toremove the brittle leaves. During thecrushing treatment, the sample positionwas changed three to four times. Severalpasses were made with a 595-g roller ateach sample position to ensure removal ofall brittle material. Several experimentswere conducted and the results comparedto find the lowest moisture content of

the brittle sample. At the same time, themoisture content of the leaves remainingon the sample gave the higher moisturecontent. The range between these two

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 1, JUNE 1973

moisture contents was expressed as thecritical moisture content range.

RESULTS AND DISCUSSIONS

Axial Force Test

The results of the tests indicated that

the petiole is connected to the stem morerigidly than the leaf is connected to thepetiole.

The axial force required to separatethe leaf from the petiole was found to beup to 150 g when the moisture content(wet basis) was at a maximum (over70%). Figures 3, 4, and 5 show thevariation of force and moisture content

for each stage of maturity. A regressionanalysis showed a quadratic relation between the moisture content and the axial

force. The variation in the shape of the

150 -LARGE R VALUE 073

-MEDIUM R VALUE 0.73

-SMALL R VALUE 0.75

0 10 20 30 40 50 60 70 80

PERCENT MOISTURE CONTENT - (DETERMINED ON WET BASIS)

Figure 3. Curve of axial force vs. moisturecontent for early bud stage of alfalfa.

150-V)

<

% 120-

z

g 90-

ou.

i 60x

<

30

LARGE R VALUE 0.69

MEDIUM R VALUE 0.57

SMALL R VALUE 0.40

0 10 20 30 40 50 60 70 80

PERCENT MOISTURE CONTENT -(DETERMINED ON WET BASIS)

Figure 4. Curve of axial force vs. moisturecontent for first-flower stage of alfalfa.

120 -

90

60

30-LARGE R VALUE 0.69

MEDIUM R VALUE 0.65

SMALL R VALUE 0.48

0 10 20 30 40 50 60 70 80

PERCENT MOISTURE CONTENT - (DETERMINED ON WET BASIS)

Figure 5. Curve of axial force vs. moisturecontent for full-flower stage of alfalfa.

21

curve and the low correlation coefficient

are due to the biological irregularities ofthe sample.

The axial force required to separatethe leaves increased with the size of

leaves. When they were compared on aforce per unit area of the leaf, however, itwas found that the larger leaves were heldsomewhat less securely than the smallerones. The leaf area of the larger leaveswas several times the area of the smaller

leaves, but the axial force of the largerleaf was, at the most, double the axialforce of the smaller ones. Thus, the largerleaves were more susceptible to beingremoved by the axial force occurring inthe field. The results also show that as the

difference in moisture content between

the stem and the leaves increased, theaxial force for leaf removal was reduced.

Hence, the leaves are more susceptiblewhen the difference in moisture content

between the stem and the leaves is high.

Bending Test

In this study, moisture content of thepetiole was considered as the moisturecontent of the plant under test. Fromeach plant, four petioles were tested tofind the ultimate bending force. Therelationship of ultimate bending forcewith (0 span to diameter ratio (l/d); (ii)d3 //; and (hi) Hnear density (weight perunit length of the sample) were foundseparately. The combined effect was alsodetermined at each stage of maturity. Aregression analysis indicated a linear relationship between ultimate bending forceand span to diameter ratio (l/d).

The results of the experiment are asfollows: (0 the ultimate bending forcerequired to separate the petiole from thestem decreases as the plants get older; and(ii) as l/d ratio increases, the bendingforce required to separate the petioledecreases.

Because the ultimate bending force isdependent on the length to diameter ratioof the petiole, the ultimate bending stressshould be calculated. Considering thepetiole as a solid circular cantilever beam,and plotting the ultimate bending forceagainst d3/I, a quadratic regression curvewas found for each stage of maturity.Using the flexural formula and the experimental equation, expressions for ultimatebending stress were determined and tabulated as in Table I for each stage ofmaturity.

Ultimate bending force was plottedagainst Hnear density of the petiole. Aregression analysis indicated a Hnear relationship between the ultimate bending

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TABLE I EXPRESSION FOR BENDING STRESS

Stage of maturity Equation

Early bud stage

First-flower stage

Full-flower stage

P = 2545 + 895.84(//d3) - 6108(tf3//)/ =4682 + 631.16(//tf3) - 10,18Q(tf3//)/ =4988 + 498.82(//tf3) - 12,216(rf3//)

t / = Bending stress (g/mm ); /(mm).

length of the petiole (mm); and d - diameter of the petiole

force and the Hnear density. Ultimatebending force increased linearly with theHnear density and decreased as the plantsmatured. Studies were also made for the

different moisture content groups at eachstage of maturity.

The correlation coefficient (R value)of the equation is low. The reasonsattributed are: (0 the petiole is notperfectly circular in cross section (cross-sectional area varies along the length ofthe petiole; i.e., from the point of support to the loading point); (ii) moisturecontent determinations are not very accurate because there is a time lag betweenthe actual weighing time and the time atwhich the test is conducted; (Hi) biological variations occur in the samples; and(iv) variation in cross section occurs fromthe loading point to the supporting point.

