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Glyphosate Tolerance in Enhanced Glyphosate-Resistant Cotton (Gossypiumhirsutum)Author(s): Jonathan A. Huff, Daniel B. Reynolds, Darrin M. Dodds, and J. Trenton IrbySource: Weed Technology, 24(3):289-294. 2010.Published By: Weed Science Society of AmericaDOI: http://dx.doi.org/10.1614/WT-08-183.1URL: http://www.bioone.org/doi/full/10.1614/WT-08-183.1

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Glyphosate Tolerance in Enhanced Glyphosate-Resistant Cotton(Gossypium hirsutum)

Jonathan A. Huff, Daniel B. Reynolds, Darrin M. Dodds, and J. Trenton Irby*

Glyphosate applied to glyphosate-resistant (RR) cotton varieties after the four-leaf stage can decrease boll retentionresulting in severe yield reductions. Enhanced glyphosate-resistant cotton (RR Flex), released for commercial use in 2006,offers a wider window of glyphosate applications without the risk of yield loss. However, no data exist regarding the effectof glyphosate application, especially late season applications, on fruit partitioning in RR Flex cotton. The objective of thisresearch was to determine the effect of glyphosate rate and application timing on RR Flex cotton yield and fruitpartitioning compared with current RR cotton. Studies were conducted during a 3-yr period (2004 to 2006), throughoutthe cotton growing regions of Mississippi. Roundup Ready (ST 4892 Bollgard/Roundup Ready [BR]) and RoundupReady Flex (Mon 171 Enhanced Roundup Ready and ST 4554 Bollgard II/Roundup Ready Flex [B2RF]) cotton wasplanted, and glyphosate was applied at various rates and cotton growth stages. Data were collected using box mapping, atechnique designed to depict yield partitioning on a cotton plant. RR Flex cotton yields were unaffected by glyphosateapplication timing or rate. Yields for ST 4892 BR were affected by application timings after the sixth leaf. ST 4892 BR hadincreased yield partitioning to position-three bolls and upper nodes with later application timings of glyphosate. Increasesin seed cotton partitioned to higher nodes and outer fruiting positions were unable to compensate for fruit shed frominnermost, lower fruiting sites. These data indicate that RR Flex cotton has excellent tolerance to late-season glyphosateapplications.Nomenclature: Glyphosate; cotton, Gossypium hirsutum L.Key words: Application timing, boll retention, box mapping, herbicide, Roundup Ready, RR Flex cotton, yieldcomponents, yield partitioning.

El glifosato aplicado a diferentes variedades de algodon resistentes al mismo (RR) despues de la etapa de cuatro hojas puededisminuir la retencion de bellotas provocando severas reducciones en el rendimiento. Una variedad mejorada de algodonresistente al glifosato (RR Flex) liberada al mercado para su uso comercial en 2006, ofrece mejores opciones en la aplicaciondel herbicida sin el riesgo de perdida en el rendimiento. Sin embargo, no existe informacion acerca del efecto de la aplicaciondel glifosato, especialmente en aplicaciones tardıas en la etapa de dehiscente de la bellota de algodon RR Flex. El objetivo deesta investigacion fue determinar el efecto de la dosis de glifosato y el tiempo de aplicacion en el rendimiento y en ladehiscencia de la bellota del algodon RR Flex, comparada con el algodon RR de uso comun en la actualidad. Los estudios sellevaron al cabo durante un perıodo de tres anos (2004 a 2006), a lo largo de las regiones de cultivo de algodon en Mississippi.Se sembraron las variedades Roundup Ready (ST 4892 Bollgard/Roundup Ready [BR]) y Roundup Ready Flex (Mon 171Enhanced Roundup Ready y ST 4554 Bollgard II/Roundup Ready Flex [B2RF]) y el glifosato se aplico a diferentes dosis yetapas de crecimiento. La informacion fue obtenida usando ‘‘Box mapping’’ una tecnica designada para representar lacanalizacion del rendimiento en una planta de algodon. Los rendimientos del algodon RR Flex no se afectaron por el tiempode aplicacion o por la dosis. Los rendimientos para ST 4892 BR si se afectaron en respuesta al tiempo de aplicacion despues dela sexta hoja. El ST 4892 Bollgard/Roundup Ready (BR) Exhibio un mayor grado de canalizacion hacia las bellotas en laposicion de tres bellotas y hacia los nodulos superiores debido al tiempo mas tardıo de la aplicacion de glifosato. Losincrementos en la semilla de algodon canalizada hacia los nodulos superiores y hacia las posiciones perifericas de las bellotas nofueron suficientes para compensar la perdida del fruto en las posiciones interiores e inferiores. Esta informacion senala que elalgodon RR Flex tiene una excelente tolerancia a las aplicaciones tardıas de glifosato.

