Control of Giant Ragweed (Ambrosia trifida) in Mesotrione-Resistant Soybean

Preemergence (PRE) applications of mesotrione, a herbicide that inhibits 4-hydroxyphenolpyruvate dioxygenase (HPPD), have recently gained regulatory approval in appropriately traited soybean varieties. Giant ragweed is an extremely competitive broadleaf weed, and biotypes resistant to acetolactate synthase inhibitors (ALS-R) can be particularly difficult to manage with soil-residual herbicides in soybean production. This study investigated control of giant ragweed from PRE applications of cloransulam (32 g ai ha-1), metribuzin (315 g ai ha-1), and S-metolachlor (1600 g ai ha-1) in a factorial design with and without mesotrione (177 g ai ha-1) at two different sites over two years. Treatments with mesotrione were also compared with two commercial premix products: sulfentrazone (283 g ai ha-1) and cloransulam (37 g ai ha-1), and chlorimuron (19 g ai ha-1), flumioxazin (69 g ai ha-1), and pyroxasulfone (87 g ai ha-1). At 42 days after planting, control and biomass reduction of giant ragweed were greater in treatments with mesotrione than any treatment without mesotrione. Giant ragweed biomass was reduced by 84% in treatments with mesotrione, while treatments without mesotrione did not reduce biomass relative to the nontreated. Following these PRE applications, sequential herbicide treatments utilizing postemergence (POST) applications of glufosinate (655 g ai ha-1) plus fomesafen (266 g ai ha-1) and S-metolachlor (1217 g ai ha-1) resulted in at least 97% control of giant ragweed at 42 days after planting, which was greater than sequential applications of glufosinate alone in 3 of 4 site-years. PRE applications of mesotrione can be an impactful addition to soybean herbicide programs designed to manage giant ragweed, with the potential to improve weed control and delay the onset of herbicide resistance by providing an additional effective herbicide site of action.