Weed control and yield response to Enlist-based herbicide programs in cotton and soybean systems in North Carolina

Abstract

Cotton and soybean with tolerance to 2,4-D, glufosinate, and glyphosate offer useful options for managing herbicide-resistant weeds, especially Palmer amaranth, but effective late-season weed management depends on herbicide program design and timing. In 2019 and 2020, field experiments at two North Carolina locations compared herbicide programs built on preemergence (PRE) herbicides (fomesafen + acetochlor), followed by an early postemergence (EPOST, glufosinate) application and/or a late postemergence (LPOST, 2,4-D + glyphosate). Control was assessed 7–8 weeks after planting, and yields were standardized within environment to the maximum observed yield to allow direct comparison of herbicide programs across crops. Overall, programs that included a herbicide applied LPOST (PRE+LPOST, EPOST+LPOST, or PRE+EPOST+LPOST) controlled weeds completely and prevented yield losses. Herbicides applied PRE+EPOST frequently provided a slightly lower control than LPOST herbicide programs, and PRE herbicide alone consistently resulted in the lowest control. Lower early-season rainfall in 2019 coincided with reduced PRE herbicide performance and wider differences among herbicide programs, whereas earlier, greater rainfall in 2020 increased weed control consistent with improved PRE herbicide soil incorporation. Weed control in soybean was often greater than in cotton with PRE+EPOST in the drier year, consistent with faster canopy closure. Weed species responded similarly across programs, though PRE-only control of morningglories was generally 20–30 % lower than for Palmer amaranth and annual grasses. Relative yield reflected differences in weed control; plots without herbicides exhibited 70–90% losses, whereas herbicide programs with LPOST treatments achieved maximum yields. Results support pairing PRE residuals with sequential herbicides applied POST in herbicide programs, and highlight the value of herbicides applied LPOST to mitigate escapes of late-emerging weeds and avoid yield losses.