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

Abstract

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 × 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 × 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.