WEED SCIENCE Tolerance of GlyTol ® and GlyTol ® + LibertyLink ® Cotton to Glyphosate and Glufosinate in the Southeastern U. S.

Cotton (Gossypium hirsutum L.) has been genetically engineered with tolerance to both glyphosate and glufosinate. This new technology will give growers an additional tool to control weeds, including glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Wats). An experiment was conducted at eight locations in five southeastern states during 2008 to determine tolerance of an experimental line of GlyTol™ + LibertyLink ® cotton to the ammonium salt of glyphosate (1.3 kg a.e. ha -1 ) or the ammonium salt of glufosinate (0.6 kg a.e. ha -1 ) applied four times alone or combined. Additional treatments included glyphosate and glufosinate in four alternating applications with glyphosate applied first or with glufosinate applied first. An experimental line of GlyTol™ cotton was included for comparison and received four glyphosate applications. Herbicides were applied topically to cotton with 1- to 3-, 6- to 8-, and 14- to 16-nodes and again at 50% cracked boll. Compared to non-treated checks, no visible injury was noted with glyphosate applied to GlyTol cotton or with glyphosate and glufosinate applied to GlyTol + LibertyLink cotton at any application timing. Herbicides had no effect on cotton height, boll morphology, lint yield, maturity, or fiber quality within cotton lines. This experiment demonstrates these new transgenic technologies have excellent tolerance of glyphosate and glufosinate.