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Response of nonglyphosate-resistant cotton to reduced rates of glyphosateAuthor(s): Donnie K. Miller, Robert G. Downer, B. Roger Leonard, E. Merritt Holman, and Steve T.KellySource: Weed Science, 52(1):178-182. 2004.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/P2002-089URL: http://www.bioone.org/doi/full/10.1614/P2002-089

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178 • Weed Science 52, January–February 2004

Weed Science, 52:178–182. 2004

Response of nonglyphosate-resistant cotton to reducedrates of glyphosate

Donnie K. MillerCorresponding author. LSU Northeast ResearchStation, LSU AgCenter, St. Joseph, LA 71366;dmiller@agctr.lsu.edu

Robert G. DownerDepartment of Experimental Statistics, LSUAgCenter, Baton Rouge, LA 70803

B. Roger LeonardMacon Ridge Location of Northeast ResearchStation, LSU AgCenter, Winnsboro, LA 71295

E. Merritt HolmanLSU Northeast Research Station, LSU AgCenter, St.Joseph, LA 71366. Current address: Arkansas CropTechnologies, Lonoke, AR 72086

Steve T. KellyScott Research and Extension Center, LSUAgCenter, Winnsboro, LA 71295

Field research was conducted in 1999 and 2000 to determine the effect of reducedglyphosate rates on growth and yield of nonglyphosate-resistant cotton. Rates of 9,18, 35, 70, 140, and 280 g ha21, representing 0.008, 0.016, 0.031 0.063, 0.125,and 0.25, respectively, of the maximum use rate per application (1,120 g ha21), wereapplied to cotton at the two-, five-, or nine-node growth stage. On the basis ofvisual injury estimates, cotton was more tolerant to glyphosate at the nine-node thanat earlier growth stages. Plant dry weight was reduced with 70 g ha21 of glyphosateor higher, when applied at the two- and five-node growth stages in two of threeexperiments. Dry weight was not affected by glyphosate at the nine-node stage. Plantheight also was unaffected by glyphosate rates below 70 g ha21, but height reductionwas noted for all growth stages by experiment combinations, with the exception ofthe nine-node application for both experiments in 2000, with herbicide rates of 70g ha21 or higher. Cotton maturity delay, as noted by an increase in node abovewhite flower number, was observed only at the highest glyphosate rate applied totwo- and five-node cotton in one of three experiments. Percent open boll dataanalysis indicated a decreased opportunity of observing an open boll with increasingglyphosate rate, and this effect was greater at the five-node compared with the two-and nine-node stages in two of three experiments. Seedcotton yield after all gly-phosate applications was equivalent to that for the nontreated control.

Nomenclature: Glyphosate; cotton, Gossypium hirsutum L. ‘Stoneville 474’,‘DP33B’.

Key words: Crop injury, nontarget application, drift.

The registration of glyphosate for use in glyphosate-resis-tant cotton in 1997 has had a tremendous impact on Lou-isiana cotton production. In 2000, estimated acreage plant-ed to glyphosate-resistant cotton varieties in Louisianaranged from 65 to 70% in most areas and was as high as85 to 90% in localized areas (Kelly et al. 2001). Researchhas shown glyphosate to control a broad spectrum of bothgrass and broadleaf weeds (Chandler and Prostko 1997; Cul-pepper and York 1999; Johnson et al. 2000; Miller et al.2001; Shaw et al. 2001), with total postemergence (POST)programs providing control that is improved or comparablewith that of standard herbicide programs (Baldwin 1995;Clay et al. 1995; Culpepper and York 1998). In addition,glyphosate exhibits favorable environmental characteristics(Franz et al. 1997) and offers the potential to reduce oreliminate soil-applied herbicides, thereby reducing total her-bicide use (Culpepper and York 1998). These factors makeglyphosate an effective and popular tool for weed manage-ment in Louisiana cotton.