Vibration Test

The regression analysis of the resultsindicated that a linear relationship existedbetween the percentage leaf loss and themoisture content. The treatment in this

test was more severe than the actual

conditions in the field. The loss increased

as the plants matured (Figure 6). Losseswere greater at the dry state than at thegreen state. The test results revealed thatthere was an increase in loss as the

difference in moisture content between

stem and leaves increased.

Tumbling Test

This study was made to find the leafloss due to impact. The regression analysis results indicated that a quadraticrelation exists between the percentageleaf loss and the moisture content of the

plant. The loss of leaves at the dry statein the early bud stage was more pronounced than the loss at the other two

stages; however, there is close similaritybetween the curves for each stage ofmaturity (Figure 7). This, in effect, reveals that the stage of maturity has noeffect on percentage loss of leaves causedby impact. A linear relationship betweenthe percentage loss of leaves due toimpact and the difference in moisturecontent between stem and leaves at each

stage of maturity was established.

55 20-

< Hz

(/) < 15-

10

-EARLY BUD STAGE R VALUE 0.53

-FIRST-FLOWER STAGE R VALUE 0.66

- FULL- FLOWER STAGE R VALUE 0.84

0 10 20 30 40 50 60 70 80

PERCENT MOISTURE CONTENT (WET BASIS)

Figure 6. Leaf loss vs. moisture content foralfalfa in vibration.

25

EARLY BUD STAGE R VALUE 0.64

FIRST-FLOWER STAGE R VALUE 0.80

FULL- FLOWER STAGE R VALUE 0.81

0 10 20 30 40 50 60 70

PERCENT MOISTURE CONTENT (WET BASIS)

Figure 7. Curve of leaf loss vs. moisture content for alfalfa in tumbling.

Wind

The alfalfa plants at various moisturecontents and different stages of maturitywere treated in the wind tunnel with a

velocity of air ranging from 2,000 to2,800 ft/min (0.61 to 0.85 km/min). Thisis above the terminal velocity reported byMenzies and Bilanski (6). The results ofthe test indicated that there was no loss

of leaves due to high-velocity wind at anystage of maturity. Even at the dry state(13%), the effect of wind on leaf loss wasnegligible. These test conditions are comparable with the normal conditions occurring in the field.

Critical Moisture Content

Critical moisture content is the mois

ture content at which the leaves changefrom a ductile to a brittle state. A studywas conducted using different methods oftreatment for various intervals of time to

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 1, JUNE 1973

separate the brittle leaves. The resultsshow that the critical moisture contentranges between 8.5 and 13.5% (wetbasis).

The leaves of the plant become brittle(susceptible to removal) as the moisturecontent decreases to 13.5%. The 8.5% issaid to be the lower limit and 13.5% theupper limit of the critical moisture content.

CONCLUSIONS

1. In each stage of maturity, the axialforce required to separate a leaf depends on the size of the leaf, moisturecontent of the plant, and the difference in moisture content between thestem and leaves.

2. Bending stress equations are reportedfor the petioles of the three stages ofmaturity.

3. The ultimate bending force of thepetiole for each stage of maturityvaries with the span to diameter ratios(l/d and d3/I) and the linear density ofthe petiole. This relationship is determined for each group of moisture-content range.

4. Leaf loss due to vibration for each

stage of maturity increases as themoisture content of the plant decreases and reaches a maximum at thevery dry state. Difference in moisturecontent between stem and leaves alsohas an effect on leaf loss.

5. Tumbling affects a maximum leaf lossof 27% at 6% moisture content of theplant. The loss decreases quadraticallyas moisture increases. In each stage ofmaturity, there is a linear increase inleaf loss as the moisture content difference between stem and leaves increases.

6. Wind, even at velocities of 30 mph(48.3 km/h) has a negligible effect onleaf loss.

7. The critical moisture content exists inthe range from 8.5 to 13.5% (wetbasis).

SUMMARY

The axial and bending force requiredto separate the leaf from the stem atvarious moisture contents of the plantwas determined for the early bud, first-flower, and full-flower stages of alfalfaplant. The effects of vibrating, tumbling,and wind forces on leaf loss were also

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 1, JUNE 1973

studied for the three stages of maturity ofthe plant. Overall results indicated anincrease of leaf loss due to mechanicalforces at low moistures and older stagesof maturity of the plant.

REFERENCES

1. Bainer, R., R.A. Kepner and E.L. Barger.1965. Hay harvesting: mowing, rakingandbailing. Page 304 in Principles of farmmachinery. John Wiley & Sons, Inc., NewYork, N.Y.

2. Currence, H.D. and W.F. Buchele. 1967.Alfalfa leaf strip harvester. Agr. Eng.48(1): 20.

3. Jones, L.G., F.P. Zscheile and R.B.Griffith. 1953. Carotene and protein inalfalfa. Hilgardia 22(6): 179-202.

4. Marx, R.E., and J.W. Birkhead. 1951. Hayharvesting methods and costs. USDA Circ.868.

5. McAulay, J.D. and W.K. Bilanski. 1968.Mechanical properties of leaf loss in birds-foot trefoil. Trans. Amer. Soc. Agr. Eng.11(4): 568-571.

6. Menzies, D.R. and W.K. Bilanski. 1968.Aerodynamic properties of alfalfa particles. Trans. Amer. Soc. Agr. Eng.11(6): 829-831.

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