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Conventional Soybean Plant and Progeny Response to GlyphosateAuthor(s): JASON K. NORSWORTHYSource: Weed Technology, 18(3):527-531. 2004.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/WT-03-066R3URL: http://www.bioone.org/doi/full/10.1614/WT-03-066R3

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527

Weed Technology. 2004. Volume 18:527–531

Conventional Soybean Plant and Progeny Response to Glyphosate1

JASON K. NORSWORTHY2

Abstract: Field studies were conducted to determine the sensitivity of conventional ‘Motte’ and‘Pioneer 9831’ soybean to simulated glyphosate drift rates applied during vegetative and reproductivedevelopment and the effect of glyphosate on progeny. Glyphosate at 8, 84, and 420 g ae/ha wasapplied to soybean at the V3, V6, R2, and R5 growth stages. Glyphosate at 8 and 84 g/ha did notreduce soybean plant population, growth, or yield or cause deleterious effects on progeny, regardlessof the growth stage at application. Soybean population, growth, and yield were reduced as much as99 to 100% after application of 420 g/ha glyphosate at the V3 growth stage. Glyphosate at 420g/ha applied at V6 was less detrimental to soybean compared with the V3 timing. Delaying theapplication of 420 g/ha glyphosate until R2 and R5 reduced soybean yields 22 to 49% and 43 to44%, respectively. Soybean injury from 420 g/ha glyphosate was generally transient or less severewhen applied at the V6 growth stage or later. However, 420 g/ha glyphosate at R5 (initial podfill)caused a 390 to 450 kg/ha yield reduction compared with the V6 application, which indicated greatersoybean vulnerability to glyphosate drift during podfill than in the late-stage vegetative development.Although glyphosate at 420 g/ha was injurious to soybean, regardless of application timing, progenywas not affected.Nomenclature: Glyphosate; soybean, Glycine max (L.) Merr. ‘Motte’, ‘Pioneer 9831’.Additional index words: Glyphosate tolerance, herbicide drift, herbicide injury, off-target move-ment, soybean injury, spray drift.Abbreviation: WAE, weeks after emergence.

INTRODUCTION

Potential off-target movement of glyphosate has in-creased because of increased hectarage of glyphosate-resistant crops and glyphosate usage. In 2001, glyphos-ate was applied to 57 and 73% of the U.S. cotton (Gos-sypium hirsutum L.) and soybean [Glycine max (L.)Merr.] hectarage, respectively (USDA 2001). Glypho-sate, broadcast postemergence, is labeled for glyphosate-resistant corn (Zea mays L.), cotton, and soybean. Theseglyphosate-resistant crops are often planted adjacent toconventional crops.

Before the release of glyphosate-resistant crops, theagronomic use of glyphosate was solely for vegetationcontrol before crop establishment. Now, glyphosate canbe applied over the top of glyphosate-resistant cottonfrom cracking through the fourth true-leaf stage or inglyphosate-resistant soybean from cracking throughoutflowering (Anonymous 2002). In addition, the recent

1 Received for publication February 28, 2003, and in revised form October13, 2003. Technical contribution 4849 from the South Carolina Agricultureand Forestry Research System.

2 Assistant Professor, Department of Crop and Soil Environmental Sciences,Clemson University, 277 Poole Agricultural Center, Clemson, SC 29634. Cor-responding author’s E-mail: jnorswo@clemson.edu.

availability of glyphosate-resistant corn hybrids adaptedto the southern United States could increase glyphosateuse, with applications possible from emergence throughthe eight-leaf stage or 76-cm height (Anonymous 2002).Because full-season and double-cropped soybean areplanted from early May to early July in the southeasternUnited States (Palmer 1999; Raymer 2003), convention-al soybean in fields adjacent to glyphosate-resistant soy-bean differ in growth stage and are prone to glyphosatedrift throughout vegetative and reproductive develop-ment. Furthermore, glyphosate-resistant corn and cottonhave increased the potential for glyphosate drift on con-ventional soybean.

