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PRODUCTION PAPER

Yield Ranks of Glyphosate-Resistant Cotton Cultivars are Unaffected by Herbicide Systems

^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. 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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