Research for selective chemical weed control in cottonbegan in the late 1940s and early 1950s with the introductionof several herbicides, including dinoseb, chlorpropham, anddiuron (Kohel and Lewis 1984). During the next half century,rapid advancements in herbicide discovery led to dramaticshifts in weed management practices throughout the U.S.cotton-producing regions. Technological discoveries in PREand POST herbicides led to a decrease in the use of in-croptillage for weed control. Glyphosate-resistant (henceforth,referred to as RR) cotton was introduced in 1997 and

revolutionized weed control in cotton. Traditionally, weedmanagement in cotton relied on a combination of soil-appliedherbicides, cultivation, and POST-directed herbicides (Wilcutet al. 1996). The introduction of RR cotton providedproducers with greater flexibility in the timing of herbicideapplications and also offered a broader spectrum of weedcontrol than other systems on the market (Askew and Wilcut1999; Culpepper and York 1999). Additional benefitsincluded less reliance on tillage, reduced herbicide costs, andminimal crop injury; subsequently, glyphosate usage increasedfrom 700,000 kg in 1997 to 3,870,000 kg in 2006 (Young2006).

Glyphosate is a nonselective, foliar-applied herbicide thattranslocates from treated foliage to metabolically active

DOI: 10.1614/WT-08-183.1* Graduate Student, Professor, Research Associate, and Graduate Student,

Department of Plant and Soil Sciences, Mississippi State University, MississippiState, MS 39762. Corresponding author’s E-mail: dreynolds@pss.msstate.edu

Weed Technology 2010 24:289–294

Huff et al.: Glyphosate tolerance in Flex cotton N 289

regions of roots, rhizomes, and apical meristems (Franz 1997;Kishore et al. 1992). Glyphosate inhibits 5-enolpyruvylshiki-mate-3-phosphate (EPSP) synthase in the shikimic acidpathway, leading to depletion of aromatic amino acidstryptophan, tyrosine, and phenylalanine (Vencill 2002).Typical herbicidal effects from glyphosate application includegeneral chlorosis, stunting, and necrosis among a wide rangeof grass and broadleaf weed species. Cotton tolerance toglyphosate was achieved by expression of a gene that encodes aglyphosate-tolerant 5-enolpyruvate-3-phosphate synthasefrom Agrobacterium sp. (CP4 EPSPS)(Nida et al. 1996).The development of RR cotton allows topical application ofglyphosate without risk of plant injury when applied inaccordance with label recommendations. Topical applicationsof glyphosate can be made to RR cotton until the four-leafstage of development. Applications after that stage arerestricted to POST-directed sprays to minimize foliar contact.