Soybean responds differently to low rates of nonla-beled herbicides in terms of yield loss associated withobserved injury and stunting. Soybean injury and yieldloss from postemergence application of MSMA, fluome-turon, and propanil increased with plant maturity (Bodeand McWhorter 1977), whereas low rates of primisul-furon and nicosulfuron caused greater reductions in soy-bean height when applied during vegetative rather thanreproductive development (Bailey and Kapusta 1993).Injury induced by nicosulfuron reduced yields when ap-plied at the V3 growth stage but not at the R1 growth

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stage. Primisulfuron, regardless of application timing,caused yield loss. Soybean height reductions from sim-ulated dicamba drift also were closely associated withyield reductions; however, visual injury symptoms suchas crinkling and cupping of terminal leaves were ob-served at rates lower than required to reduce yield(Weidhamer et al. 1989).

Al-Khatib and Peterson (1999) reported that glyphos-ate at 280 g/ha applied at the V3 growth stage causedtransient injury to conventional soybean but did not re-duce yields. Similarly, conventional soybean injury fromV2 to V3 or R1 glyphosate application at rates up to 105g/ha was transient and did not affect yield (Ellis andGriffin 2002). Glyphosate applied at 1,280 to 2,550 g/hato conventional soybean approximately 28 d before har-vest did not reduce crop yield but affected seed qualitywith reduced germination and increased atypical plants(Azlin and McWhorter 1981; Whigham and Stoller1979). However, Cerkauskas et al. (1982) found that gly-phosate applied at 2,870 g/ha at the R7 growth stage ofconventional soybean had no effect on seed germinationwhen seeds were harvested 9 d after treatment. The ef-fect of glyphosate drift on harvested seed and its progenyis unknown.

The objectives of this research were to (1) evaluatethe response of conventional soybean to simulated driftrates of glyphosate applied at vegetative and reproduc-tive stages and (2) assess the growth characteristics ofprogeny from glyphosate-treated plants.

MATERIALS AND METHODS

Field experiments were conducted in 2000 and 2001under dryland conditions at the Edisto Research and Ed-ucation Center in Blackville, SC, to evaluate conven-tional soybean response to three glyphosate rates appliedat four soybean growth stages. The soil at the test sitewas a Dunbar sandy loam (fine, kaolinitic, thermic AericPaleaquults), containing 0.6% organic carbon with a pHof 6.0. Preplant incorporation of trifluralin at 840 g ai/ha, manual weeding, and mechanical cultivation pre-vented weed interference from confounding soybeangrowth and yield loss results. The conventional maturitygroup VIII soybeans ‘Pioneer 9831’ and ‘Motte’ wereseeded on May 22, 2000, and May 6, 2001, respectively.Both cultivars were seeded in 97-cm-wide rows at 30seeds/m of row in 9-m-long four-row plots. The exper-imental design was an augmented factorial with a non-treated control added in a randomized complete blockwith four replications. Factor A consisted of glyphosateapplied at 8, 84, and 420 g/ha, and factor B consisted of

application timings at the V3, V6, R2, and R5 soybeangrowth stages (Fehr and Caviness 1977). These rates cor-respond to 1:100, 1:10, and 1:2 of a labeled use rate of840 g/ha for most glyphosate-resistant crops (Anony-mous 2002). Glyphosate3 was applied over the two cen-ter rows using a CO2-pressurized, handheld backpacksprayer equipped with four 11001 XR nozzles spaced 48cm, delivering 94 L/ha at 276 kPa.

Percentage soybean injury and groundcover of alltreatments were visually rated 2 wk after the R5 gly-phosate application on a 0 to 100 scale with 0 5 noinjury or groundcover and 100 5 crop death or completecanopy coverage of the soil surface. Height and widthof three randomly selected living soybean plants andnumber of live soybean plants per meter of row per plotwere recorded 2 wk after the R5 glyphosate application.Using the height and width data, soybean canopy volumeper plant was calculated as a cylindrical volume:

2SV 5 Ph(w/2) ,

where SV represents soybean canopy volume, P is theconstant 3.14, h is soybean height, and w is soybeanwidth. The aboveground soybean plants in the meter ofrow used for population determination were oven-driedat 55 C for 3 wk for biomass dry weight.

