In Wisconsin, nearly 70% of farmers perceive that weeds have become more difficult to control with glyphosate over time, including both common lambsquarters and giant ragweed. Many have reported variable or inconsistent response of common lambsquarters to glyphosate. One of our goals has been to investigate the variable response of common lambsquarters to glyphosate, including potential resistance to glyphosate. We have characterized the response of more than 40 common lambsquarters populations to glyphosate from across southern Wisconsin. We have not found any of these populations to be resistant to glyphosate. However, we have observed variable responses among these populations to glyphosate. Our results suggest that variability of common lambsquarters to glyphosate is most apparent following treatment with low rates of glyphosate (e.g., 0.375 lb ae/acre). Such variability is much less or not apparent following treatment with higher rates of glyphosate (e.g., 1.5 lb ae/acre), at which shoot biomass is greatly reduced and injury is severe relative to non-treated check plants. We’ve also found that the relationship between a field history of exposure to glyphosate and less sensitivity to glyphosate was inconsistent. That is, in some instances less sensitivity (to low rates of glyphosate) was associated with a field history of previous glyphosate use, but in other instances, such a relationship was not apparent. We think it’s likely that our results reflect natural or inherent variability among common lambsquarters populations to glyphosate.
Soil Science Extension
University of Wisconsin Madison