Agronomy Journal 94:889-894 (2002)
© 2002 American Society of Agronomy
PRODUCTION PAPER
Yield Ranks of Glyphosate-Resistant Cotton Cultivars are Unaffected by Herbicide Systems
O. Lloyd May*,a and
Edward C. Murdockb
a Dep. of Crop and Soil Sci., Univ. of Georgia, Coastal Plain Exp. Stn., P.O. Box 748, Tifton, GA 31793-0748
b Dep. of Crop and Soil Environ. Sci., Clemson Univ., Pee Dee Res. and Educ. Cent., 2200 Pocket Rd., Florence, SC 29506-9706
* Corresponding author (lmay{at}tifton.cpes.peachnet.edu)
Received for publication August 20, 2001.
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ABSTRACT
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Official Cultivar Trials (OCTs) evaluate transgenic, glyphosate [N-(phosphonomethyl)-glycine]-resistant and nonresistant cotton (Gossypium hirsutum L.) cultivars with a nonglyphosate herbicide regime. Thus, yields of glyphosate-resistant cotton cultivars in OCTs may not reflect cultivar variation in glyphosate resistance or the intended production system because all cultivars are produced under a common herbicide regime. Our objective was to assess in field trials main effects and interaction among glyphosate-resistant cotton cultivars and herbicide systems with and without glyphosate for lint yield. Fourteen glyphosate-resistant cultivars were tested in 1998 while 10 were tested in 1999. Herbicide systems were (i) Standard, preplant soil-applied and postdirect-applied herbicides potentially used in OCTs but no glyphosate; (ii) Residual + Glyphosate, preplant soil-applied and postdirect-applied herbicides including a four-leaf-stage topical glyphosate application; and (iii) Glyphosate Only, a four-leaf-stage topical glyphosate application followed by an eight-leaf directed spray. We found significant (P < 0.05 and P < 0.10, earlier- and later-maturity trials, respectively) herbicide system main effects for lint yield but nonsignificant cultivar x herbicide system interactions. Averaged over years and cultivars, the Glyphosate Only herbicide system produced significantly (P < 0.05) greater yields (597 and 839 kg ha-1 for earlier- and later-maturity trials, respectively) than the Standard herbicide system (419 and 644 kg ha-1 for earlier- and later-maturity trials, respectively). The lack of cultivar x herbicide system interactions for yield suggests OCTs determine relative yield potential among glyphosate-resistant cultivars. The lower yield in the Standard system suggests OCTs can impose a yield penalty on glyphosate-resistant cultivars; thus, comparisons with non-glyphosate-resistant cultivars may not be valid.
Abbreviations: OCTs, Official Cultivar Trials
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INTRODUCTION
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GROWER ADOPTION of glyphosate-resistant cotton cultivars reached nearly 70% of the 2001 U.S. cotton hectarage (USDA-AMS, 2000). Topical and postdirected glyphosate applications have enhanced control of highly competitive weed species such as Palmer amaranth (Amaranthis palmeri L.), contributing to expansion of glyphosate-resistant cotton cultivar hectarage since introduction in 1997. The current glyphosate resistance gene in cotton allows topical applications to the four-leaf growth stage or earlier, followed by directed applications where leaf contact is minimized (Kerby and Voth, 1998). Topical glyphosate applications after the four-leaf development stage can impact gamete development and fruit retention, resulting in yield loss (Kerby and Voth, 1998; Jones and Snipes, 1999). Glyphosate resistance occurs through insertion of sequences imparting glyphosate insensitivity by allowing for the production of key amino acids needed for plant growth (Johnson, 1996; Thompson et al., 1987). Weed management in cotton production has traditionally relied on soil-applied and postdirected herbicides, reflecting few broad-spectrum overtop options. In contrast, glyphosate-resistant cultivars were intended for production systems in which glyphosate applications replace or augment soil-applied or postdirected herbicides (Askew and Wilcut, 1999; York, 1997). For example, one or two glyphosate applications up to the four-leaf growth stage can replace preplant-incorporated and/or pre-emergence herbicides, depending on prevalent weed species and densities (Culpepper and York, 1998, 1999).