At soybean maturity, the center 6 m of two rows ofeach plot was machine harvested with a small-plot com-bine. The percent seed moisture was determined for eachplot, and yields were adjusted to 13% moisture. Seedweight of a 100-seed sample from each plot was used toestimate seed size and numbers.

Soybean seeds collected from nontreated and gly-phosate treatments were planted the next year to deter-mine emergence, seedling development, and yield ofprogeny plants. These studies were conducted under ir-rigation on a Dothan loamy sand (fine-loamy, siliceous,thermic Plinthic Paleudults), containing 0.5% organiccarbon with a pH of 6.1. Field preparation, seeding rates,and weed control were the same as the previous fieldexperiment. Soybean was seeded in four-row plots witha cone planter on May 11, 2001, and May 10, 2002.Soybean population and plant heights were determined2 and 4 wk after emergence (WAE) in addition to har-vesting aboveground biomass from 1 m of row and vi-sually assessing soybean injury 4 WAE. The harvestedbiomass was oven-dried for 3 wk and then weighed. Atphysiological maturity, the center 6 m of two rows ofeach plot were harvested, and yields were adjusted to13% moisture.

3 Glyphosate, Roundup Ultra, Monsanto, St. Louis, MO 63167.

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Table 1. Percent soybean injury and groundcover from postemergence gly-phosate at four soybean developmental stages.

RateGrowthstagea

Injuryb,c

2000 2001

Groundcoverb,d

2000 2001

g ae/ha %

Nontreatede

8—V3V6R2R5

—1 c4 c4 c3 c

—1 c1 c4 c1 c

92 a89 a84 a88 a84 a

95 a97 a95 a96 a96 a

84 V3V6R2R5

6 c6 c

15 bc1 c

15 c6 c5 c5 c

80 a86 a80 a93 a

87 ab93 a94 a96 a

420 V3V6R2R5

99 a35 bc48 b23 bc

79 a54 ab43 b34 b

1 c60 b50 b91 a

39 e58 d71 cd79 bc

a Soybean growth stages at glyphosate application (V3, three trifoliateleaves; V6, six trifoliate leaves; R2, full bloom; R5, initial podfill).

b Means within a column followed by the same letter are not significantlydifferent at P # 0.05 with Fisher’s protected LSD test.

c Injury included chlorosis, necrosis, and growth reduction at 2 wk after theR5 glyphosate application.

d Groundcover was the percent soil surface area covered by soybean foliageassessed 2 wk after the R5 glyphosate application.

e ‘‘—’’ Indicates that the nontreated control was not included in the statis-tical analysis for injury.

All data were subjected to analysis of variance, andmeans were separated using Fisher’s protected LSD testat P # 0.05. All percentage data were arcsine squareroot transformed before data analysis to homogenize var-iances among experimental treatments. Visual injurydata for the nontreated plots were excluded from thisanalysis. Nontransformed percentage data are presentedwith mean separation based on transformed data. Be-cause of a significant year by treatment effect for mostdependent variables, data were analyzed by year and re-sults for each year presented separately.

RESULTS AND DISCUSSION

Rainfall Patterns. From May through June, 14 cm ofrainfall occurred in 2000, whereas twice this amount fellduring the same period in 2001 (data not shown). Be-cause of lack of rainfall after planting in 2000, early-season soybean vegetative growth was delayed com-pared with 2001, but soybean was not visually stressedduring the V3 or V6 glyphosate timings in either year.In 2000, 1 to 2 cm of rainfall occurred 3 d before gly-phosate treatment; however, moisture conditions wererelatively dry in August and September with only 9.5cm of rainfall during both months, excluding a 14-cmrainfall event in late September. In 2001, sufficient rain-fall occurred in July during the later phase of vegetativedevelopment, but dry conditions persisted during mid-September, coinciding with the R5 application.

