Evaluation of foliar-applied post-emergence corn-soybean herbicides on giant ragweed and waterhemp control in Wisconsin

Giant ragweed (Ambrosia trifida L.) and waterhemp (Amaranthus tuberculatus [Moq.] J.D.Sauer) are highly competitive and the most troublesome weeds in corn (Zea mays L.) and soybean (Glycine max Merr.) cropping systems in Wisconsin. Field experiments were conducted to evaluate the efficacy of multiple foliar-applied POST-emergence herbicides on giant ragweed at Janesville, WI (2020 and 2021), and on groups 2- and 9-resistant waterhemp at Brooklyn, WI (2019 and 2020), respectively. Several herbicides from site of action groups 2, 4, 5, 6, 9, 10, 14, and 27 were evaluated. Herbicide treatments were applied when giant ragweed and waterhemp reached similar to 10 cm in height. Herbicide efficacy (0-100% visual control) was evaluated 14 days after treatment (DAT). Group 2 herbicides provided poor (<45%) control of giant ragweed and waterhemp 14 DAT. Several herbicides encompassing different sites of action provided >90% giant ragweed control. Certain herbicides from groups 4, 10, 14, and 27 provided the best control (74-87%) of waterhemp, but no herbicide evaluated in this study provided = 90% waterhemp control. Weed management programs containing multiple effective herbicide sites of action PRE- and POST-emergence accompanied by non-chemical strategies are recommended to extend the spectrum of chemical control and reduce the risk of selection of further herbicide-resistant biotypes for both giant ragweed and waterhemp in Wisconsin and beyond.