When I first started working in the field crop Integrated Pest Management Program at MSU in the 1980s, an important component of farm scouting was year-end weed mapping. In the past, we used to hand-draw the boundaries of a field and record weed populations that were not controlled with the farmer’s current herbicide program. With the technology available today, farmers can map weeds with much better precision than what we did in the past.

The development of glyphosate-resistant crops has renewed the need for weed mapping. As I drive across Michigan and observe the landscape, most of the fields look almost weed-free. The adoption of glyphosate-resistant crops has enhanced the farmer’s ability to control weeds. However, continuous use of glyphosate-resistant crops can ultimately result in the build-up of glyphosate-resistant weeds.

Fortunately in Michigan, glyphosate-resistant weeds have been slow in developing. Currently, marestail, also called horseweed, is the only common Michigan weed that has been found to be resistant to glyphosate (Gratiot and Ionia counties). This low incidence of resistance in Michigan could be due to the diversity of crops grown here. Many Michigan farmers continue to have a three- or four-year rotation where glyphosate-resistant crops are not grown every year, thus decreasing the amount of glyphosate applied to these fields. Including herbicides with different modes of action in the crop rotation can eliminate weeds that survived glyphosate applications. However, we must remain on the lookout as the registrations of glyphosate-resistant sugarbeets and alfalfa could increase the risk and speed in which glyphosate-resistant weed populations appear and spread throughout Michigan.

Though weed mapping has declined over the past 10 years, it could be an important tool in combating herbicide resistance in the future. Mapping areas where weeds have escaped management can be the key to identifying resistance and slowing down the spread. If resistance is suspected, contact your local MSU Extension educator. It will be important to keep seeds from being transported to other fields on equipment. Ways to combat glyphosate-resistant weeds include using alternative herbicide chemistries other than or in addition to glyphosate and increasing the number of crops in your rotation, including putting wheat back into the system, which currently doesn’t have glyphosate-resistant varieties available for production.

As harvest begins for corn and soybeans this year, pay attention to the weeds in your field. Map them, and if you suspect the weeds are resistance to glyphosate or other herbicides, contact your local MSU Extension educator. MSU Diagnostic Service on MSU’s campus, 107 Center for Integrated Plant Systems (517-355-4536), is another good source for the detection of herbicide resistance in weeds.