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Diversify strategies of weed control to keep glyphosate working effectively

ST. PAUL -- If using multiple applications of glyphosate at increasingly higher rates is not improving weed control in your crops, you must stop following glyphosate failure with more glyphosate.

Early September is a good time to assess fields for the presence of common waterhemp and giant and common ragweed that have survived this year's glyphosate applications, and to develop a strategy that prevents this from happening next year.

Glyphosate has been used extensively on Minnesota corn and soybean acres since the late 1990s. Unfortunately, sole reliance on glyphosate has resulted in giant and common ragweed and common waterhemp plants that are resistant to glyphosate. Glyphosate-resistant weeds are often injured by the herbicide, yet many plants still go to seed, increasing their numbers in following years.

To maintain the usefulness of glyphosate in corn, soybean and sugar beet cropping systems in the future, it is important to reduce total reliance on glyphosate by diversifying weed management practices, putting more emphasis on spring and early-summer weed control and focusing glyphosate use in the crop where it 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:

- Diversification of your chemical weed control practices is critical to managing resistance. Select herbicide partners for glyphosate that will effectively control the weeds that have become difficult for glyphosate to control. This partnership could include a pre-emergence herbicide applied when the crop is planted, followed by glyphosate or an early-season postemergence herbicide tank mixed with glyphosate. The advantage of the pre-emergence herbicide is the residual weed control for multiple flushes of early-emerging weeds and reduction of the potential for crop yield loss due to weed competition from a delayed postemergence glyphosate application.

- Postemergence tank-mix partners are often preferred by farmers because they reduce the number of field operations, but timing of application is critical as they must be applied to 3- to 4-inch weeds for maximum effectiveness.

- Liberty Link corn and soybeans offer another postemergence herbicide strategy -- the use of Ignite herbicide, which controls weeds without killing the Liberty Link crops. Application to weeds 3 to 4 inches tall is critical and more consistent results are achieved following a pre-emergence herbicide.

- As other herbicides are introduced into your weed control plan, crop rotation restrictions associated with each herbicide could influence your crop rotation; this is especially true for sugar beets.

- Also, consider using glyphosate only in the crop where effective herbicide alternatives to glyphosate are lacking.

- Finally, consider rotation to early-season competitive crops, such as small grains.

For details, go online to to view the "PRE and POST Herbicide Diversification Options for Glyphosate-Resistant Corn" table created by University of Minnesota Extension and North Dakota State University Extension.

For more information about managing weeds in crops, visit the University of Minnesota Applied Weed Science website at

Jeffrey L. Gunsolus is an agronomist in weed science with University of Minnesota Extension.