Evolution and rapid spread of herbicide-resistant (HR) kochia has become a significant challenge for growers in the U.S. Great Plains. The main objectives of this research were to confirm and characterize the response of putative auxinic HR (Aux-HR) kochia accessions (designated as KS-4A, KS-4D, KS-4H, KS-10A, KS-10-G, and KS-10H) collected from two different corn fields near Garden City, KS, to dicamba and fluroxypyr and to determine the EPSPS gene copy number to detect whether those accessions were also resistant to glyphosate. Single-dose experiments indicated that putative Aux-HR kochia accessions had 78% to 100% and 85% to 100% survivors when treated with dicamba (560 g ae ha ^{– 1}) and fluroxypyr (235 g ae ha^–1), respectively. Whole-plant dicamba dose–response studies revealed that the selected Aux-HR accessions had 2.9- to 15.1- and 3.1- to 9.4-fold resistance to dicamba relative to two susceptible accessions (MT-SUS and KS-SUS). In a separate fluroxypyr dose–response experiment, the selected Aux-HR accessions also exhibited 3.8- to 7.3- and 3.0- to 8.6-fold resistance to fluroxypyr on the basis of shoot fresh and dry weight responses, respectively. The confirmed Aux-HR kochia accessions also had 3 to 13 EPSPS gene copies relative to MT-SUS and KS-SUS accessions (each with 1 EPSPS gene copy). These results suggest that the putative Aux-HR kochia accessions from Kansas had developed moderate to high levels of cross-resistance to dicamba and fluroxypyr and low to high levels of resistance to glyphosate. This is the first confirmation of kochia accessions with cross-resistance to dicamba and fluroxypyr in Kansas. Growers should use diverse kochia control programs, including the proper use of dicamba and fluroxypyr stewardship, use of cover crops, occasional tillage, diversified crop rotations, and alternative effective herbicides to prevent further evolution and spread of Aux-HR kochia on their fields.

Nomenclature: Dicamba; fluroxypyr; glyphosate; kochia, Bassia scoparia (L.) A. J. Scott