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Northern Illinois Agriculture

University of Illinois Extension
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Giant ragweed control update

Posted by Russel Higgins - Weeds

In 2014 a northern Illinois agronomist reported that labeled glyphosate applications had failed to successfully control Giant ragweed (GRW) in a LaSalle County field. With the agronomists and growers cooperation an area of the field was left untreated and seed collected from the GRW at maturity. The seed sample was delivered to the University of Illinois Department of Crop Sciences and allowed to go through a cold period before it was germinated in a replicated greenhouse study. Lisa Gonzini, Senior Research Specialist, reported that the germination was variable within the population, but she categorized the plants to be treated by size. The GRW was treated at recommended plant heights with rates of 1.5 and 3 pounds acid equivalent of glyphosate per acre. Dr. Aaron Hager, Weeds Specialist at the University of Illinois evaluated the population post application. He shared that the response to the herbicide application to this population was mixed. Most of the plants were killed by the lower rate of glyphosate, some not killed but so severely injured they likely would not interfere with yield if in a crop setting. However, several of the plants survived the high (3 lbs. ae/acre) glyphosate rate application. While not ready to announce glyphosate resistant Giant ragweed from this individual trial, it was evident that at least several plants were clearly not controlled by glyphosate. The take home message for Illinois farmers, you may be at risk if you are dependent upon glyphosate to control this weed, especially in areas that have exhibited GRW increased tolerance or decreased sensitivity to glyphosate. For existing GRW now in our crops, timely post herbicide applications utilizing multiple modes of action are encouraged.

Learn more about the Biology and Management of Giant ragweed

https://www.extension.purdue.edu/extmedia/bp/gwc-12.pdf


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