Weed Management in Enhanced Glyphosate-Resistant Cotton

Abstract

Field experiments were conducted in 2003 and 2004 to evaluate glyphosate rates and timings on control of Palmer amaranth, devil’s-claw, ivyleaf morningglory, and silverleaf nightshade in enhanced glyphosate-resistant cotton.  Treatments based on cotton growth stage (CS) were compared to as-needed (ASN) treatments based on weed population and size.  Palmer amaranth, devil’s-claw, and silverleaf nightshade were controlled (> 90%) with postemergence (POST) treatments based on CS or ASN applications in both years.  These weeds were controlled with glyphosate at 0.75 lb ae/A and no benefit was observed with an increased glyphosate rate.  Ivyleaf morningglory control, in both years, improved with increased glyphosate rates to 1.5 lb ae/A.  When the first application was delayed to 11-leaf cotton, three glyphosate applications at 1.5 lb ae/A were required to achieve control.  Ivyleaf morningglory in 2003 was controlled with four glyphosate applications applied ASN beginning at two-leaf cotton and ending with the last treatment applied at 20-leaf cotton.  In 2004 with increased rainfall and weed pressure, five applications of glyphosate at 1.5 lb ae/A were required for effective control (> 90%).