RR cotton exhibits a high level of vegetative tolerance toglyphosate; however, reproductive tissues may be less tolerant(Nida et al. 1996; Pline et al. 2002). Monopodial limbs orvegetative branches begin formation at the time the main stembegins unfolding and continue through the development of thethird or fourth true leaf (Mauney 1984). Upon development ofthe third or fourth true leaf, subsequent growth is in the form ofsympodial or reproductive branches. Growth of sympodiallimbs continues for the duration of the life cycle of the plant.Exposing RR cotton to glyphosate after the four-leaf stage canresult in yield reductions due to a lack of reproductive tolerance(Viator 2004). Glyphosate applications inconsistent with labeldirections may affect boll retention, square abscission, delaymaturity, and subsequently, yield reductions (Ferreira et al.1998; Kalaher and Coble 1998; Kalaher et al. 1997; Martens etal. 2003; Matthews et al. 1998). Cotton may compensate forearly season boll abscission on lower nodes by producing morecotton at fruiting sites higher on the plant (Ferreira et al. 1998;Kalaher and Coble 1998). Pline et al. (2002) attributed this bollabscission to lower expression of the CP4-EPSP synthase inmale reproductive tissues, resulting in altered floral morphologyin RR plants and poor pollination. Under favorable growingconditions, cotton can compensate for fruit abscission at lowernodes by setting more fruit at higher nodes (Jenkins et al. 1990;Jones and Snipes 1999). However, maturity will be delayed andlate-season bolls are typically nonharvestable, resulting in yieldreductions (Kalaher and Coble 1998). Martens et al. (2003)reported significant yield reductions to RR cotton whenglyphosate was applied at 1.68 kg ae ha21 and 2.52 kg ha21 atthe 3-, 6-, 10-, and 14-leaf growth stages. Greater than 60%yield reductions were observed in RR cotton when glyphosatewas applied at 0.84 kg ae ha21 at multiple intervals from 4- to16-leaf growth stages (Jones et al. 2006).

Enhanced glyphosate-resistant (henceforth, referred to asRR Flex) cotton was introduced in 2006. RR Flex cottonexhibits both vegetative and reproductive tolerance toglyphosate, allowing glyphosate to be applied POST over-the-top (POT), at any growth stage, without risk of bollabortion (May et al. 2004). RR Flex technology uses a new‘‘transformation event’’ (MON 88913) with a differentpromoter; however, the same CP4 EPSPS gene provides bothvegetative and reproductive tolerance in cotton (Anonymous

2005). RR Flex has potential benefits, including an expandedwindow for POT glyphosate applications, enhanced applica-tion flexibility and convenience, increased production effi-ciencies, less dependence on selective spray equipment, andthe ability to tailor herbicide applications to weed heightsinstead of cotton growth stage (Murdock and Mullins 2006).

Previous research indicates no significant yield response tomultiple, topical applications of glyphosate to RR Flex cotton(Jones et al. 2006; Martens et al. 2003). However, researchaddressing the effect of glyphosate application on fruit set anddistribution in RR Flex cotton is lacking. Therefore, theobjectives of this research were to evaluate the effect ofglyphosate applied to RR Flex cotton at various rates andgrowth stages and to determine whether rates and timing ofapplications affect crop maturity, plant development, bollretention, and crop yield.

Materials and Methods

Experiments were conducted from 2004 to 2006 toevaluate the effects of POT glyphosate applications onRoundup Ready and Roundup Ready Flex cotton. Experi-mental locations were chosen to represent various growingregions throughout Mississippi. Experiments were conductedat the following locations during the 3-yr period: MonsantoResearch Farm, Leland, MS (2005); Mississippi StateUniversity Delta Research and Extension Center, Stoneville,MS (2006); 4/5 Plantation, Greenwood, MS (2005); R.R.Foil Plant Science Research Center, Starkville, MS (2005 and2006); and the Black Belt Branch Experiment Station,Brooksville, MS (2004, 2005, and 2006). Plot size, varieties,and planting dates are shown in Table 1. Experiments wereconducted as a three-factor factorial arrangement of treat-ments in a randomized, split-split block design. Factor Aconsisted of glyphosate1 application rates of 0.84 and1.68 kg ae ha21, which represent a common 13 and 23rate of application. Factor B consisted of application timings,including 3-leaf; at 3-, followed by (fb) 6-leaf; at 3-, fb 6-, fb9-leaf; and at 3-, fb 6-, fb 9-, fb 12-leaf cotton growth stages.Factor C represented cotton variety. ST 4892 B/R2 and MON171 ERR (Flex)3 were planted at all locations in 2004 and2005; however, ST 4554 B2RF4 replaced MON 171 ERR atall locations in 2006. An untreated check was included forcomparison purposes. Seeds were planted at a rate of 118,500seeds ha21. All treatments were applied with a tractor-mounted, compressed-air sprayer or a CO2-pressurizedbackpack sprayer5 delivering a volume of 169 L ha21. Allplots were kept free of weeds for the duration of the growingseason to prevent any weed interference. Insecticides, plantgrowth regulators, and defoliants were all applied according tostandard management practices in Mississippi.

