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Tolerance of Roundup Ready Cotton to Topical and Post-Directed Glyphosate

William B. McCloskey and Hal S. Moser


ABSTRACT

Roundup Ready cotton tolerance to topical and post-directed glyphosate applications was evaluated in a series of experiments conducted in 1997 to 2000 at three University of Arizona Experiment Stations. In the transgenic comparison experiments, glyphosate treated split-plots received a 0.75 lb ae/A topical application at the 2 to 4 true-leaf growth stage, followed by two 0.75 lb ae/A post-directed applications at the base of the cotton at the 10 to 12 node and 18 to 20 node growth stages. In side by side paired plots, there were no significant yield differences between unsprayed and sprayed varieties containing the Roundup Ready gene except for DP458BRR in 1999. On average, varieties containing the Roundup Ready gene yielded about 1% more than the conventional parent variety in these trials. Topical applications of glyphosate at 0.75 lb ae/A made before the fifth true leaf growth stage had no effect on cotton yield. Two sequential topical glyphosate applications at 0.75 lb ae/A with the second application made after the fifth true leaf growth stage had variable effects on cotton yield depending on the year and location. At Marana in 1999, a second 0.75 lb ae/A topical glyphosate application at the 7, 11, and 12 node growth stage did not significantly reduce yield, however, at MAC in 1997, 0.75 lb ae/A topical glyphosate applications made at the 6, 11, and 14 node growth stages reduced yield 5.9 (not statistically significant), 60, and 20.6%, respectively. In Marana in 1999 and 2000 and in Safford in 2000, post-directed glyphosate applications at labeled (0.75 lb ae/A) and “off-label” (1.12 and 1.5 lb ae/A) rates did not result in yield reductions. Similarly, in Marana and Safford in 2000, sloppy post-directed glyphosate applications at labeled(0.75 lb ae/A) and “off-label” (1.5 lb ae/A) rates did not result in yield reductions even when they followed an “off-labeled topical application (1.12 lb ae/A). In contrast, all treatments at Maricopa in 2000 with two or more glyphosate applications yielded significantly less than the unsprayed control (Table 4). These data indicate that Roundup Ready cotton varieties possess significant tolerance to “off-label” or above label rate glyphosate applications but that yield losses can result from such applications when environmental conditions limit mid-season and late-season boll production and retention (i.e., compensation).





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Document last modified May 20, 2002