Concurrent with the commercialization of transgenic cultivars has been the realization that agronomic performance of transgenic cultivars may vary substantially compared with the nontransgenic cultivar or germplasm line from which they were developed (Jenkins et al., 1997). Additionally, expression of the transgene can be influenced by transgene x genetic background effects, in part because of the nature of transformation (Sachs et al., 1998). Transformation randomly incorporates a gene into the host genome, which can influence expression of native genes as well as the transgene (Altman et al., 1991). Typically, with upland cotton, the transgene is added through Agrobacterium spp. or particle bombardment–mediated transformation of meristem tissue of the obsolete cultivar Coker 312, as most cotton genotypes are recalcitrant to regeneration from tissue culture (Trolinder and Xhixian, 1989). Following plant regeneration, transgenic plants derived from one or more transformation events are evaluated for transgene expression and nontarget phenotypic effects, followed by backcross introgression of the transgene into target cultivars. After transgene introgression into an elite genetic background, screening lines for transgene expression and agronomic performance occurs. Because all donor DNA from the original transformed line is not eliminated through backcrossing, particularly in chromosomal regions flanking the transgene (Falconer, 1989), the resulting transgenic cultivar may exhibit different performance from linkage drag effects. Additionally, a host of factors related to the transformation process and background genotype may contribute to agronomic and transgene expression variation in derived germplasm (Sachs et al., 1998). Thus, candidate transgenic cultivars require testing to evaluate gene x background effects on agronomic performance and transgene expression. The glyphosate-resistant cultivars in our study were all derived through backcross breeding with popular nontransgenic cultivars (Calhoun et al., 1997), with the glyphosate donor parent an Agrobacterium sp.–mediated transformed line selected from Coker 312. Therefore, trials designed to measure main effects and interaction of glyphosate-resistant cultivar and herbicide system can demonstrate the value-added herbicide resistance trait and reveal transgene x genetic background interactions influencing yield.
Glyphosate-resistant cotton cultivars were introduced into the U.S. cottonseed market in 1997 with little or no public testing in either Official Cultivar Trials (OCTs) or trials designed to evaluate such cultivars in the intended production system (May et al., 2000). Consequently, growers have chosen glyphosate-resistant cultivars largely without access to performance data where the trials included glyphosate applications. Instead, glyphosate-resistant cultivar choice has been driven by grower experience with the corresponding nontransgenic recurrent parent cultivar. Despite the general popularity of glyphosate-resistant cultivars, crop growth abnormalities including fruit shed, incompletely developed fruit, and yield reductions were reported in 1997 and 1998 (Kerby and Voth, 1998), raising concerns about the suitability of OCTs for testing glyphosate-resistant cultivars. Following reports of yield losses by growers producing glyphosate-resistant cotton have been demands for OCTs to evolve in tandem with transgenic technologies to include systems-type trials employing cultivar-specific pest management regimes. Because OCTs generally do not impose cultivar-specific production regimes, all cultivars are produced with a common herbicide system that does not include glyphosate application to resistant cultivars (Bowman, 1998; Creech et al., 1998; May et al., 1998, 1999; Raymer et al., 1999). Thus, the validity of OCT performance data for glyphosate-resistant cultivars has been questioned.
Research on glyphosate-resistant cotton has focused on effects of glyphosate applied topically at growth stages exceeding the recommended four-leaf stage of cotton development (Vargas et al., 1998; Jones and Snipes, 1999). These studies revealed fruit set skewed toward nodes borne higher on the plant from nonrecommended glyphosate applications compared with control treatments but variable effects on yield. When the growth season permits, cotton can often compensate for fruit loss at lower nodes by maturing fruit at higher nodes and/or fruiting sites more distal to the main stem and frequently not incur yield reductions (Mann et al., 1997). Cultivar variation in resistance to glyphosate applications as directed by Monsanto was, however, not the focus of these studies.
We conducted this study to examine main effects and interaction for lint yield among glyphosate-resistant cotton cultivars and herbicide systems with and without glyphosate. These data should suggest if OCTs determine relative yield potential among glyphosate-resistant cotton cultivars or if systems-type trials are needed.
