Giant Ragweed – A Giant Headache for Midwestern Growers

Giant ragweed is one of the most troublesome weeds to control in U.S. grain crops. This pernicious weed emerges throughout the growing season and carries tolerance to a wide range of herbicide chemistries. Giant ragweed has exhibited glyphosate resistance and, in some cases, ALS inhibitors.

“We’ve had trouble controlling giant ragweed in soybeans this year,” reports Minnesota crop consultant Bruce Nowlin. “The main problem with it is that last year we were so wet. We had a lot of drowned out areas in corn and soybeans. Then farmers went back in and planted soybeans and didn’t get good weed control.

“A lot of giant ragweed went to seed.  So now in some low spots you have giant ragweed that’s knee high or bigger and it’s really hard to kill. I just came from a meeting where they said if you can keep it from going to seed for about 2 years you can have about 90% control. Clearly, we’re not keeping it from going to seed. You’ve got to actually get out there and do some hand weeding. We’ll definitely be doing some of that this year,” said Nowlin.

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Giant Ragweed Facts  

  • Grows very quickly. Left alone, giant ragweed can grow to 17 feet. In the field, it’s often 1 to 5 feet taller than the crop. In one Illinois study, giant ragweed emerged with corn and soybeans planted on the same day. The weed topped out at 9 feet in corn and 6 feet in soybeans.*
  • A prolific seed producer. A single giant ragweed plant in a soybean field can produce up to 5,100 seeds, or up to 3,500 seeds per square yard when growing in competition with corn.*
  • A “quick study” when it comes to survival. Giant ragweed has changed its emergence patterns and adapted to row crop production.  Studies in the 1960s and 1970s showed most giant ragweed emerged by early May. Today, new research shows, this persistent weed can start emerging in early March and continue “coming out” until July, creating a huge management challenge for growers and their crop advisors.

Giant Ragweed Battle Plan Weed scientists from Corn Belt universities gathered their best information in the publication “Biology and Management of Giant Ragweed.” Here are key recommendations:

  • Use a combination of pre and post herbicide applications that incorporates at least 2 herbicide modes of action.
  • Control weeds that emerge prior to planting with tillage or a preplant burndown herbicide application.
  • Apply preemergence herbicides with activity on giant ragweed to reduce competition with crops, provide flexibility in the timing of post herbicides and minimize the need for a second post glyphosate application.
  • Spray post herbicides before plants are more than 6 to 10 inches tall.
  • Where pre herbicides are not used, control giant ragweed with post herbicides when weeds are less than 6 inches tall.

*Information from Purdue University’s The Glyphosate, Weeds, and Crop Series – Biology and Management of Giant Ragweed