Briefly, new cases of glyphosate resistance continue to be reported. Glyphosate-resistant Johnsongrass has been confirmed in Arkansas and Mississippi, which brings the total number of glyphosate-resistant weeds in the U.S. to 9 species. Johnsongrass is notable as it is another example that resistance can develop in grass weeds and it makes a person wonder when resistance might develop in the related sorghum weed, shattercane.

 

To our west, Minnesota has confirmed glyphosate-resistant waterhemp and glyphosate-resistant giant ragweed. New reports of glyphosate-resistant giant ragweed have also been made for Arkansas and Tennessee.  Both of these weeds should be of special concern for soybean growers because of the potential for multiple resistance, which means a simple tank mix may not always fix the problem. Most waterhemp is likely to be resistant to ALS-inhibiting herbicides already. If glyphosate-resistance develops, non-ALS preemergence herbicides or PPO-inhibiting postemergence herbicides like Cobra, Flexstar, or Ultra Blazer are likely solutions. One caution is the potential for PPO-resistant waterhemp, which also exists to our south. Triple-stacked resistant waterhemp also exists (resistant to glyphosate, ALS- and PPO-inhibiting herbicides).

 

Giant ragweed also has the potential for ALS resistance. ALS resistance is important because FirstRate is probably the most effective non-glyphosate herbicide for giant ragweed control and it is an ALS-inhibiting herbicide.  Fortunately, I believe the amount of ALS-resistant giant ragweed is limited in Wisconsin. However, in fields where ALS-resistance also exists, the most likely option to control glyphosate and ALS-resistant giant ragweed are the PPO-inhibiting herbicides like Cobra and Flexstar. These herbicides must be applied to small giant ragweed for optimum control.

 

This map illustrates the reported distribution of glyphosate-resistant weed species at the state level. The actual number of acres infested will vary greatly among weed species and among states.