Evaluation of single versus multiple site of action preemergence herbicides for waterhemp and giant ragweed control in Wisconsin soybean

Abstract The use of preemergence (PRE) herbicides with soil residual activity is recognized as an effective strategy for early‐season weed management in Midwest US soybean [ Glycine max (L.) Merr.] production. The efficacy of soybean PRE herbicides with single and multiple sites of action (SOAs), including acetolactate synthase (ALS), photosystem II, protoporphyrinogen oxidase (PPO), and very long‐chain fatty acid (VLCFA) inhibitors, was assessed in a randomized complete block design across 4 site‐years (Brooklyn‐2019, Fond du Lac‐2019, Lancaster‐2018, and Lancaster‐2019) for waterhemp [ Amaranthus tuberculatus (Moq.) J.D. Sauer] and three site‐years (Janesville‐2019, Janesville‐2020, and Janesville‐2021) for giant ragweed ( Ambrosia trifida L.) in Wisconsin. The data were pooled across site‐years, and the results are presented by species to highlight the most effective herbicide treatments across varying environmental conditions. Several single‐ and multiple‐SOA PRE herbicides from the photosystem II (PS II), PPO, and VLCFA groups effectively (>90%) controlled waterhemp at 21 and 42 days after treatment (DAT) and significantly reduced biomass (>86%) 42 DAT. Similarly, single ALS PRE herbicides group, as well as premixes containing ALS + PPO, ALS + PS II + PPO, and ALS + PPO + VLCFA, provided 79%–92% giant ragweed control at 21 and 42 DAT and reductions in biomass by 80%–95%. Our data show a positive correlation between biomass reduction and visual weed control assessments for both waterhemp and giant ragweed, suggesting that quality visual assessments can serve as a reliable proxy for evaluating PRE herbicide efficacy. Our results suggested that PRE herbicide containing two or three SOAs provided greater and more consistent control and biomass reduction of waterhemp and giant ragweed compared to single‐SOA PRE herbicides.