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 herbicides (fomesafen + acetochlor), followed by an early postemergence application of glufosinate and/or a late postemergence application of 2,4-D + glyphosate. Control was assessed at 7 to 8 wk after planting, and yields were standardized within environment to the maximum observed yield to allow direct comparison of herbicide programs across crops. Overall, treatments that included a herbicide applied late postemergence (preemergence+ late postemergence, early postemergence + late postemergence, or preemergence + early postemergence + late postemergence) provided complete control of weeds and prevented yield losses. Herbicides applied preemergence + early postemergence frequently provided a slightly lower control than late postemergence herbicide applications, and a preemergence herbicide applied alone consistently resulted in the lowest control. Lower early season rain in 2019 coincided with reduced preemergence herbicide performance and wider differences among herbicide programs, whereas an earlier, higher amount of rain in 2020 provided greater weed control that was consistent with improved preemergence herbicide soil incorporation. Weed control in soybean was often greater than in cotton with a preemergence + early postemergence regimen in the drier year, consistent with faster canopy closure. Weed species responded similarly across programs, although preemergence-only control of morningglories was generally 20% to 30% lower than for Palmer amaranth and annual grasses. Relative yield reflected differences in weed control; plots without herbicides exhibited 70% to 90% losses, whereas late postemergence treatments achieved maximum yields. Results support pairing preemergence residuals with sequential herbicides applied postemergence and highlight the value of herbicides applied late postemergence to mitigate escapes of late-emerging weeds and avoid yield losses.