Data collection consisted of end-of-season box mapping.Box-mapping data were taken from a 3-m section of row asdescribed by Jenkins et al. (1990). Plants were taken fromeach 3-m section, and each boll was harvested individually byhand, keeping each fruiting site and nodal position separate(Jenkins et al. 1990). The total number of bolls and totalweight of seed cotton produced at each fruiting site weredetermined. Plants that had lost apical dominance were

290 N Weed Technology 24, July–September 2010

categorized as aborted plants, and seed-cotton was handharvested from the entire plant with no regard to position.Bolls from vegetative branches were also separated and addedto the total weight to determine total yield. Box mapping dataincluded total plants, total bolls, and seed cotton weight ateach position. Fruit partitioning and yield data were analyzedby horizontal fruiting position by calculating the amount ofseed cotton located on sympodial branches in fruitingpositions 1, 2, and 3. Position 1 represents the first positionclosest to the main stem at each node on a fruiting branch;position 2 represents the second position (the second squareor floral bud to develop on the branch), and position 3represents any position beyond the second position on a givenfruiting branch. Fruit partitioning and yield data were alsoanalyzed by fruiting zone. Seed cotton partitioned to fruitingzones were determined by combining the amount of seedcotton located on all fruiting sites of sympodial branches atvarious nodes. Zone 1 represents all seed cotton collectedfrom nodes 5 to 8, zone 2 represents all seed cotton collectedfrom nodes 9 to 12, and zone 3 represents all seed cottoncollected from above the 12th node. The assumption wasmade that cotton has a 3-d vertical, and 6-d horizontal,fruiting interval (Jenkins et al.,1990). Using that time interval,plants were also sectioned into cohorts, according to maturity.Cohort positions represent the fruiting sites that are the sameage, for example: cohort 4 represents seed cotton collected fromnode 8, position 1; node 6, position 2; and node 4, position 3.Analyzing the data in this manner isolates fruiting sites ofsimilar age to determine whether they were affected byglyphosate application. Data were analyzed up to the 16thcohort. Total yields from box mapping data were calculated bysumming the weight of all seed cotton collected from each plot.

Yield data were also collected from the remaining plants inthe plots, at all locations, using a mechanical harvester. Boxmapping and yield data were combined over environments,analyzing environment as a random effect, and subjected to anANOVA, and the means were separated by Fisher’s ProtectedLSD at the 0.05 level of significance.

Results and Discussion

No significant interactions were present regarding glypho-sate application rate; therefore, all application timing datawere pooled over glyphosate application rates of 0.84 and

1.68 kg ae ha21. The percentage of seed cotton partitioned tohorizontal fruiting positions, vegetative branches, and abortedplants, based on the timing by variety interaction, is displayedin Table 2. No difference in the percentage of seed cottonpartitioned to any position among all application timings wasobserved in RR Flex cotton. The ST 4892 BR that received noglyphosate application partitioned 72% of the seed cotton toposition 1 bolls. A 7 and 17% reduction in seed cottonpartitioned to position 1 bolls was observed in ST 4892 BRwhen glyphosate was applied at 3, fb 6, fb 9 nodes and at 3, fb6, fb 9, fb 12 nodes, compared with the ST 4892 BR plot thatreceived no glyphosate application (Table 2). RR Flex cottonpartitioned more seed cotton to position 1 bolls, regardless ofglyphosate application timing, than did ST 4892 BR cotton.ST 4892 BR partitioned 5.5% more seed cotton to position 2fruiting sites when glyphosate applications were made at 3, fb6, fb 9 nodes and at 3, fb 6, fb 9, fb 12 nodes. Increasedpartitioning to position 3 fruiting sites was observed with ST4892 BR when glyphosate was applied at 3, fb 6, fb 9, fb 12nodes. Seed cotton partitioning increased from 7 to 11 and19% when applications were made at 3, fb 6, fb 9 nodes andat 3, fb 6, fb 9, fb 12 nodes, respectively, when compared withST 4892 BR plots that received no glyphosate application.The percentage of cotton partitioned to vegetative braches andto aborted plants exhibited no differences that were due toglyphosate application timing for both RR Flex and ST 4892BR cotton.

