ST. PAUL -- I saw many corn and soybean fields with prominent patches of giant or common ragweed, common waterhemp or volunteer corn during my travels in Minnesota, Wisconsin and Iowa this fall. These weed patches were likely the product of glyphosate herbicide overuse.
This fall is the time to develop a plan and take control of herbicide-resistant weeds before they take control of you. Due to the long-term exposure to glyphosate in the corn and soybean cropping system, we are now in a situation where the probability of finding a glyphosate-resistant giant or common ragweed or waterhemp is high.
Currently, glyphosate-resistant weeds are a serious problem in the southern United States, resulting in significant increases in herbicide and labor costs, and increased field applications. The situation isn't as dire in the Upper Midwest now, but the trend with herbicide-resistant weed development in a field is a slow start that can grow exponentially by the next season.
To address glyphosate-resistant weeds, you must reduce your total reliance on glyphosate and diversify your weed-management practices, with more emphasis on spring and early summer weed control and a more focused use of glyphosate in the crop where its weed control is of greatest value to you.
I recommend the following strategies based on experience and extensive research conducted by University of Minnesota Extension and other land-grant institutions:
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In all fields:
- Select herbicide sequential and tank-mix partners for glyphosate that will effectively control the weeds that have become difficult for glyphosate to control.
- Start with a pre-emergence residual herbicide to control early-emerging weeds and reduce the potential for crop yield loss due to weed competition from a delayed postemergence glyphosate application.
- Glyphosate and postemergence tank-mix partners should be applied to 3- to 4-inch weeds for maximum effectiveness.
In fields currently infested with glyphosate-resistant weeds:
- Liberty Link corn and soybeans offer another postemergence herbicide strategy -- the use of Ignite herbicide. Application to weeds 3 to 4 inches tall is critical, and best results are achieved when following a pre-emergence herbicide.
- Limit glyphosate use in crops where effective herbicide alternatives to glyphosate exist. Target glyphosate use in crops where its weed control is of greatest value to you.
- Use inter-row cultivation.
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- Rotate to early-season competitive crops, such as small grains.
Take control of the situation now, so your weed seed bank doesn't remind you of your mistakes for many cropping seasons.
Jeff Gunsolus is a weed scientist with University of Minnesota Extension.