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MATERIALS AND METHODS
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Glyphosate-resistant cotton cultivars (Table 1) entered into the 1998 and 1999 South Carolina OCTs (May et al., 1998, 1999) were produced with three herbicide systems for 2 yr at the Clemson University Pee Dee Research and Education Center near Florence, SC. Soil types were Norfolk loamy sand (fine-loamy, kaolinitic, thermic Typic Kandiudults) in 1998 and Goldsboro loamy sand (fine-loamy, siliceous, subactive, thermic Aquic Paleudults) in 1999. The 1998 and 1999 experiments were conducted in fields planted to soybean [Glycine max (L.) Merr.] the previous crop season. Glyphosate-resistant cultivars included in our study were the majority of those available to growers in 1998 through 2000 (USDA-AMS, 1998, 1999, 2000). Early and later-maturity cultivars were evaluated in separate trials (Table 1), with relative cultivar maturity being the choice of the cultivar owner. Planting dates were 15 May 1998 and 16 May 1999. Conventional tillage, including subsoiling and disking, followed by bedding was practiced both years.
With the advent of glyphosate-resistant cotton cultivars, many combinations of herbicides can be used in their culture (Murdock, 1999). We wanted to focus on two herbicide regimes recommended for glyphosate-resistant cultivars and one employed in OCTs (May et al., 1998, 1999). Therefore, we defined the following herbicide regimes: Residual + Glyphosate, a single overtop glyphosate application at four-leaf stage, followed by postdirected soil-applied herbicides at layby; Glyphosate Only, one overtop glyphosate spray at four-leaf stage, followed by a postdirected glyphosate application at eight-leaf stage; and Standard, an herbicide regime employing only soil-applied herbicides applied at usual cotton growth stages. Herbicide systems including chemical and common names of herbicides, cotton growth stage at application, and application rates are listed in Table 2. All herbicides were applied at broadcast rates recommended by the manufacturer and verified by Murdock (1999) for cotton production in South Carolina. Herbicides were applied with a tractor-mounted, pressurized air applicator delivering 187 L ha-1 carrier volume. The applicator was modified as needed to accomplish the recommended manner of application (e.g., broadcast or postdirected) for the herbicide(s) specified in Table 2. Postdirected applications of herbicides were carefully applied to minimize leaf contact.
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Table 2. Herbicide systems, including herbicides, rates, dates, and methods of application for cultivar x herbicide system trials conducted for 2 yr at Florence, SC, in 1998 and 1999.
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Cultivars and herbicide systems were arranged in a strip-plot treatment design (Gomez and Gomez, 1984, p. 108–115), with cultivar as the horizontal factor and herbicide system as the vertical factor. Four replicates were arranged in a randomized complete block design. The experimental unit (plot) was four 10.6-m (35 feet)-long rows, spaced 0.96 m (38 inches) apart. Plots were thinned to two plants per 0.3 m (12 inches) at the two-leaf cotton growth stage. At maturity, the center two rows of each plot were mechanically harvested with a commercial spindle-type cotton picker. About 1 kg of seed cotton per plot was retained at harvest for ginning on a laboratory model 10-saw gin to determine lint fraction. Lint fraction was then multiplied by plot seed cotton yield to determine lint yield. Trials were not irrigated. The remaining production practices were those recommended by the Clemson University Cooperative Extension Service (Jones, 1999).
Trials were not seeded with weeds in either year as the focus of this study was not weed management as affected by herbicide regime or weed species and densities. Weed density by plot (e.g., each cultivar x herbicide system x replicate combination) was not determined in 1998 because volunteer weed growth was sparse to nonexistent. Weed densities were observed to be minimal or nonexistent the remainder of the 1998 season. More weeds were observed in 1999 during the critical period of 6 to 8 wk after emergence for wide-row cotton (Buchanan and Burns, 1970, 1971; Patterson et al., 1980); thus, weed biomass for each plot was characterized by species and aboveground dry matter on 28 and 29 June 1999. Volunteer sicklepod [Senna obtusifolia (L.) H. Irwin & Barneby], pitted morningglory (Ipomoea lacunosa L.), and yellow nutsedge (Cyperus esculentus L.) were observed sporadically in the trials. Biomass of pitted morningglory and sicklepod was quantified for the 10.6-m length of the row width separating the two rows mechanically harvested from each plot in 1999. Yellow nutsedge biomass was quantified by randomly selecting a 1-m section of the row adjacent to that sampled for broadleaf weed species. The aboveground dry weight (kg ha-1 after forced air-drying to constant moisture for 12 h) of sicklepod, pitted morningglory, and yellow nutsedge was summed for ANOVA of weed biomass production. Weeds were not removed from plots before initiation of fruiting in 1999 because their biomass was deemed too low to cause yield loss through competition effects (Buchanan and Burns, 1970, 1971; Buchanan et al., 1980).