Increased use of glyphosate-resistant technology by pro-ducers creates the possibility of off-target movement ontoadjacent nonglyphosate-resistant cotton varieties or misap-plication due to sprayer contamination. Previous researchhas indicated that crop growth stage is important in deter-mining potential negative effects of herbicides applied atreduced rates that may be encountered in drift or misappli-cation events. Crawford et al. (1990) reported that 0.05 kgha21 of quinclorac applied POST at cotyledon, two- tothree-leaf, or early square growth stages of cotton reducedyield 13, 33, and 42%, respectively. Snipes et al. (1992)

observed greatest quinclorac effect on cotton at pinheadsquare growth stage, with yield losses ranging from 4.5 to12.3 kg ha21 for each 1 g ha21 of quinclorac applied. Row-land et al. (1999) reported that glyphosate rates rangingfrom 0.026 to 0.42 g ha21 applied to cotton at cotyledon,pinhead square, or early bloom stages resulted in no visualinjury; however, a 20% yield reduction was observed at thehighest rate. In addition to visual injury and yield reduction,other growth parameters may be affected by reduced ratesof herbicides.

Research was initiated to quantify the effects of reducedrates of glyphosate herbicide applied at various crop stageson growth and yield of nonglyphosate-resistant cotton. Thisresearch was conducted to simulate the possible negativeeffects from glyphosate spray drift or misapplication.

Materials and Methods

Field studies were conducted at the Macon Ridge locationof the Northeast Research Station near Winnsboro, LA, in1999 and 2000 and at the Northeast Research Station nearSt. Joseph, LA, in 2000 to determine the effect of reducedglyphosate rates on growth and yield of nonglyphosate-re-sistant cotton. Reduced rates of 9, 18, 35, 70, 140, and 280g ha21, representing 0.008, 0.016, 0.031 0.063, 0.125, and0.25, respectively, of the maximum use rate per applicationof 1,120 g ha21, were applied to cotton at the two-, five-,or nine-node growth stage. A nontreated control was in-cluded for comparison. Experimental design was a random-ized complete block with a factorial arrangement of gly-

Miller et al.: Glyphosate and nonresistant cotton • 179

TABLE 1. Odds ratio estimates and 95% confidence interval esti-mates for observing visual injury 14 d after treatment with a 56 gha21 increase in glyphosate rate.a

Odds ratioestimate Lower limit Upper limit

Two-nodeFive-nodeNine-node

3.43.51.7

1.82.11.3

6.56.92.3

Winnsboro 1999St. Joseph 2000Winnsboro 2000

2.81.83

1.71.31.7

4.52.55.2

a Glyphosate applied at 9, 18, 35, 70, 140, and 280 g ha21. Resultspresented follow logistic regression with herbicide rate as a continuous pre-dictor.

phosate rates and cotton growth stage replicated four times.Applications were broadcast to each 2-m-wide by 12-m-longplot with a CO2-pressurized backpack sprayer delivering140 L ha21 at 255 kPa. Two 0.5- by 1-m plastic shieldswere carried parallel to plot rows and on the outer edge ofthe spray pattern to prevent drift to adjacent plots. ‘Stone-ville 474’ cotton was planted on a Gigger silt loam (fine-silty, mixed, thermic Typic Fragiudalf ) at the Macon Ridgelocation on April 29, 1999 and May 8, 2000. ‘DP 33B’cotton was planted on a Mhoon silt loam (fine-silty, mixednonacid, thermic Typic Fluvaquent) at St. Joseph on May11, 2000. Because of the extreme droughty nature of thesoil at Winnsboro, furrow irrigation was supplied as needed.Plots were maintained weed-free throughout the season byusing trifluralin preplant incorporated at 840 g ha21 andfluometuron preemergence at 1,340 g ha21 and by mechan-ical cultivation and hand hoeing. Insects were controlledaccording to entomologists’ recommendations.

To assess possible negative herbicide effects, the followingmeasurements were taken: a visual assessment of injury 14d after treatment (DAT), plant height 30 DAT, whole plantdry weight 30 DAT, node above white flower (NAWF) mid-season, percent open boll late-season, final plant populationbefore harvest, and seedcotton yield. Visual injury ratingswere based on a scale of 0 (no injury) to 100 (plant death).Plant height and NAWF were determined from 10 random-ly selected plants in each plot. Whole plant dry weight wasrecorded after drying plants removed from a 1-m section ofrow at 60 C for 7 d. Percent open boll was determined bycounting green and open bolls from a 1-m section of row.Final plant population was established by counting all plantsin each 12-m row. Seedcotton yield was calculated afterchemical defoliation and machine harvest of the entire ex-perimental plot.

