MANKATO, Minn. — For farmers, the enemies are many: waterhemp, Palmer amaranth, kochia, common ragweed, giant ragweed, barnyard grass.

They’re among the weeds in Minnesota that are growing resistant to weed killers, including the commonly used herbicide glyphosate.

David Nicolai, of the University of Minnesota, told farmers at the Ag Expo in Mankato, Minn., on Jan. 23 that waterhemp, which has long been common in southern Minnesota, is a headache statewide.

“I asked a guy in Pine County what the three top weed problems were in his county. He said No. 1, waterhemp, No. 2, waterhemp, No. 3, waterhemp.”

Andrew Lueck, owner of consulting firm Next Gen Ag, said a monster of a weed that has begun to spread to Minnesota from the deep south has the potential to be a bear to control.

“Palmer amaranth grows 2 to 3 inches a day and it can produce a half million seeds on a plant,” Lueck said. The weed, which grows up to 8 feet tall, has been found in several counties in Minnesota.

Lueck said even if the insidious amaranth plants are killed early in the season when farmers concentrate on weed prevention, any plants that come up even in August grow so fast they can produce seeds by fall that can produce many more of the plants the following years.

Dan Miller, of United Farmers Cooperative, said farmers are taking new approaches to try and keep up with the new threats from resistant weeds. “One of the big questions I get is what are we going to need for weeds this year.” He said many farmers last season had difficulty with spraying effectively because of the frequent and often heavy rains.

One of the challenges is that farmers overwhelmingly now plant genetically modified soybean seeds that produce a variety of different traits in the plant to withstand different weather and pest conditions in different parts of the country. The beans also tolerate herbicide sprays like the popular Roundup and dicamba, which contain glyphosate.

Those modified beans allowed farmers to use less pesticides for bugs and initially less herbicide for weeds. But research is showing that herbicide use is increasing because more and more weeds have found ways to survive glyphosate, leaving chemical companies and farmers racing to find new and different herbicides.

Nicolai said their is growing interest among some farmers to return to growing conventional soybeans, which are substantially cheaper seeds to buy and offer different weed-prevention options that could help reduce resistant weeds.

Conventional seed popularity could grow more as demand for them is increasing in the marketplace as many consumers are looking for foods and oils made with non-GMO soybeans, making the conventional food-grade soybeans more valuable. But the speakers said it can be difficult for farmers to find a supply of conventional soybean seeds and noted that if a lot of farmers switch to conventional beans, the premium price when sold by farmers could fall.

The speakers said other approaches to limiting resistant weeds includes crop rotation, cover crops and a return to cultivating between soybean rows during the summer, a practice that has faded with the advent of improved herbicides. They also promoted a “layer residual herbicide program,” where herbicides are applied before and after plants emerge from the ground, not just relying mostly on pre-emergent herbicides.

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