Recent greenhouse testing confirms an increase in the number of North Dakota and Minnesota fields with glyphosate-resistant common and giant ragweed. In Minnesota, glyphosate resistant common ragweed was recently confirmed in Pennington County. Since the 2008 and 2009 growing seasons, glyphosate-resistant common ragweed has been confirmed in a total of 4 fields in Clay County (Hawley to Hitterdal area), MN, 3 fields in Cass County (Casselton to Kindred area), ND, 2 fields in Red Lake County (Plummer area), MN and Traill County (Buxton area), ND, and one field in Pennington, Stearns, and Todd Counties, MN.
In Minnesota, glyphosate resistant giant ragweed was recently confirmed in Renville and Sibley Counties. Since 2006, glyphosate-resistant giant ragweed has been confirmed in 15 fields in the following counties: Meeker, McLeod, Renville, Sibley, and Yellow Medicine. The greatest concentration (11 fields) of glyphosate-resistant giant ragweed can be found in McLeod County and eastern Renville County. It is likely that 60 to 95% of all fields in this area have some frequency of glyphosate-resistant giant ragweed.
Based upon recent greenhouse testing by Michael Moechnig at South Dakota State University, glyphosate-resistant kochia is likely present in south central North Dakota.
What does all this mean? Glyphosate-resistant weeds continue to increase in North Dakota and Minnesota due to the exclusive use of glyphosate. Utilizing as many weed control methods as soon and as often as possible is the best way to reduce the frequency of all herbicide-resistant weeds.
It has been about 14 days since glyphosate was first applied to Roundup Ready sugarbeet or in preplant applications. The best time period to scout fields following a glyphosate application is 14 to 21 days, with 18 to 21 days ideal. Scouting earlier than 14 days after application may not allow enough time for glyphosate to completely kill plants. Scouting beyond 21 days after the glyphosate application may allow plants emerging shortly after the application to be confused with surviving plants.
When scouting for glyphosate resistance look for dead plants in the field. Most likely all weed species will be controlled, but a few plants of a single or maybe two species may survive the glyphosate application. If some plants are normal in appearance with dead plants of the same species in close proximity and all in-between responses are observed, then resistance is likely present. Identification of this range in plant responses is illustrated in a video entitled “Scouting for Glyphosate Resistance”. It can be found by visiting the following website:
At this website, click the button “Scouting for Glyphosate Resistance”. This will direct you to another website in which you can view and/or download two versions of the video. The long version has some introductory remarks about glyphosate resistance and shows the range in response of individual plants. The short version only shows the response of individual plants. Additional information about glyphosate resistance can be found at the website above.
How important is this video? Last year we visited two fields near Plummer, MN and two fields near Hawley, MN and predicted that the common ragweed in these fields were resistant to glyphosate because the range of responses shown in the video were present in these fields. Recent greenhouse testing confirmed the presence of glyphosate-resistant common ragweed at three of the four sites. One (near Hawley, MN) of the three confirmed sites is where the video was recorded. Visually evaluating a field for the presence of glyphosate-resistant plants can be successful.
Scouting for glyphosate resistance is important for several reasons. The sooner a field is identified as being resistant, the sooner more drastic management strategies can be adopted to reduce the number of resistant plants. Identifying glyphosate resistance in a field when just a few plants are resistant is better than waiting for large patches to develop over time. Waiting until there is a definite problem will cause greater loss of profit and make weed control more difficult in the future compared to identifying those few resistant plants and slightly adjusting management strategies to eliminate those plants. The sooner in the season surviving plants are identified in a field, the more likely herbicides with alternative modes/sites of action can effectively control those surviving plants.
Jeff Stachler, U of MN and NDSU
Mike Christoffers, NDSU
Carlyle Holen, U of MN Extension