Precipitation and temperature data were recorded daily from planting to harvest. The weather station reporting these data was located proximal to the 1998 and 1999 trial sites.
The lint yield data were subjected to ANOVA within and over years while weed biomass ANOVAs were conducted for 1999 only. One replicate of the 1999 later-maturity trial was discarded because of nonuniform stands. Within-year ANOVAs of lint yield with PROC GLM (SAS Inst., 2001) for all trials revealed that Error B (replication x herbicide system) was nonsignificant at P > 0.25. Similarly, combined ANOVAs over years with PROC GLM revealed that Error B [herbicide system x replication(year)] was nonsignificant at P > 0.41 and P > 0.65 for earlier- and later-maturity trials, respectively; year x herbicide system was nonsignificant at P > 0.7 and P > 0.9, respectively, for earlier- and later-maturity trials; and the year x herbicide system x cultivar interaction was nonsignificant at P > 0.9 and P > 0.7, respectively, for earlier- and later-maturity trials. One approach to conducting ANOVA when sources of variation for random effects are nonsignificant at P > 0.25 is to use the expected values of mean squares and pool nonsignificant random sources accordingly (Carmer et al., 1969), followed by recomputing mean squares and F-ratios to test significance of fixed effects. However, the validity of expanding degrees of freedom for error sources through pooling to then test significance of fixed effects remains in question. Instead of this approach, we chose to subject the lint yield data within and over years to mixed-model analyses with SAS PROC MIXED (SAS Inst., 2001). A key feature of this approach is that PROC MIXED automatically constructs the F-ratio to test fixed effects when the model for the treatment and experimental design in question is correctly entered and sources of variation in the ANOVA are appropriately identified as fixed or random, but degrees of freedom for the denominator of F-ratios of fixed effects are not increased when PROC MIXED determines one or more random effects to be negligible. We used the PDIFF option of the LSMEANS procedure to separate main effects with a t-test when they were significant at P < 0.10, but cultivar x herbicide system interactions were nonsignificant (P > 0.10). The 1999 weed biomass ANOVAs were conducted similar to the within-year lint yield ANOVAs.
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RESULTS AND DISCUSSION
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Hectarage planted to glyphosate-resistant cotton cultivars has nearly doubled every year since commercial introduction in 1997 (USDA-AMS, 1998, 1999), testifying to the popularity of the technology. Indeed, the cultivars in our study accounted for 16 and 42% of the southeastern U.S. cotton hectarage in 1998 and 1999, respectively, and all of that planted to glyphosate-resistant cultivars, except the small portion planted to newly developed candidate cultivars. Producers, however, have chosen glyphosate-resistant cultivars without performance data reflecting one of the intended glyphosate herbicide systems or knowledge of cultivar variation in resistance to glyphosate. We are not aware of studies that have evaluated main effects and interaction of both glyphosate-resistant cultivars and herbicide systems with and without glyphosate when glyphosate was applied according to Monsanto's instructions for production of glyphosate-resistant cotton cultivars.
The ANOVA of lint yield from the early- and later-maturity trials within and over years indicated significant effects of herbicide system but not cultivar (Table 3). There were no significant interactions among year, cultivar, or herbicide systems in the combined ANOVA of cultivars produced in both years. The absence of cultivar x herbicide system interactions in the early- and later-maturity trials indicates that relative cultivar yield ranks were similar, regardless of herbicide system. This finding suggests that OCTs can discriminate among glyphosate-resistant cultivars for relative yield potential.
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Table 3. Mean squares from the within-year and combined ANOVAs over years of early and later-maturity, glyphosate-resistant cotton cultivar yields produced in three herbicide systems for 2 yr at Florence, SC.
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Before the development of cotton cultivars containing transgenically imparted pest management traits, conduct of OCTs to assess relative cultivar yields was simpler because cultivars generally did not require specific production practices to estimate genetic yield potential. Scientists conducting OCTs must therefore decide on the appropriate protocol to conduct trials with the objective of determining relative yield potential among transgenic cultivars and to also fairly compare yields between transgenic and nontransgenic cultivars.
We found in both the early and later-maturity trials within and over year that the Glyphosate-Only herbicide system produced more yield than the Residual + Glyphosate or the Standard system (Table 4). Additionally, the Residual + Glyphosate herbicide system outyielded the Standard system within and over years in the early maturity trials.
