Comparison of residual activity of pre-emergence herbicides for control and seed production of multiple herbicide-resistant Palmer amaranth in food-grade white corn

Nebraska is the number-one producer of food-grade white corn (Zea mays L.) in the United States. Food-grade white corn has not been genetically engineered; therefore, non-selective herbicides such as glyphosate or glufosinate cannot be used. Multiple herbicide-resistant (MHR) Palmer amaranth (Amaranthus palmeri S. Watson) populations have been reported in multiple counties in Nebraska, and their management is a challenge, particularly for white corn producers. The objectives of this study were to evaluate the residual activity of pre-emergence (PRE) herbicides for acetolactate synthase inhibitor (halosulfuron-methyl)/atrazine/glyphosate-resistant Palmer amaranth control, density, biomass, seed production, and grain yield in food-grade white corn. Field experiments were conducted during summer 2020 and 2021 in a grower's field infested with MHR Palmer amaranth near Carleton, NE. All the PRE herbicides resulted in similar control (>90%) 30 days after PRE application (DAPRE) apart from atrazine (64%). At 45 DAPRE, acetochlor/mesotrione, atrazine/bicyclopyrone/mesotrione/S-metolachlor, and acetochlor/clopyralid/mesotrione controlled 90%–95% Palmer amaranth. Acetochlor/clopyralid/mesotrione and atrazine/bicyclopyrone/mesotrione/S-metolachlor provided 96%–99% MHR Palmer amaranth control and reduced Palmer amaranth density and biomass to 2–4 plants m⁻² and 5–12 g m⁻² 60 DAPRE. The highest corn yields of 12,139 kg ha⁻¹ and 12,093 kg ha⁻¹ in 2020 and 2021, respectively, were obtained with acetochlor/clopyralid/mesotrione. Palmer amaranth seed production was least with acetochlor/clopyralid/mesotrione (32,894 seeds m⁻²). Tested residual PRE herbicides did not show corn injury and were safe to use in food-grade white corn. It is concluded that acetochlor/clopyralid/mesotrione and atrazine/bicyclopyone/mesotrione/S-metolachlor are the best options for early season control of MHR Palmer amaranth in food-grade white corn.