Control of Multiple-Herbicide-Resistant Green Pigweed (Amaranthus powellii) With Preemergence and Postemergence Herbicides in Ontario Corn Production

Green pigweed [Amaranthus powellii S.Wats.] is a prolific annual dicot weed that is a prominent weed of crop production in northeastern North America. Green pigweed interference has been documented to reduce corn yields up to 54% in the absence of control strategies. In 2021, a green pigweed biotype from a field near Dresden, Ontario, Canada was determined to be resistant to MCPA, mecoprop, dichlorprop-p, aminocyclopyrachlor (synthetic auxins), and imazethapyr (acetolactate synthase-(ALS)-inhibitor), further impacting control of this weed biotype. Two field studies, with herbicides applied preemergence (PRE) or postemergence (POST), were conducted in 2020 and repeated in 2021. The objective of the research was to determine the most effective PRE and POST herbicides for the control of multiple-herbicide-resistant (MHR) green pigweed in Ontario corn production. 18 PRE and 18 POST herbicide treatments were evaluated in separate studies. Visible crop injury, visible green pigweed control at specified timepoints after herbicide application, green pigweed density, green pigweed biomass, and corn yield at harvest maturity were collected. In the PRE study, rimsulfuron + mesotrione was identified as the most efficacious treatment providing 88% control at 8 WAA. In the POST study, atrazine was identified as the most efficacious treatment providing 94% control at 8 WAA. Control with all PRE herbicide treatments was impacted by rainfall following application. MCPA ester applied POST controlled green pigweed 30% at 8 WAA; reduced control is attributed to herbicide resistance in this biotype. When compared to similar studies, control of green pigweed was reduced with some of the POST herbicides tested. While MHR green pigweed represents an additional challenge for growers, there are efficacious herbicide treatments that would allow it to be managed in corn production.