The percentage of seed cotton partitioned in vertical zones,as well as the box mapping total weights (kg ha21) is displayedin Table 3. RR Flex showed no differences in the percentageof seed cotton partitioning to any zone related to glyphosateapplication. Decreased percentage of seed cotton partitionedin zone 1 at the 3-, fb 6-, fb 9-node and at the 3-, fb 6-, fb 9-,fb 12-node application timings was observed with ST 4892BR cotton. Up to 8% decreases in seed cotton partitioned inzone 1 was observed when these application timings werecompared with the ST 4892 BR cotton that received noglyphosate. RR Flex partitioned 44 to 46% of total seedcotton weight, regardless of glyphosate application timing, inzone 1; 39 to 41% in zone 2; and 12 to 13% in fruiting zone3. Decreased seed cotton partitioning in zone 2 was observedin ST 4892 BR when glyphosate was applied at 3, fb 6, fb 9,fb 12 nodes, compared with the ST 4892 BR cotton thatreceived no glyphosate. However, increased seed cottonpartitioned in zone 3 at all glyphosate timings was observed.

Table 1. Experimental test sites and specifications.a

Location Year Roundup Ready variety Roundup Ready Flex variety Plot size (width 3 length) Planting rate Planting date Geographic location

m seed m21 of row

Brooksville, MS 2004 ST 4892 BR MON 171 ERR 1.99 3 12.19 12.4 May 7, 2004 HillBrooksville, MS 2005 ST 4892 BR MON 171 ERR 1.99 3 12.19 12.4 May 16, 2005 HillBrooksville, MS 2006 ST 4892 BR ST 4554 B2RF 3.86 3 12.19 12.4 May 9. 2006 HillGreenwood, MS 2005 ST 4892 BR MON 171 ERR 1.99 3 12.19 12.4 May 20, 2005 DeltaLeland, MS 2005 ST 4892 BR MON 171 ERR 1.99 3 12.19 12.4 May 17, 2005 DeltaStarkville, MS 2005 ST 4892 BR MON 171 ERR 1.99 3 12.19 12.4 May 16, 2005 HillStarkville, MS 2006 ST 4892 BR ST 4554 B2RF 3.86 3 12.19 12.4 May 16, 2006 HillStoneville, MS 2006 ST 4892 BR ST 4554 B2RF 2.03 3 12.19 12.4 May 9, 2005 Delta

a Abbreviations: ST 4892 BR, Stoneville 4892 Bollgard/Roundup Ready2; MON 171 ERR, Roundup Ready Flex3; ST 4554 B2RF, Stoneville 4554 Bollgard II/Roundup Ready Flex.4 (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)

Huff et al.: Glyphosate tolerance in Flex cotton N 291

Glyphosate applied to ST 4892 BR at 3, fb 6, fb 9 nodes andat 3, fb 6, fb 9, fb 12 nodes resulted in 23 and 31% seedcotton partitioned to zones 2 and 3, respectively, comparedwith 18% for the ST 4892 BR cotton that received noglyphosate. ST 4892 BR partitioned more seed cotton to zone3, regardless of glyphosate application and timing, comparedwith RR Flex cotton. Box mapping total weights werereflective of changes in partitioning by positions and by zones,with a decrease in the total weight observed with ST 4892 BRfrom glyphosate application at 3, fb 6, fb 9, fb 12 nodes. RR

Flex yields exhibited no differences in total weight fromglyphosate application timing.

The percentage of seed cotton partitioned according tocohorts is displayed in Table 4. No differences betweenapplication timings for all cohorts were observed with RR Flexcotton. Additionally, no differences were observed in cohort 1from glyphosate application in ST 4892 BR. However, incohorts 2, 3, 4, and 5, reductions in seed cotton were observedbecause of glyphosate application at 3, fb 6, fb 9 nodes and at3, fb 6, fb 9, fb 12 nodes, compared with the ST 4892 BR

Table 2. Percentage of total seed cotton that was partitioned in horizontal positions, that had aborted plants, and that had vegetative branches, averaged over theglyphosate application rate.a

