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Posted 28 May 2015. PMN Crop News.

Diversified Management Crucial to Giant Ragweed Management

Weed scientist recommends burndown, two post-emergence applications

Source: United Soybean Board Press Release.

Chesterfield, Missouri (May 19, 2015)--Giant ragweed is making a comeback as one of the most competitive weeds farmers deal with every growing season. If not controlled, its ability to grow faster and taller than the planted crop threatens yields. Once easily managed by herbicides, giant ragweed is beginning to show resistance to several popular herbicides, emphasizing the need for farmers to adapt their management techniques.


According to a Focus on Soybean webcast presented by Bill Johnson, Ph.D., a professor of weed science at Purdue University, farmers should include more variety in their weed-management plans.

“It’s re-emerged as one of our most problematic weeds because of its resistance to ALS herbicides and glyphosate,” he says.

According to Johnson, effective management plans include:

• Thorough tillage or a burndown application of a residual herbicide. Burndown treatment plans could include glyphosate in combination with 2,4-D, dicamba or Sharpen®.

• Two applications of post-emergence herbicides:

- For ALS-resistant giant ragweed (Group 2), use glyphosate.

- For glyphosate-resistant giant ragweed (Group 9), use FirstRate® or Classic®.

- For populations resistant to both ALS herbicides and glyphosate, use Flexstar®, Cobra®, Phoenix™ or Liberty®.

• Spraying plants when less than 6 inches tall. Younger plants are more susceptible to herbicides. The key is to eliminate the threat before the giant ragweed plants reach maturity. “Lower seed production means fewer problems for future years,” said Johnson.

• Scouting fields for plants that emerge late or escape herbicide applications.

Watch the full webcast for more information on giant ragweed management in soybeans. A printable version of the Management of Herbicide-Resistant Giant Ragweed fact sheet referenced in the webinar, as well as other free resources for herbicide-resistance management, can be found on the Take Action website.

Executive summary versions of all Focus on Soybean webcasts are available for free to all U.S. soybean farmers through a partnership between the Plant Management Network and the soy checkoff.