Giant ragweed has increased as a problem weed in both corn and soybean in Wisconsin over the past decade. It is certainly one of our most competitive weeds and has also proven to be quite adaptive. Originally, giant ragweed was categorized as an early emerging weed that was controlled reasonably well with burndown treatments in no-till or with seedbed tillage. However, giant ragweed adapted to that management system by emerging later into the growing season. As a consequence of late emergence and its rapid growth, giant ragweed is a weed that frequently escapes control and may push through the crop canopy by mid-summer. Giant ragweed tends to be a “2-pass” weed because preemergence herbicides in corn or soybean often do not provide complete control and a postemergence application is needed for complete control.

 

None of this information is particularly new. However, the evolution of glyphosate-resistant giant ragweed is new and worth note. Dr. Mark Loux provided an excellent overview of the development of glyphosate-resistant giant ragweed in Ohio and Indiana at last January’s Fertilizer, Aglime and Pest Management Conference. Visit http://www.soils.wisc.edu/extension/wfapmc/ and search the “Online database” by author using “Loux” to find his article and presentation. Ohio and Indiana may seem to be a comfortable distance away from Wisconsin to worry about glyphosate resistance. However, isolated reports of giant ragweed surviving glyphosate applications are being made in Minnesota, which is much closer to home. At one Minnesota site, the response appears similar to the low level resistance described by colleagues in Indiana and Ohio. At lower levels of resistance, Ohio and Indiana colleagues have plants where the main growing point is stunted and axillary branches grow. One population with a higher level of resistance has burning of treated leaves, but the main stem continues to grow after a delay. 

 

Although we have concerns about resistance, poor management such as applications of low glyphosate rates or applications of glyphosate to large giant ragweed may still be the more likely reason for poor giant ragweed control. Therefore, it is important to follow good giant ragweed management practices in Wisconsin. A new giant ragweed bulletin has been printed and has excellent management recommendations. I will be distributing this bulletin at field days this summer, but you can also view this bulletin online at http://www.glyphosateweedscrops.org/ Just click “Publications” to get to the bulletin. Management recommendations near the end of the bulletin are divided between situations where previous control has been acceptable and where poor glyphosate performance has occurred in previous years (i.e. potential for resistance). Key management recommendations in soybeans with prior acceptable control are to use a minimum of 1.1 lb ae/a glyphosate on giant ragweed less than 6 inches tall and 1.5 lb ae/a glyphosate on larger ragweed (non-glyphosate options are also given). Where poor performance has occurred, increased management is recommended and is outlined below.

            1. Apply a preemergence herbicide with residual activity on giant ragweed.

            2. Apply 1.5 lb ae/a glyphosate or a full rate of Flexstar, Phoenix, or Cobra before ragweed exceeds 10 inches.

            3. Apply 0.75 lb ae/a glyphosate 3 weeks after the first postemergence application.

Please view the bulletin for greater detail on these management recommendations.