Application timingb

Position 1c Position 2d Position 3e Vegetativef Abortedg

Flexh ST 4892 BRi Flex ST 4892 BR Flex ST 4892 BR Flex ST 4892 BR Flex ST 4892 BR

nodes -------------------------------------------------------------------------------------------------------------------------------------------% ------------------------------------------------------------------------------------------------------------------------------------------

— 78 72 15 17 5 7 2 4 0.20 0.803 76 70 16 18 5 10 3 3 0.16 0.183, 6 76 70 15 18 5 9 2 3 0.38 0.423, 6, 9 76 65 16 21 5 11 3 3 0.28 0.233, 6, 9, 12 77 55 15 22 5 19 2 4 0.21 0.83LSD (0.05)j 4 2 3 NSDk NSD

a Glyphosate was applied at 0.84 and 1.68 kg ae ha21; however, no significant interactions were present that were due to glyphosate application rate. Therefore, datawere pooled over glyphosate application rate.

b Application timing (e.g., 3, 6, 9) indicates applications were applied sequentially at 3, 6, and 9 nodes of growth; — indicates no glyphosate was applied.c Position 1, the percentage of total seed cotton weight located at the position 1 fruiting site.d Position 2, the percentage of total seed cotton weight located at the position 2 fruiting site.e Position 3, the percentage of total seed cotton weight located at the position 3 and beyond fruiting sites.f Vegetative, the percentage of total seed cotton weight on the monopodial (vegetative) branches.g Aborted, the percentage of total seed cotton weight on aborted plants.h Flex–Roundup Ready Flex varieties used included MON 171 ERR3 in 2004 and 2005. MON 171 ERR was supplied by Monsanto, and no other variety name was

provided. In 2006, ST 4554 Bollgard II/Roundup Ready Flex4 was used in this study. (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)i ST 4892 BR,2 Stoneville 4892 Bollgard/Roundup Ready. (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)j Least significant differences were separated by Fisher’s Protected LSD at the 0.05 level of significance.k NSD, no significant difference among treatments

Table 3. Percentage of total seed cotton partitioned in vertical zones and total box mapping weights averaged over glyphosate application rate.a

Application timingb

Zone 1c Zone 2d Zone 3e Box mapping total weights

Flexf ST 4892 BRg Flex ST 4892 BR Flex ST 4892 BR Flex ST 4892 BR

nodes -----------------------------------------------------------------------------------------------------% ---------------------------------------------------------------------------------------------------- kg seed cotton ha21

— 45 35 40 42 12 18 2,083 1,9983 45 35 40 41 12 21 2,004 1,9433, 6 46 34 40 42 12 21 2,043 1,9533, 6, 9 44 28 41 46 13 23 2,119 1,9423, 6, 9, 12 46 27 39 36 12 31 1,985 1,326LSD (0.05)h 4 3 3 168

a Glyphosate was applied at 0.84 and 1.68 kg ae ha21; however, no significant interactions were present that were due to glyphosate application rate. Therefore, datawere pooled over glyphosate application rate.

b Application timing (e.g., 3, 6, 9) indicates applications were applied sequentially at 3, 6, and 9 nodes of growth; — indicates no glyphosate was applied.c Zone 1, the percentage of total seed cotton weight located at zone 1 (nodes 5–8).d Zone 2, the percentage of total seed cotton weight located at zone 2 (nodes 9–12).e Zone 3, the percentage of total seed cotton weight located at zone 3 (nodes 12 and greater).f Flex–Roundup Ready Flex varieties used included MON 171 ERR3 in 2004 and 2005. MON 171 ERR was supplied by Monsanto, and no other variety name was

provided. In 2006, ST 4554 Bollgard II/Roundup Ready Flex4 was used in this study. (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)g ST 4892 BR,2 Stoneville 4892 Bollgard/Roundup Ready. (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)h Least significant differences were separated by Fisher’s Protected LSD at the 0.05 level of significance.

