Glyphosate Resistance News

Chris Boerboom, Ext. Weed Scientist

Extension weed scientists at Ohio State University and Purdue University have recently reported confirmed cases of glyphosate-resistant giant ragweed in Ohio and Indiana.  This will certainly be important to the growers who have this problem, especially in years when they are growing soybeans in these fields.  This should be a warning notice to Wisconsin soybean growers as the complaints about giant ragweed continue to increase in Wisconsin.  At this time, I don’t believe we have any glyphosate resistance problems with giant ragweed.  Our problems are just because it is a difficult weed to control with a single pass and it seems to be spreading into more fields.  

The concern that I have for glyphosate-resistant giant ragweed is greater in soybean than corn because there are better alternative postemergence herbicides options for effective giant ragweed control in corn than soybean.  The other concern is that we know ALS-resistant giant ragweed in exist in Wisconsin.  ALS resistance ragweed makes a highly effective herbicide like FirstRate a useless option in soybean.  The concern is that giant ragweed would happen to become resistant to both ALS and glyphosate herbicides.  This would limit postemergence options to herbicides like Flexstar or Cobra.  This would be similar to the multiple resistant waterhemp in Missouri (ALS, PPO, and glyphosate resistant) that I discussed during the Pest Management Update meetings.  Some of these problem weeds with multiple resistance will be major challenges.

Fortunately, Wisconsin ag-professionals and growers are concerned about glyphosate resistance.  At the Pest Management Update meetings, 81% of attendees expressed moderate or high levels of concerns (question 1).  A high percentage of Wisconsin agriculturalists also recognize the importance of herbicide rotations to reduce the selection pressure from glyphosate (question 2).  If these concerns and knowledge result in diverse weed management programs, I believe this bodes well for Wisconsin to slow the evolution of glyphosate resistance.

(Both questions based on 321 responses).

If any agents, consultants, or agronomists are interested providing education on glyphosate resistance or glyphosate performance issues, I have free copies of two new bulletins that will be useful.  Here are the titles:  

* Facts about Glyphosate Resistant Weeds (GWC-1)

* Understanding Glyphosate to Increase Performance (GWC-2)

The GWC bulletin code stands for “The Glyphosate, Weeds, and Crops Series” and you can view, print, or buy bulletins in  this series at http://glyphosateweedscrops.org.  The two bulletins above will be posted to the site shortly as they have just been printed.  If you want multiple free copies for a grower meeting, please contact me.  (These bulletins are not available through UWEX publications).