As of March of this year glyphosate-resistant Russian thistle has been identified in Chouteau County with more counties likely to be added, according to a new publication from MSU Extension on Russian thistle. Glyphosate-resistant Russian thistle has resulted from the repeated use of a single mode of action herbicide, i.e. glyphosate, which is in Group 9, for weed control for decades. Group 2-resistant Russian thistle was first identified in Montana wheat fields in 1987. Since then resistance to this group of herbicides has continued to increase.
To combat glyphosate-resistant Russian thistle, it is critical that a multi-faceted approach be utilized. Prevent seed production, destroying any plants that survive herbicide application prior to flowering using alternative herbicides, handpulling, or tillage. Consider diversifying your crop rotation and integrating pulse crops into wheat rotations to add diversity to weed control options. Also, consider increasing crop competitiveness by decreasing row spacing, increasing seeding rate, and adjusting planting dates. You could use green manure and incorporate legume green manure into the rotation instead of fallow to reduce the number of burndown applications needed. Manage field borders as allowing resistance-prone weeds to produce seed in field borders may make resistance problems worse. If you know of weed problems, clean planting, tilling, and harvesting equipment to reduce the spread of resistant weed seeds into uninfested fields. Lastly, limit bare fields to reduce the spread of Russian thistle if it has gone to seed.
When applying herbicides, do so at the labelled rates as repeated exposure to low doses of herbicide allows weeds to develop resistance. Scout fields regularly as scouting prior to and after spraying can help you find and destroy any surviving weeds immediately. As you diversify herbicides and rotate herbicide modes of action or groups and use tank mixes with multiple modes of action you can hopefully avoid developing resistance. Last, use recommended rates of adjuvants, water conditioners/pH buffers to enhance herbicide efficacy.When using herbicides use best herbicide application techniques by reading and following the pesticide product label as improperly calibrated sprayers, incorrect nozzles, low (< 10 GPA) spray volumes, poor water quality, dusty and hot/dry weather conditions decrease herbicide effectiveness and can perpetuate resistance. Target Russian thistle in the early spring for effective control. Plants should be sprayed as early as 3-4 inches of growth. Use fall-applied soil-residual for extended residual activity into the following season and reducing selection pressure from glyphosate burndown. When applied in the fall, these herbicide programs also minimize the risk of injury to dry pea under unfavorable conditions in the spring.