292 N Weed Technology 24, July–September 2010

that did not receive a glyphosate application. Cottonpartitioned to cohorts 2 and 3 in RR Flex was higher at allapplication timings when compared with ST 4892 BR. Nodifferences were observed in cohorts 6 through 9 fromglyphosate application timing or by cotton variety. ST 4892BR exhibited an increase in the percentage of cottonpartitioning in cohorts 10 and 11 that was due to glyphosateapplications at 3, fb 6, fb 9 nodes and at 3, fb 6, fb 9, fb 12nodes. Increased percentage of seed cotton partitioning in ST4892 BR was also observed in cohort 12 when glyphosate wasapplied at 3, fb 6, fb 9, fb 12 nodes. Seed cotton partitionedin cohorts 10 through 12 was higher in ST 4892 BR than inRR Flex for all application timings. Higher percentages ofseed cotton partitioned in cohorts 13 through 16 fromglyphosate application at 3, fb 6, fb 9, fb 12 nodes wereobserved in ST 4892 BR.

These data indicate that RR cotton technology issusceptible to glyphosate during the reproductive stages ofgrowth when applied beyond the labeled application timings

at an 13 or 23 rate of application. Additionally, cotton thatexperienced fruit abortion early in the growing season tendedto compensate for that loss by partitioning more fruit onupper and outer portions of the plant under growingconditions that were conducive. These results support thecompensatory effect described by Ferreira et al. (1998) andKalaher. However, cotton plants could not fully compensatefor fruit loss from the glyphosate application, as was evidentwith decreased yields in the RR cotton where glyphosate wasapplied later in the season. Applications of glyphosateinconsistent with label directions can lead to boll abscissionand subsequent yield loss (Ferreira et al. 1998). These dataalso clearly demonstrate vegetative and reproductive toleranceof RR Flex to glyphosate applications. Applications ofglyphosate at the 13 and 23 rate resulted in no yieldreduction in RR Flex varieties across all application timings.Yield partitioning was also unaffected in RR Flex at allglyphosate application timings and rates. RR Flex allows awider window of application without the risk of yield loss.

Table 4. Percentage of seed cotton partitioned in cohorts as affected by glyphosate application timing and variety averaged over glyphosate application rate.a

Cohortb

Nodes of growth at application timingc

LSDf

None 3 3, 6 3, 6, 9 3, 6, 9, 12

Flexd ST4892 BRe Flex ST4892 BR Flex ST4892 BR Flex ST4892 BR Flex ST4892 BR

----------------------------------------------------------------------------------------------------------------------------------------- % ----------------------------------------------------------------------------------------------------------------------------------------

1 1.84 0.61 1.65 0.64 1.77 0.61 1.43 0.41 1.76 0.43 NSDg

2 2.60 1.64 2.33 1.79 2.61 1.51 2.16 0.98 2.62 1.00 0.373 3.63 2.86 3.75 2.54 3.74 2.42 3.68 1.79 3.71 1.44 0.574 4.06 3.03 4.13 2.90 4.28 2.95 4.04 2.02 4.05 1.74 0.515 4.28 4.20 4.34 4.08 4.38 4.02 4.53 3.54 4.43 3.02 0.476 3.97 4.10 4.00 3.99 3.85 3.90 3.81 4.08 3.85 3.27 NSD7 3.35 3.66 3.28 3.44 3.31 3.79 3.51 4.16 3.29 3.13 NSD8 2.75 3.20 2.76 3.25 2.69 3.42 2.95 4.17 2.82 3.14 NSD9 1.95 2.68 2.06 2.67 2.06 3.00 2.10 3.52 1.98 3.08 NSD10 1.55 2.05 1.43 2.11 1.40 2.30 1.50 2.65 1.41 2.93 0.3411 0.92 1.43 0.79 1.77 0.81 1.54 0.95 1.87 0.88 2.23 0.3012 0.48 0.83 0.47 1.12 0.49 1.05 0.46 1.27 0.38 2.02 0.3213 0.20 0.44 0.24 0.73 0.20 0.50 0.22 0.64 0.25 1.51 0.2614 0.11 0.22 0.14 0.30 0.08 0.26 0.10 0.22 0.13 0.86 0.1915 0.04 0.10 0.06 0.17 0.02 0.08 0.05 0.10 0.06 0.43 0.1116 0.01 0.02 0.02 0.06 0.01 0.04 0.02 0.04 0.02 0.34 0.12

a Glyphosate was applied at 0.84 and 1.68 kg ae ha21; however, no significant interactions were present that were due to glyphosate application rate. Therefore, datawere pooled over glyphosate application rate.