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Table 4. Herbicide system main-effect lint yield means averaged over cultivars for glyphosate-resistant cotton cultivar x herbicide system trials conducted for 2 yr at Florence, SC.
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All herbicides in our study were applied at labeled rates and at recommended crop growth stages (Murdock, 1999). We did not observe leaf chlorosis or other visible symptoms of herbicide injury (data not shown) in the Standard or Residual + Glyphosate herbicide systems. Therefore, the lower yield of the Standard system may not reflect herbicide injury per se but instead yield drag from soil-applied and postapplied herbicides, potentially exacerbated by poorly distributed rainfall (Table 5).
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Table 5. Mean maximum day and night temperatures and precipitation totals at 5-d intervals from seeding to 31 Aug. 1998 and 1999 for glyphosate-resistant cotton cultivar x herbicide system trials conducted at Florence, SC.
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Welch et al. (1997) and Sanders et al. (2000) found in single-year field trials that glyphosate-resistant cotton cultivars produced with soil-applied herbicides yielded less compared with glyphosate-only herbicide systems. Greenhouse and growth chamber studies report reduced plant or root growth when cotton was grown in the presence of dinitroaniline herbicides or fluometuron (Kappelman and Buchanan, 1968; Kappelman et al., 1971; Murray et al., 1979), possibly explaining yield reduction in the presence of such herbicides.
Although weed species were not seeded into plots in either year, weed competition effects from volunteer populations were considered in interpreting yield variation among herbicide system main-effect means. In 1998, weed densities were visually assessed season-long to be so low (data not shown) as to not warrant quantification by species, frequency, or dry weight. In 1999, we observed greater volunteer weed growth that was characterized 6 wk after planting by species and aboveground dry weight for each plot in the early and later-maturity trials. Yellow nutsedge, sicklepod, and pitted morningglory were observed sporadically in the trials, but herbicide system main-effect yield means were apparently not influenced by weed competition effects (Buchanan and Burns, 1970, 1971; Patterson et al, 1980) because we found nonsignificant (P > 0.3) differences in total weed dry weights among herbicide systems for all weed species (data not shown).
Another hypothesis to explain the response of glyphosate-resistant cotton cultivars to herbicide systems in our study is that glyphosate-resistant cotton cultivars may be more sensitive to soil-applied herbicides compared with nonresistant cultivars, reflecting effects of insertion of a foreign gene into the host genome. Our finding that glyphosate-resistant cultivars produced with the Standard herbicide system yielded less than the two herbicide systems with fewer or no soil-applied herbicides suggests further study to determine if OCTs allow valid comparisons between nontransgenic and glyphosate-resistant cultivars.
Growers in Mississippi in 1997 and Georgia in 1998 reported fruit abortion and, in some instances, yield losses on portions of their glyphosate-resistant cotton hectarage. Extreme low day and night temperatures have been implicated in the performance of glyphosate-resistant cultivars in these instances (Kerby and Voth, 1998). For example, mean May through June temperatures in Mississippi in 1997 were 4°C below the 100-yr mean, leading to speculation that the transgene conferring resistance to glyphosate was not expressed at levels it would normally be expressed at under more typical temperature regimes for cotton production. In our study, examination of mean minimum and maximum temperatures at 5-d intervals from seeding to late boll filling (Table 5) indicates no unusual temperatures; thus, the transgene imparting glyphosate resistance apparently functioned as intended.
In summary, this study suggests that OCTs remain relevant vehicles with which to determine relative yields among glyphosate-resistant cultivars. Commercialization of new transgenic traits will require additional studies to assess the veracity of OCTs to provide relevant cultivar performance data. While OCTs are not fail-safe methods to determine relative cultivar yields, they are a cost-effective means of testing cultivars and much simpler to accomplish than trials employing cultivar-specific pest management regimes. Still, the finding of herbicide system main effects favoring glyphosate-resistant cultivars when produced without soil-applied herbicides prompts further research to determine if OCTs generally provide fair comparisons between glyphosate-resistant and nonresistant cultivars. As additional transgenic input traits become commercialized and combined into the same cultivar, it may ultimately be necessary to adopt systems trials imposing cultivar-specific insect and weed pest management.
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ACKNOWLEDGMENTS
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The authors thank the South Carolina Cotton Board for sponsoring this research and Debbie Cottingham, Ryan Graham, and Charles Parker for technical assistance.
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TOP
ABSTRACT
INTRODUCTION
