Evolution of kochia resistance to glyphosate and dicamba is a concern for growers in the US Great Plains. An increasing use of glyphosate and dicamba with the widespread adoption of glyphosate/dicamba-resistant (GDR) soybean in recent years may warrant greater attention. Long-term stewardship of this new stacked-trait technology will require the implementation of diverse weed control strategies, such as the use of soil-residual herbicides (PRE) aimed at effective control of GDR kochia. Field experiments were conducted in Huntley, MT, in 2017 and 2018, and Hays, KS, in 2018 to determine the effectiveness of various PRE herbicides applied alone or followed by (fb) a POST treatment of glyphosate plus dicamba for controlling GDR kochia in GDR soybean. Among PRE herbicides tested, sulfentrazone provided complete (100%), season-long control of GDR kochia at both sites. In addition, PRE fb POST programs tested in this study brought 71% to 100% control of GDR kochia throughout the season at both sites. Pyroxasulfone applied PRE resulted in 57% to 70% control across sites at 9 to 10 wk after PRE (WAPRE). However, mixing dicamba with pyroxasulfone improved control up to 25% at both sites. Kochia plants surviving pyroxasulfone applied PRE alone produced 2,530 seeds m^–2 compared with pyroxasulfone + dicamba (230 seeds m^–2) at the Montana site. No differences in soybean grain yields were observed with PRE alone or PRE fb POST treatments at the Montana site; however, dicamba, pyroxasulfone, and pendimethalin + dimethenamid-P applied PRE brought lower grain yield (1,150 kg ha^–1) compared to all other tested programs at the Kansas site. In conclusion, effective PRE or PRE fb POST (two-pass) programs tested in this research should be proactively utilized by the growers to manage GDR kochia in GDR soybean.

Nomenclature: Glyphosate; dicamba; dimethenamid-P; pendimethalin; pyroxasulfone; sulfentrazone; kochia, Bassia scoparia (L.) A.J. Scott; soybean, Glycine max (L.) Merr