b Cohort 1, percentage of total seed cotton weight of all bolls on node 5, position 1; cohort 2, percentage of total seed cotton weight of all bolls on node 6, position 1;cohort 3, percentage of total seed cotton weight of all bolls on node 7, position 1, and node 5, position 2; cohort 4, percentage of total seed cotton weight of all bolls onnode 8, position 1, and node 6, position 2; cohort 5, percentage of total seed cotton weight of all bolls on node 9, position 1, and node 7, position 2, and node 5,position 3; cohort 6, percentage of total seed cotton weight of all bolls on node 10, position 1, and node 8, position 2, and node 6, position 3; cohort 7, percentage oftotal seed cotton weight of all bolls on node 11, position 1, and node 9, position 2, and node 7, position 3; cohort 8, percentage of total seed cotton weight of all bolls onnode 12, position 1, and node 10, position 2, and node 8, position 3; cohort 9, percentage of total seed cotton weight of all bolls on node 13, position 1, and node 11,position 2, and node 9, position 3; cohort 10, percentage of total seed cotton weight of all bolls on node 14, position 1, and node 12, position 2, and node 10, position 3;cohort 11, percentage of total seed cotton weight of all bolls on node 15, position 1, and node 13, position 2, and node 11, position 3; cohort 12, percentage of totalseed cotton weight of all bolls on node 16, position 1, and node 14, position 2, and node 12, position 3; cohort 13, percentage of total seed cotton weight of all bolls onnode 17, position 1, and node 15, position 2, and node 13, position 3; cohort 14, percentage of total seed cotton weight of all bolls on node 18, position 1, and node 16,position 2, and node 14, position 3; cohort 15, percentage of total seed cotton weight of all bolls on node 19, position 1, and node 17, position 2, and node 15, position3; cohort 16, percentage of total seed cotton weight of all bolls on node 20, position 1, and node 18, position 2, and node 16, position 3.

c Application timing (e.g., 3, 6, 9) indicates applications were applied sequentially at 3, 6, and 9 nodes of growth; — indicates no glyphosate was applied.d Flex–Roundup Ready Flex varieties used included MON 171 ERR3 in 2004 and 2005. MON 171 ERR was supplied by Monsanto, and no other variety name was

provided. In 2006, ST 4554 Bollgard II/Roundup Ready Flex4 was used in this study. (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)e ST 4892 BR,2 Stoneville 4892 Bollgard/Roundup Ready. (Superscript numbers refer to the ‘‘Sources of Materials’’ section of the text.)f Least significant differences were separated by Fisher’s Protected LSD at the 0.05 level of significance.g NSD, no significant difference among treatments.

Huff et al.: Glyphosate tolerance in Flex cotton N 293

Producers now have greater flexibility in crop management bynot restricting glyphosate applications to cotton with fiveleaves or less.

Sources of Materials1 Roundup WeathermaxH, Monsanto, 800 N. Lindbergh Blvd.,

St. Louis, MO 63167.2 ST 4892 Bollgard/Roundup Ready cotton seed, Bayer

CropScience, 2 T.W. Alexander Drive Research Triangle Park,NC 27709.

3 MON 171 ERR Roundup Ready Flex cotton seed, Monsanto,800 N. Lindbergh Blvd. St. Louis, MO 63167.

4 ST 4554 Bollgard II/Roundup Ready Flex cotton seed, BayerCropScience, 2 T.W. Alexander Drive, Research Triangle Park, NC27709.

5 CO2-pressurized backpack sprayer, BellSpray, Inc., R & DSprayers, P.O. Box 267, Opelousas, LA 70571-0267.

Acknowledgments

The authors would like to thank the Cotton IncorporatedMississippi State Support Committee and Monsanto, forpartial financial support of this research, as well as Delta andPine Land Company and Stoneville Pedigreed Seed forproviding cotton seed. Approved for publication as JournalArticle No. J-11787 of the Mississippi Agricultural andForestry Experiment Station, Mississippi State University.

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Received December 22, 2008, and approved April 7, 2010.

294 N Weed Technology 24, July–September 2010