Weather Forecast


Time weed control in corn, soybean for maximum yields

Recent rain will result in increased weed seedling growth and weed seed emergence for many Minnesota corn and soybean fields. Understanding the principles of herbicides will help producers control weeds that are competing with these and other row crops.

Many growers ask, "Will preemergence herbicides still have activity after remaining on the soil surface for several weeks?" The answer is generally yes--some activity is expected from herbicide once rain finally moves it down into the soil. Herbicide can be lost from the surface, but dry soil conditions will limit the rate of degradation, which causes this loss. The important issue is the weeds that emerge between application and rainfall, which will generally require some type of postemergence herbicide treatment even if the preemergence herbicides are finally "activated."

Research done by Jeff Gunsolus, University of Minnesota Extension weed scientist, indicates that weeds emerging with corn must be controlled within two to five weeks after weed emergence to prevent a yield loss. Soybeans can tolerate four to six weeks of weed/crop competition. This period of crop tolerance to weed competition is decreased under high weed densities or environmental stresses, such as low moisture or nitrogen levels. For example, common lambsquarters control by glyphosate is often poor due to lack of timely application, reduced rates of glyphosate, extended hot/dry weather patterns and type and rate of an adjuvant system.

University of Minnesota research over a three-year period of 2004 to 2006 at five locations in the state found that, in corn, a soil preemergence herbicide followed by a glyphosate application to 5-inch weeds gave the best economic return. Glyphosate applications alone from the 3- to 5- inch weed height resulted in the highest yield. In soybeans, glyphosate applications for both the pre/post and postemergence timings gave the highest yields at the 5-inch weed height. Application of only glyphosate applied too early (less than 5-inch weeds) can reduce crop yield due to later emerging weeds.

University of Minnesota weed control recommendations may be found at More educational information for corn producers can be found on Extension's corn website at Soybean growers may visit the collaborative "Just for Growers: Minnesota Soybean Production" website at