Statistical AnalysisLinear models with block (replicate), herbicide rate,

growth stage, experiment, and interactions involving the lat-ter three factors were fit for each variable. Initial analysisindicated good linear fit for plant dry weight and heightvariables (r2 5 0.82 and 0.94, respectively). Further analysisof the relationship of these variables with herbicide rate re-vealed a response in which a significant negative responsewas noted for herbicide rates higher than or equal to 70 gha21 but not for lower rates. Because of significant growthstage by experiment and growth stage by herbicide rate in-teractions in the initial full linear model for dry weight, aseparate regression analysis was performed on rates lowerthan 70 g ha21 and rates including and higher than 70 gha21 (Neter et al. 1996). The same analysis was performedon the plant height variable within each of the combinationsof growth stage and experiment because of the significantgrowth stage by herbicide rate by experiment interaction inthe full linear model for that variable.

After the fit of the initial full model, regressions on her-bicide rate were investigated for NAWF, plant population,and yield (Neter et al. 1996). As displayed by the signifi-cance of effects in the full model, some differences did existin the nature of the relationship of each response with her-bicide rate (either significant main effect or an interaction).However, the directional relationships were not consistentamong blocks, so regressions within the combinations of

growth stage and experiment over blocks did not show prac-tically significant relationships similar to those found for dryweight and height early. As a result, the focus changed toan investigation of differences in means for NAWF, plantpopulation, and yield. Herbicide rate was considered as acategorical variable and differences in means separated usingTukey adjustment at the 0.05 level of significance in theselinear models. A significant growth stage by herbicide rateby experiment interaction was noted with respect to NAWF.Analysis of plant population data yielded a significantgrowth stage by herbicide rate interaction, whereas growthstage by experiment interaction and herbicide rate main fac-tor were significant for yield.

The discrete nature of the percentages recorded for injuryand open boll evaluations made standard regressions of orig-inal response questionable. Problems were observed withnormality of errors and heterogeneity of error variance.Transformation of original percentages did not overcomeobserved problems. Percentages were recoded as binary var-iables to absence or presence of injury or open or closed boll(weighted by total number of bolls) and logistic regressionsperformed (Collett 1991). The probability of injury or openboll was modeled as a function of the same factors as thelinear models, with herbicide rate considered a continuouspredictor. With respect to percent injury, interactions of her-bicide rate with growth stage and experiment were signifi-cant and led to separate logistic regressions for levels of thesefactors. The growth stage by herbicide rate by experimentinteraction was significant for percent open boll.

Results and Discussion

Visual injury 14 DAT in the range of 0 to 15%, 20 to35%, and 40 to 65%, expressed primarily as chlorosis andgrowth reduction, was observed in 85, 7, and 8%, respec-tively, of the total plots evaluated in the three experiments(data not shown). Logistic regression analysis results for vi-sual injury are presented in Table 1. Parameter estimates forthe effect of glyphosate rate can be viewed as the odds ofobserving injury for a 56 g ha21 increase in rate. Averagedacross experiments, the estimated odds of injury increased3.4 times for each increase in herbicide rate when cottonwas treated at the two- or five-node growth stage comparedwith only 1.7 times for the nine-node stage, indicatinggreater tolerance to glyphosate at the later growth stage (Ta-ble 1). Keeling et al. (2002) reported cotton injury rangingfrom 0 to 50%, 10 to 90%, and 0 to 85% at three locations

180 • Weed Science 52, January–February 2004

TABLE 2. Regression equations and r2 demonstrating dry weight (g) response of nonglyphosate-resistant cotton 30 d after POST applicationof glyphosate.

Growth stage Location

Glyphosate ratea

, 70 $ 70

g ha21

Two-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

NSNSNS

Y 5 2 476.17x 1 150.7 (0.75)NSb

Y 5 2 379x 1 172.5 (0.44)Five-node Winnsboro 1999

St. Joseph 2000Winnsboro 2000

NSNSNS

NSY 5 2 443.4x 1 637.25 (0.51)Y 5 2 482.26x 1 199.67 (0.74)

Nine-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

NSNSNS

NSNSNS

a Glyphosate applied at 9, 18, 35, 70, 140, and 280 g ha21. Within equation, x 5 herbicide fraction expressed as rate g ha21/1,120 g ha21. r2 valuesfollow equation in parentheses.

b Abbreviations: POST, posttreatment; NS, not significant.