Plant Measurements. Interveinal chlorosis was visibleonly on newly developed leaves at 8 and 84 g/ha gly-phosate after V3, V6, and R2 applications; whereas oldand new leaves developed chlorosis followed by necrosisafter all applications of 420 g/ha glyphosate, with symp-toms similar to those observed by Al-Khatib and Peter-son (1999). Maximum late-season soybean injury of 4and 15% occurred both years after 8 and 84 g/ha gly-phosate (Table 1), which is comparable with the injurylevel observed in other studies after a V3 glyphosateapplication to conventional soybean (Al-Khatib and Pe-terson 1999; Ellis and Griffin 2002). A V3 applicationof 420 g/ha glyphosate resulted in 79 to 99% soybeaninjury, whereas injury ranged from 23 to 48% and 34 to54% after the V6, R2, and R5 applications in 2000 and2001, respectively (Table 1). Furthermore, crop maturitywas delayed 5 to 7 d after 420 g/ha glyphosate was ap-plied at the R2 or R5 growth stages (data not shown),which also can result from other herbicides that causetransient soybean injury (Nelson and Renner 2001).

Applications of 8 and 84 g/ha glyphosate at all timings

failed to reduce soybean groundcover compared withnontreated soybean in both years (Table 1). Glyphosateat 420 g/ha was most detrimental to soybean ground-cover after the V3 application, resulting in late-seasongroundcover of 1 and 39% in 2000 and 2001, respec-tively (Table 1). Soybean groundcover was generally lessaffected by glyphosate applied during reproductive com-pared with vegetative development. In 2000, the R5 ap-plication of 420 g/ha glyphosate had no detrimental ef-fect on soybean groundcover. The R5 application of 420g/ha glyphosate caused a 16% reduction in soybeangroundcover in 2001. Soybean plants surviving the V3and V6 applications were severely stunted, which retard-ed canopy development. Soybean had produced an ex-tensive canopy (approximately 90% groundcover) by thetime of R2 and R5 applications; thus, reductions ingroundcover at these timings were attributed to leaf se-nescence after treatment.

Soybean plant height after 8 and 84 g/ha glyphosateat all developmental stages was not significantly differ-ent from nontreated soybean in both years; however,plant height was reduced 16 to 26% by 420 g/ha gly-phosate when applied at V6 or R2 in 2000 and 37 to39% at V3 or V6 in 2001 (Table 2). Later applicationsfailed to reduce plant height because soybean had al-ready reached maximum height before glyphosate treat-ment. Similarly, Al-Khatib and Peterson (1999) found

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530 Volume 18, Issue 3 (July–September) 2004

Table 2. Soybean height, canopy volume, plant population, and aboveground biomass after postemergence glyphosate treatment at four soybean developmentalstages.a,b

Rate Growth stagec

Plant height

2000 2001

Canopy volume

2000 2001

Population

2000 2001

Biomass

2000 2001

g ae/ha cm m3 plants/m row g dry wt/m row

Nontreatedd — 71 ab 81 ab 0.30 abc 0.47 ab 19.8 a 22.5 a 680 ab 1,060 ab8 V3

V6R2R5

74 a66 ab69 ab70 ab

82 ab77 abc85 a83 a

0.29 abcd0.28 abcd0.31 abc0.33 ab

0.35 bc0.35 bc0.55 a0.47 ab

19.0 a18.8 a21.3 a21.2 a

22.0 a20.8 ab23.8 a25.0 a

660 abc680 ab590 abc670 abc

1,120 a990 ab960 ab

1,010 ab84 V3

V6R2R5

65 abc76 a69 ab75 a

71 bc74 abc76 abc77 abc

0.24 bcd0.39 a0.35 ab0.40 a

0.32 bcd0.39 ab0.37 bc0.34 bcd

19.0 a21.0 a19.4 a19.0 a

22.0 a24.5 a22.3 a21.0 a

540 bcd740 a640 abc740 a

920 ab890 b950 ab940 ab

420 V3V6R2R5

0 d52 c59 bc70 ab

51 d50 d70 c84 a

0 e0.17 d0.18 cd0.30 abc

0.19 d0.21 cd0.33 bcd0.34 bc

0 c14.6 b19.9 a20.3 a

6.8 c16.5 b23.5 a21.8 a

0 e390 d500 cd640 abc

460 c450 c840 ab990 ab

a Means within a column followed by the same letter are not significantly different at P # 0.05 with Fisher’s protected LSD test.b Measurements were taken 2 wk after the R5 glyphosate application.c Soybean growth stages at glyphosate application (V3, three trifoliate leaves; V6, six trifoliate leaves; R2, full bloom; R5, initial podfill).d ‘‘—’’ Indicates that the nontreated control was not included in the statistical analysis for injury.

