Evolution of Multiple Herbicide-Resistant Kochia: A Threat to Montana Wheat-Fallow Cropping System.

Herbicide-resistant (HR) kochia is an increasing concern for growers in the Northern Great Plains (NGP), including Montana. Winter wheat after summer chemical-fallow dominates > 90% of the dryland cropping systems of this region, where soil moisture (< 30 cm of average annual precipitation) is often the limiting factor for continuous cropping. Glyphosate is the most predominant herbicide chemistry used for weed control in chemical-fallow, pre-plant burndown, and post-harvest wheat, and sulfonylurea (SU; acetolactate synthase (ALS)-inhibitor herbicide) and dicamba/fluroxypyr are the major chemistries for weed control in wheat. In summer of 2012, kochia control failures even with repeated applications of glyphosate (at the recommended field-use rate) were reported in chemical-fallow fields (wheat-fallow rotation) in Hill and Liberty Counties, Montana.  

To fulfill objective 1 of this research, kochia seeds (accessions) were collected from the survivors (suspected resistant plants) in those fields.  Based on whole-plant dose-response assays, those accessions (GIL01, JOP01, CHES01, and CHES02) were 4.5- to 11-folds more resistant relative to a known glyphosate-susceptible kochia accession from Montana.  The tested glyphosate-resistant biotypes were also found to be resistant to the ALS-inhibitor herbicides (SU), but susceptible to dicamba and fluroxypyr.

Our objective 2 was to investigate the molecular mechanism(s) conferring resistance to glyphosate and ALS-inhibitor herbicides in those multiple HR kochia accessions from Montana. No target-site mutations were detected at Pro₁₀₆ of EPSPS gene in glyphosate-resistant plants. All glyphosate-resistant accessions had increased EPSPS gene copies (~ 4 to 10) compared with the susceptible accession (single EPSPS gene copy). Furthermore, glyphosate-resistant kochia plants accumulated higher EPSPS protein in western blot. Elevated levels of EPSPS protein in glyphosate-resistant plants correlated with increased EPSPS gene copy numbers. Resistance to the ALS-inhibitor herbicides was conferred by Pro₁₉₇amino acid substitution (proline to glutamine).  

To further determine the distribution and frequency of HR [glyphosate-, dicamba-, and ALS-inhibitor herbicides] kochia in northern Montana (objective 3), we conducted a random field survey in fall of 2013, when kochia plants produced fully matured seeds. Almost 150 kochia populations were collected from wheat-fallow fields. Based on the greenhouse screening of 128 kochia populations so far, 4 populations  (Toole County) were resistant to all three herbicide modes of action (glyphosate, ALS-inhibitor, and dicamba)), 17 populations (Hill, Liberty, and Toole Counties) were multiple resistant to glyphosate and ALS-inhibitor, and 16 populations (Liberty, Toole, Glacier, and Chouteau Counties) showed multiple resistance to dicamba and ALS-inhibitor herbicides. ALS-inhibitor-resistance (ALS-R) was found in >90% of the surveyed kochia populations. Results from field survey suggest that kochia populations with multiple herbicide resistance to glyphosate, ALS-inhibitor, and/or dicamba have evolved in northern Montana wheat-fallow fields. Evolution of moderate to high frequencies of resistance warrants growers to adopt reactive herbicide resistance management strategies. Furthermore, resistance at low frequencies suggests the need for proactive resistance education to growers in MT and adjacent states in the NGP region.