Scientists at Purdue University have just answered an important question that has plagued many corn and soybean growers. Why does mixing glyphosate with other selective herbicides sometimes fail to control glyphosate-resistant giant ragweed plants?

New research featured in the journal Weed Technology provides an answer.  And it has to do with how some giant ragweed plants react to glyphosate. Certain biotypes of glyphosate-resistant giant ragweed are known for their rapid response to the herbicide. Within hours of an application, mature foliage will die. But immature leaves remain uninjured and keep growing.

Researchers found the same rapid response impacts how the weed reacts to other selective herbicides that might be mixed with glyphosate for broader-spectrum control.  The more glyphosate contained in the mixture, the more diminished the control provided by other herbicides.

What’s the answer? Simply apply selective herbicides separately – a least two days prior to any glyphosate treatment. Want to know more? Read the full text of the article, “Glyphosate-Induced Antagonism in Rapid Response Giant Ragweed (Ambrosia trifida)” in Weed Technology Vol. 32, Issue 1.