TABLE 3. Regression equations and r2 demonstrating plant height (cm) response of nonglyphosate-resistant cotton 30 d after POSTapplication of glyphosate.

Growth stage Location

Glyphosate ratea

, 70 $ 70

g ha21

Two-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

NSNSNS

Y 5 2 122.5x 1 62.3 (0.8)Y 5 2 48.2x 1 82.3 (0.75)Y 5 2 116.3x 1 60.5 (0.68)

Five-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

Y 5 272x 1 69.5 (0.3)NSNS

Y 5 2 102.7x 1 80.7 (0.7)Y 5 2 121x 1 104.5 (0.88)Y 5 2 170.2x 1 72.2 (0.77)

Nine-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

NSNSNS

Y 5 2 92x 1 119.8 (0.48)NSNS

a Glyphosate applied at 9, 18, 35, 70, 140, and 280 g ha21. Within equation, x 5 herbicide fraction expressed as rate g ha21/1,120 g ha21. r2 valuesfollow equation in parentheses.

b Abbreviations: POST, posttreatment; NS, not significant.

after glyphosate application ranging from 25 to 425 g ha21

to cotton at the cotyledon to two-leaf, four- to five-leaf,pinhead square, and first bloom stages. Early-season injurywas generally most severe at the highest rates applied atearlier growth stages. Rowland et al. (1999), however, re-ported no visual injury with a glyphosate rate range of 26to 420 g ha21 applied to nonglyphosate-tolerant cotton atthe cotyledon, pinhead square, and early bloom growth stag-es. Averaged across growth stages, odds of injury increasedonly 1.8 times for each increase in herbicide rate at the St.Joseph location compared with 2.8 and 3.0 times at theMacon Ridge location in 1999 and 2000, respectively (Table1). This difference could not be attributed to differing en-vironmental conditions at St. Joseph. Previous research in-dicated no difference in tolerance to glyphosate between va-rieties Stoneville 474 and DP 33 B used in Winnsboro andSt. Joseph, respectively (Ellis and Griffin 2002).

Linear regression analysis indicated no effect of glyphosaterates below 70 g ha21 on plant dry weight 30 DAT (Table2). Dry weight was reduced 0.4 and 0.33 g for each gramincrease in glyphosate rate beginning at 70 g ha21 at theWinnsboro location in 1999 and 2000, respectively, whenapplied at the two-node growth stage. A reduction of 1.29and 0.43 g per gram herbicide increase was observed with

the five-node growth stage application at the St. Joseph andWinnsboro locations in 2000. Dry weight was not affectedby glyphosate application at the nine-node growth stage.

No effect was noted on plant height 30 DAT for herbi-cide rates below 70 g ha21, with the exception of the five-node growth stage at the Winnsboro location in 1999, whenheight increased with glyphosate application (Table 3). A 1-g increase in glyphosate rate beginning at 70 g ha21 resultedin a 0.11-, 0.04-, and 0.1-cm decrease in plant height atWinnsboro in 1999 and St. Joseph and Winnsboro in 2000,respectively, at the two-node growth stage. Results were sim-ilar for the five-node application with a 0.09-, 0.11-, and0.15-cm decrease in height for the respective locations.Height was reduced 0.08 cm only at the Winnsboro locationin 1999 at the later timing. Thomas et al. (2002) reportedthat nonglyphosate-resistant cotton height was not nega-tively influenced at glyphosate rates of 70 g ha21 or lesswhen applied at the four-leaf stage.

A delay in cotton maturity, as indicated by greater NAWFnumber compared with the nontreated control (4.8), wasobserved only for the two- (7.3) and five-node (8) growthstages with the 280 g ha21 glyphosate rate at Winnsboro in2000 (data not shown). Ellis and Griffin (2002) reportedno difference in NAWF number after glyphosate application

Miller et al.: Glyphosate and nonresistant cotton • 181

TABLE 4. Odds ratio estimates and 95% confidence interval esti-mates for observing an open boll with a 56 g ha21 increase inglyphosate rate.a

Growth stage LocationOdds ratio

estimateLowerlimit

Upperlimit

Two-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

0.8970.9160.760

0.8410.8670.693

0.9570.9680.833

Five-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

0.8070.7260.658

0.7580.6860.612

0.8610.7690.708

Nine-node Winnsboro 1999St. Joseph 2000Winnsboro 2000

0.8311.1750.762

0.7811.090.699

0.8851.30.830

a Glyphosate applied at 9, 18, 35, 70, 140, and 280 g ha21. Resultspresented follow logistic regression with herbicide rate as a continuous pre-dictor.

