Weeds: Focus on Giant Ragweed

Early emergence, rapid growth characterize this aggressive competitor 

Giant ragweed (GRW) is an aggressive weed that poses a significant threat to corn and soybean yields. Season-long competition from just one GRW plant per 110 square feet can cut soybean yields by 50 percent. Two GRW plants in the same amount of space will reduce corn yields by 13 percent. This annual weed: 

  • Emerges early— as early as March— and can continue emerging until late July 
  • Grows rapidly above crops. GRW plants typically grow one to five feet taller than crops they are competing with. Its large leaf area forms a dense canopy that shades young plants and leads to yield losses, even at low densities.  
  • Produces large seeds,allowing weeds to emerge from as deep as four inches in the soil, thus evading some pre-emergence herbicides in the upper soil layer. 
  • Is resistant to two classes of chemistry. There are widespread populations resistant to glyphosate or ALS inhibitors. Populations with resistance to both chemistries have been found in Ohio, Minnesota, Missouri and Indiana. The loss of these two classes of chemistries is significant because they have been the most effective herbicides for control of giant ragweed. 

Best management practices for GRW include the following: 

  • Start weed-free.Control weeds that emerge before planting with tillage or apreplant burndown herbicide application. 
  • Apply pre-emergence residual herbicideprior to or at planting to suppress GRW that emerges with the crop and improve post-emergence herbicide control. 
  • Apply post-emergence herbicides by the time GRW plants are four inches tall.  Try to use effective modes of action that are different than the modes in your residual herbicides. Apply a second post-emergence application, if needed, three to four weeks after the first post application.  
  • Prevent weed seeds.GRW seeds are short-lived in the soil, a trait that can be exploited for control.  University of Minnesota research found that maintaining zero GRW seed deposits for two years depleted 96% of the GRW seeds in the soil. 
  • Diversify crop rotation.Add alfalfa or another crop, such as a small grain, to your corn and soybean rotation.The same University of Minnesota research found that GRW emergence dropped by about half in wheat and alfalfa, compared to emergence in corn or soybeans.  


Photo Credit © iStock bungoume 2017

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