Over the past two years we have been conducting a survey of soybean fields at harvest to determine the extent and distribution of herbicide-resistant weeds throughout Missouri. The majority of weed samples we have collected over the past two years have been waterhemp, but in 2009 we observed an increase in the number of fields with giant ragweed that survived treatment and were present at harvest. Recently, we have completed our initial greenhouse screen of these plants and believe we have identified several more giant ragweed populations that are resistant to glyphosate. Some of these populations so showed signs of multiple resistance to both glyphosate and ALS-inhibiting herbicides like Firstrate (Figure 1).
Clearly, our most significant weed problem in Missouri right now is glyphosate-resistant waterhemp. However, this doesn't mean we should overlook giant ragweed and the increase in the number of problems we have seen with giant ragweed over the past two seasons. One of the characteristics of giant ragweed is that it is usually one of the first summer annual weeds to emerge in Missouri so this can be beneficial in that we have the "opportunity" to deal with this weed prior to corn or soybean planting.
For control of glyphosate-resistant giant ragweed populations prior to corn or soybean planting, consider tank-mixes of 2,4-D or Sharpen with your glyphosate burndown. Gramoxone plus 2,4-D and Ignite plus 2,4-D are also options for the control of giant ragweed in a burndown situation. If the population is not ALS-resistant, the addition of products that contain chlorimuron, like Canopy EX, Synchrony, Valor XLT, or Envive, or products that contain cloransulam, like Authority First, Sonic, Firstrate or Gangster, can provide good control of giant ragweed in soybeans. Assuming the population is not ALS-resistant, these same herbicides should also provide some residual control of giant ragweed plants that emerge later in the season.
In soybeans, if giant ragweed is present in-crop, the key to effective control is application timing. Giant ragweed that has reached 1 foot or more in height is not only more difficult to control but at this height has more than likely already caused a significant yield loss. If the population is glyphosate-resistant, one of the most effective herbicide options is Firstrate. If the population is both glyphosate- and ALS-resistant, then the remaining herbicide options are far less effective and application timing becomes critically important. Giant ragweed populations with multiple resistance should be treated with tank-mixes of glyphosate and Cobra, Phoenix, or Flexstar (or other products with fomesafen).
For more information, you can visit this website: http://www.extension.purdue.edu/extmedia/BP/GWC-12.pdf and download a free publication authored by several university weed scientists from around the Midwest that details the biology and management of giant ragweed in corn and soybean production systems.
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REVISED: October 2, 2015