WEED SCIENCE Evaluation of GlyTol ™ and GlyTol ™ + LibertyLink ® Cotton in the Mid-South

Historically, weed control in cotton (Gossypium hirsutum L.) relied upon a combination of tillage, soil-applied herbicides, postemergencedirected herbicides, and hand weeding. More recently, weed control in cotton has become heavily reliant on transgenic technologies. Glyphosate-resistant and enhanced glyphosateresistant cotton were commercialized in 1997 and 2006, respectively. Glufosinate-resistant cotton was commercialized in 2004. Although Monsanto Company has been the traditional provider of glyphosate-resistant technology, Bayer CropScience identified a novel glyphosateresistant gene and released this technology to the market in 2011. In addition, Bayer CropScience introduced glyphosate/glufosinate-resistant cotton containing this new glyphosate-resistant trait plus the existing glufosinate-resistance trait. The new glyphosate-resistant technology is known as GlyTol ™ , whereas the glyphosate/ glufosinate-resistant technology is known as GlyTol ™ + LibertyLink ® . Field experiments were conducted at 14 locations across Arkansas, Louisiana, Mississippi, and Tennessee from 2007 through 2009 to determine cotton response to multiple glyphosate and/or glufosinate applications. Glyphosate-resistant cotton was not visually injured by sequential glyphosate applications. Glyphosate/glufosinate-resistant cotton visual injury was 2% or less when treated with glyphosate, glufosinate, or glyphosate plus glufosinate. A reduction in plant height of up to 4 cm was observed only with the glyphosate/glufosinateresistant cotton after two or three glufosinate applications were made; heights were not reduced by late season. Herbicide applications did not affect boll development or cotton yield. These data indicate GlyTol cotton has excellent tolerance to glyphosate applied topically and GlyTol + LibertyLink cotton has excellent tolerance to topical applications of glyphosate, glufosinate, and glyphosate plus glufosinate.