Cotton and grain sorghum response to drift rates of glyphosate

With the increase in the number of hectares planted to herbicide tolerant crops, cotton and grain sorghum could be subjected to glyphosate drift from applications made to glyphosate-tolerant crops. The objectives of this study were to determine the effects of glyphosate drift to non-tolerant cotton and grain sorghum. Glyphosate at reduced rates was applied to cotton at the cotyledon to 1-leaf, 4- to 5-leaf, pinhead square (PHSQ), and first bloom (FBLM) growth stages and to grain sorghum at the 4-inch, 12-inch, boot, and bloom growth stages.;In 2002, soon after treatment, little difference in irrigated cotton injury was observed from glyphosate applications at any growth stage. By the end of the season, glyphosate applied later in the season showed more injury and cotton lint yield was affected more than from earlier applications. In 2003, glyphosate was less injurious to irrigated cotton when applied at FBLM; however, cotton lint yield was reduced more by applications made at FBLM than at other growth stages.;In 2002, glyphosate applications made to dryland cotton early in the season were more injurious than those made later in the season. Cotton lint yield was reduced more dramatically from applications made to later season dryland cotton. In 2003, similar injury was observed after applications of glyphosate made at 4- to 5-leaf, PHSQ, and FBLM. Cotton lint yield was reduced more from applications made early season compared to later season applications.;Early season applications were consistently more injurious to grain sorghum throughout the season compared to later season applications in 2002, which correlated to grain yield reductions. Injury was not as significant in 2003, but a similar trend was observed as in 2002. Yields were not reduced as significantly from bloom applications as from 4-inch, 12-inch, and boot applications.;These data show that crop conditions prior to and immediately following herbicide drift impact the severity of injury and yield loss. Cotton can tolerate higher glyphosate rates earlier in the season, while grain sorghum can tolerate higher rates later in the season without a significant impact on yield.