Table 3. Influence of postemergence glyphosate and soybean growth stage on seed weight, seed number, and seed yield in 2000 and 2001.a

Rate Growth stageb

Seed weight

2000 2001

Seed number

2000 2001

Seed yield

2000 2001

g ae/ha g/100 seeds seeds/m2 kg/ha

Nontreatedc — 17.1 a 15.2 abc 1,800 a 1,930 ab 3,070 a 2,930 a8 V3

V6R2R5

17.2 a17.6 a16.9 a18.5 a

15.0 abc15.1 abc15.1 abc15.0 abc

1,840 a1,580 ab1,790 a1,700 a

2,040 a1,880 ab2,020 a1,690 bcde

3,160 a2,780 a3,020 a3,140 a

3,060 a2,830 ab3,040 a2,530 abc

84 V3V6R2R5

17.8 a17.5 a18.1 a17.3 a

15.1 abc14.4 c15.6 a15.4 ab

1,550 ab1,660 a1,520 ab1,850 a

1,760 abcd1,950 ab1,810 bc1,970 a

2,750 a2,910 a2,750 a3,210 a

2,660 abc2,810 ab2,820 ab3,040 a

420 V3V6R2R5

11.0 b17.4 a17.7 a17.2 a

14.7 abc14.6 bc15.0 abc14.6 bc

10 d1,210 bc

880 c1,000 c

940 g1,450 ef1,530 cdef1,150 g

10 c2,100 b1,560 b1,710 b

1,380 e2,120 cd2,290 bc1,670 de

a Means within a column followed by the same letter are not significantly different at P # 0.05 with Fisher’s protected LSD test.b Soybean growth stages at glyphosate application (V3, three trifoliate leaves; V6, six trifoliate leaves; R2, full bloom; R5, initial podfill).c ‘‘—’’ Indicates that the nontreated control was not included in the statistical analysis for injury.

soybean height was reduced by a V2 to V3 applicationof 280 g/ha glyphosate, whereas lower rates failed toreduce plant height.

Late-season canopy volume and soybean plant popu-lation were not reduced by 8 or 84 g/ha glyphosate ap-plied at any growth stage in either year (Table 2). Fur-thermore, canopy volume and density after 420 g/ha gly-phosate at either R2 or R5 were similar to nontreatedsoybean. Soybean biomass was reduced 100, 42, and28% in 2000 by 420 g/ha glyphosate applied at V3, V6,and R2, respectively, whereas biomass from all otherrates and timings were similar to that of nontreated soy-bean (Table 2). In 2001, soybean biomass was reduced

43% by 420 g/ha glyphosate applied at the V3 or V6growth stage.

Seed Measurements. Seed weight was influenced lessby glyphosate rate or timing than were whole-plant mea-surements. Seed weight was reduced only by the V3 ap-plication of 420 g/ha glyphosate in 2000 (Table 3), andbecause yields comprise seed weight and number, yieldsclosely followed trends in seed numbers rather than seedweight. In either year, neither seed number nor seedyields were reduced by 8 or 84 g/ha glyphosate, regard-less of the growth stage at application (Table 3). Appli-cations of 420 g/ha glyphosate reduced seed numbers

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and yields 21 to 99%. Soybean yields after V3, V6, R2,and R5 applications of 420 g/ha glyphosate were reduced99, 32, 49, and 44% in 2000 and 53, 28, 22, and 43%in 2001, respectively. In other studies, applications of420 g/ha glyphosate at 23 to 29 d before harvest did notreduce soybean yields (Azlin and McWhorter 1981), butglyphosate at 1,280 and 2,550 g/ha applied 21 and 28 dbefore harvest reduced seed weight and yield (Whighamand Stoller 1979).

