Synergistic interactions of 2,4-D, dichlorprop-p, dicamba, and halauxifen/fluroxypyr for controlling multiple herbicide-resistant kochia (Bassia scoparia L.)

Multiple herbicide-resistant (MHR) kochia is a serious concern in the U.S. Great Plains and warrants alternative herbicide mixtures for its control. Greenhouse and field experiments were conducted at Kansas State University research and extension centers near Hays and Garden City, KS, to investigate the interactions of 2,4-D, dichlorprop-p, dicamba, and halauxifen/fluroxypyr premix in various combinations for MHR kochia control. Two previously confirmed MHR (resistant to glyphosate, dicamba, and fluroxypyr) populations and a susceptible population were tested in a greenhouse study. Kochia at the Hays field site was resistant to glyphosate and chlorsulfuron, whereas the population at Garden City was resistant to glyphosate, dicamba, and fluroxypyr. Results from a greenhouse study indicated that 2,4-D, dicamba, dichlorprop-p, and a halauxifen/fluroxypyr premix provided 26% to 69% control of both MHR populations at 28 d after treatment (DAT). However, the control increased to 85% to 97% when these herbicides were applied in three-way mixtures. Synergistic interactions were observed when dicamba was mixed with dichlorprop-p, 2,4-D, dichlorprop-p + 2,4-D, and halauxifen/fluroxypyr + 2,4-D for shoot dry weight reductions (86% to 92%) of both MHR populations. Results from a field study also indicated synergistic interactions when dicamba was mixed with dichlorprop-p + 2,4-D, halauxifen/fluroxypyr + dichlorprop-p, and halauxifen/fluroxypyr + 2,4-D, resulting in 84% to 95% control of MHR kochia at 28 DAT across both sites. These results indicate that synergistic effects of mixing dicamba with other auxinic herbicides in two- or three-way mixtures can help control MHR kochia.