Evaluation of spring and fall burndown application timings on control of glyphosate-resistant horesweed in no-till cotton

Field studies were conducted in 2007 and 2008 to evaluate fall applications of herbicides to control GR horseweed prior to planting cotton. Fall treatments were compared with spring treatments for control of GR horseweed and effect on seed cotton yield. Fall and spring treatments with and without residual herbicides were also compared. No differences were observed for control of GR horseweed or seed cotton yield for both fall and spring application timings. However, a difference was observed between fall applications with and without a residual herbicide. Fall applications that contained residual herbicides provided 86% control of GR horseweed and yielded 2355 kg/ha of seed cotton. Fall applications that did not contain a residual herbicide only provided 70% control of GR horseweed and yielded 2008 kg/ha of seed cotton. No benefit was observed from spring applications that contained a residual herbicide. This research indicates that glyphosate-resistant horseweed can be controlled with fall or spring applied burndown herbicides, and fall applications should include a residual herbicide for best results.