Progeny from Glyphosate-Treated Plants. The emer-gence of progeny from glyphosate-treated soybeans wasnot affected, and .50% emergence was observed within7 d after planting in both years. Glyphosate had no ob-servable or detectable effect on population, height,aboveground biomass, seed yield, or visual injury ofprogeny from glyphosate-treated plants (data notshown). A few atypical soybean plants were observed inall treatments, including the nontreated control. How-ever, Azlin and McWhorter (1981) and Whigham andStoller (1979) reported deleterious effects on progenyfrom 1,280 g/ha glyphosate applied approximately 28 dbefore soybean maturity. The lack of a detrimental effecton soybean progeny in this study may be attributed tothe lower glyphosate rate.

When early-season injury from glyphosate did notpersist, conventional soybean yield was not reduced be-cause of the compensatory ability of the plant. Conven-tional soybean yields were unaffected by glyphosaterates as high as 84 g/ha applied from V3 through R5development, which is 10% of a 840-g/ha use rate formost glyphosate-resistant crops. Foliage and plant char-acteristics after 420 g/ha glyphosate at R2 or R5 weregenerally not significantly different from the nontreatedcontrol; however, yields were significantly reduced. Thisresponse indicates that conventional soybean is morevulnerable to glyphosate during podfill than in the laterstages of vegetative development. However, the lack of

injury to progeny from conventional soybeans treated tosimulate glyphosate drift rates of 1, 10, and 50% of thenominal commercial rate indicated that seed viabilitywas not affected by nominal glyphosate drift. In addi-tion, emergence, growth, and yield of progeny from con-ventional soybean plants treated with glyphosate weresimilar to progeny from nontreated plants, indicating thatthere is little potential for yield reductions from gly-phosate drift from neighboring crops or detrimental ef-fects on progeny.

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simulated drift from selected sulfonylurea herbicides, dicamba, glyphos-ate, and glufosinate. Weed Technol. 13:264–270.

Anonymous. 2002. Crop Data Management Systems. Web page: http://www.cdms.net. Accessed: January 22, 2003.

Azlin, W. R. and C. G. McWhorter. 1981. Preharvest effects of applyingglyphosate to soybeans (Glycine max). Weed Sci. 29:123–127.

Bailey, J. A. and G. Kapusta. 1993. Soybean (Glycine max) tolerance to sim-ulated drift of nicosulfuron and primisulfuron. Weed Technol. 7:740–745.

Bode, L. E. and C. G. McWhorter. 1977. Toxicity of MSMA, fluometuron,and propanil to soybeans. Weed Sci. 25:101–105.

Cerkauskas, R. F., O. D. Dhingra, J. B. Sinclair, and S. R. Foor. 1982. Effectof three desiccant herbicides on soybean (Glyine max) seed quality. WeedSci. 30:484–490.

Ellis, J. M. and J. L. Griffin. 2002. Soybean (Glycine max) and cotton (Gos-sypium hirsutum) response to simulated drift of glyphosate and glufosi-nate. Weed Technol. 16:580–586.

Fehr, W. R. and C. E. Caviness. 1977. Stage of Soybean Development. Ames,IA: Iowa State University of Science and Technology, Special Rep. 80.12 p.

Nelson, K. A. and K. A. Renner. 2001. Soybean growth and development asaffected by glyphosate and postemergence herbicide tank mixtures.Agron. J. 93:428–434.

Palmer, J. 1999. Planting considerations. In J. Palmer, ed. South CarolinaSoybean Production Guide. Clemson, SC: Clemson University, ExtensionCircular 501. Pp. 22–24.

Raymer, P. 2003. Cultural practices. In P. Raymer, ed. 2000 Soybean Produc-tion Guide. Web page: http://www.griffin.peachnet.edu/caes/soybeans/03prod/03soyprod.html. Accessed: June 3, 2003.

[USDA] U.S. Department of Agriculture, National Agricultural Statistics Ser-vice. 2001. Agricultural Chemical Usage—2001 Field Crops Summary.July 2002. Web page: http://usda.mannlib.cornell.edu/. Accessed: June 3,2003.

Weidhamer, J. D., G. B. Triplett, Jr., and F. E. Sobotka. 1989. Dicamba injuryto soybean. Agron. J. 81:637–643.

Whigham, D. K. and E. W. Stoller. 1979. Soybean dessication by paraquat,glyphosate, and ametryn to accelerate harvest. Agron. J. 71:630–633.