Palmer amaranth is a summer annual weed native to the southwestern United States and is particularly troublesome across the Cotton Belt. The use of soil-residual herbicides is crucial for cotton producers in the Texas High Plains attempting to achieve early season control of Palmer amaranth and other problematic weeds. A bare-ground field study was conducted in 2022 and 2023 in Halfway, Texas, to evaluate weed control following isoxaflutole + pendimethalin applied preplant incorporated (PPI) or pendimethalin PPI followed by (fb) isoxaflutole applied preemergence alone or combined with seven other cotton residual herbicides, and to determine whether using isoxaflutole PPI would be a useful option for rainfed production systems. In 2022, Palmer amaranth was completely controlled at 28 d after application (DAA) following isoxaflutole + pendimethalin applied PPI fb acetochlor or S-metolachlor applied preemergence, and pendimethalin applied PPI fb isoxaflutole in a tank-mix with diuron, fomesafen, acetochlor, or S-metolachlor applied preemergence. At 42 DAA, PPI treatments that contained isoxaflutole + pendimethalin fb preemergence applications of acetochlor or S-metolachlor, and a PPI treatment with pendimethalin fb isoxaflutole + acetochlor or S-metolachlor applied preemergence increased Palmer amaranth control by ≥12% compared with isoxaflutole + pendimethalin applied PPI fb prometryn applied preemergence and pendimethalin applied PPI fb isoxaflutole + prometryn applied preemergence. In 2023, when assessed at 42 DAA, Palmer amaranth was controlled by >90% with isoxaflutole + pendimethalin applied PPI fb fomesafen or S-metolachlor applied preemergence, and pendimethalin applied PPI fb isoxaflutole applied alone or in a tank-mixture with diuron, fluridone, fomesafen, acetochlor, or S-metolachlor applied preemergence. End-of-season aboveground Palmer amaranth fresh biomass did not vary among treatments in either year. These results suggest that soil-residual control of Palmer amaranth greater than 90% can be achieved for 7 to 9 wk using isoxaflutole-based treatments applied either PPI or preemergence. Furthermore, applying isoxaflutole PPI can be a useful option for achieving early season weed control in cotton production systems.