TABLE 5. Seedcotton yield as influenced by POSTa application ofglyphosate to nonglyphosate-resistant cotton.b

Herbicide rate Seedcotton yield

g ha21 kg ha21

09

183570

140280

4,160 ab4,360 a4,240 a4,390 a4,280 a4,190 a3,810 b

a Abbreviation: POST, posttreatment.b Means followed by the same letter are not statistically different with

Tukey’s adjustment at the 0.05 level of probability.

to nonglyphosate-resistant cotton at rates ranging from 9 to140 g ha21. Again, as with injury, differences observed couldnot be attributed to drastic differences in environmentalconditions at that location.

Logistic regression analysis results with respect to percentopen boll are presented in Table 4. The odds ratio estimatesgiven are estimates of the probability of observing an openboll with a 56 g ha21 increase in glyphosate rate and arecalculated as 100(1 2 odds ratio estimate). For example, forthe two-node growth stage at Winnsboro in 1999, the oddsof observing an open boll decrease 100(1 2 0.897) or 10%for each increase in herbicide rate. Alternatively, with a re-duction in herbicide rate of 56 g ha21, the odds of an openboll increase by this same percentage. Odds of an open bollwere reduced 10, 19, and 17% for each 56-g ha21 increasein glyphosate rate at the two-, five-, and nine-node growthstages, respectively, at Winnsboro in 1999. Odds reductionsof 8, 27, and 75% were observed for the respective growthstages at St. Joseph in 2000. Observed reductions were 24,34, and 24% at Winnsboro in 2000. Open boll odds ratioestimates increased slightly at the five-node growth stagecompared with the two- and nine-node applications, withthe exception of the St. Joseph location in 2000.

Averaged across experiments, final plant population wasreduced 9% only for the 280 g ha21 glyphosate rate appliedto cotton in the five-node growth stage (data not shown).All other combinations of growth stages and herbicide ratesdid not affect final plant population.

Glyphosate applied at 280 g ha21 resulted in a seedcottonyield reduction ranging from 9 to 13% when compared withlower rates (Table 5). Yield, however, was not reduced afterglyphosate application when compared with the nontreatedcontrol (3,810 to 4,390 kg ha21). Results agree with thoseof Ellis and Griffin (2002), who reported that nonglyphos-ate-resistant cotton was able to recover from early-seasoninjury effects after application of simulated drift rates ofglyphosate and yield comparably with the nontreated con-trol.

Reduced rates of glyphosate did result in various negativeeffects on growth parameters measured in this study. Dif-ferences between experiments regarding plant response couldnot be attributed to application or to environmental con-ditions. Cotton was able to recover, and yield was equivalentto that of nontreated cotton. However, late-season growing

conditions were optimum at both locations because of sup-plemental irrigation at Winnsboro and adequate rainfall atSt. Joseph. Cotton recovered because of a full growing sea-son and intense management of insect pests. Under less thanoptimal environmental conditions, a shortened growing sea-son, or extensive insect damage, cotton may not have fullyrecovered and resulted in a yield reduction. Rates used inthis study may represent the lower end of those that wouldbe expected in sprayer contamination or drift situations, andrates higher than 280 g ha21 may result in more seriousdeleterious effects. Rowland et al. (1999) reported a 20%reduction in yield of nonglyphosate-tolerant cotton withglyphosate applied at 420 g ha21 at the cotyledon, pinheadsquare, or early bloom stage. Also, carrier volume was main-tained constant in these studies, and previous research hasindicated greater injury and yield reduction with reducedglyphosate rates applied to cotton in varying carrier volumescompared with constant volumes (Banks and Schroeder2000). Therefore, adequate measures should be taken toavoid glyphosate misapplication from drift or sprayer con-tamination.

AcknowledgmentsThe authors thank Donna R. Lee, A. Lawrence Perritt, and C.

F. Wilson for their assistance with this research. The authors alsowish to thank Cotton Incorporated for providing partial fundingfor this project.

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Received May 30, 2002, and approved September 10, 2003.