The commercial release of herbicide-resistant cotton (Gossypium hirsutum L.) has dramatically changed weed management practices. Roundup Ready cotton is tolerant of glyphosate applied topically through the 4-leaf stage. Cotton with enhanced glyphosate-resistance is available with Roundup Ready Flex cotton, which is tolerant of glyphosate from emergence until harvest. Studies were conducted with Roundup Ready Flex cotton to determine the response of cotton and weeds to single and multiple glyphosate applications, the effect of early season weed competition on cotton yield, and the best timing for glyphosate applications. No cotton injury or yield loss was observed with glyphosate applied past the 4-leaf stage. Any treatment combination with multiple postemergence (POST) glyphosate applications provided essentially complete control of Palmer amaranth (Amaranthus palmeri L.), sicklepod (Senna obtusifolia L.), pitted morningglory (Ipomoea lacunose L.), and annual grasses. Single glyphosate applications from the 4- to 12-leaf cotton stage provided inconsistent weed control, allowed for competitive losses, and provided variable cotton lint yield. Optimum weed control and lint yield was achieved with multiple glyphosate applications. Roundup Ready Flex cotton can provide producers with acceptable weed control without compromising cotton yield. Early season weed interference had inconsistent effects on cotton yield. Multiple postemergence glyphosate applications can preserve cotton yield potential with acceptable weed control.
