University Researchers Confirm Two Cases of Glyphosate-Resistant Weeds in Minnesota

University of Minnesota and Syngenta researchers have confirmed two cases of glyphosate-resistant weeds in the state. Giant ragweed and common waterhemp were both determined resistant in separate studies. These are the first two instances of confirmed glyphosate resistance in the state.

Both waterhemp and giant ragweed can significantly reduce yields. Giant ragweed can reduce yields by at least 30 percent and, according to university experts, a dense population (3 to 4 plants per square yard) can affect yields by as much as 70 percent.

“We’re seeing many different cases of weeds exhibiting signs of resistance throughout the U.S. due to the widespread use of glyphosate and glyphosate-tolerant cropping systems,” said Chuck Foresman, manager of weed resistance strategies for Syngenta. “States throughout the Midwest continue to report problems with weeds that are difficult to control with glyphosate. These Minnesota cases should serve as an important indicator for growers in the upper Midwest to add diversity to their weed control programs.”

According to Dr. Jeffrey L. Gunsolus, professor of agronomy and weed science at the University of Minnesota, at least two of the three resistant weed cases were in continuous glyphosate programs for more than five years.

Giant Ragweed

According to Brett Miller, research and development scientist for Syngenta, the first case of giant ragweed (Ambrosia trifida) survived 8-times the rate of glyphosate during field testing in a patch of about 40 acres in McLeod County, Minn. Field management records indicate glyphosate was used at least once each year since 1998, with the first four years in a continuous soybean program.

Just four miles away, another field once farmed by the same grower has populations found to be tolerant to glyphosate, surviving three applications in one season. “Extension agronomists estimate that 320 total acres are affected in McLeod County,” Gunsolus said.

A second site for giant ragweed was investigated in 2005 and seed was collected for lab tests, Miller reported. Two separate Syngenta greenhouse experiments indicate a 4x rate of resistance. This approximately 25-acre field was in glyphosate systems continuously since at least 2002.

Common Waterhemp

In Renville County, Minn., the confirmed glyphosate-resistant waterhemp affects approximately 50 acres, Gunsolus said. Field management records did not indicate continuous glyphosate use, but researchers found many different biotypes present in the field, possibly transferred by field equipment.

University records indicate several other reports of tough-to-control waterhemp throughout the state. In fact, Gunsolus said a Cottonwood County agronomist estimates that 70 to 80 percent of area glyphosate applications were greater than 22 oz/A in 2007.

Syngenta encourages all growers to practice integrated weed management to preserve yield and delay onset of glyphosate resistance. “We recommend using a pre-emergence residual herbicide in all glyphosate-tolerant crops,” Foresman said. In soybeans, that includes an application of Prefix™ herbicide. In corn, Syngenta recommends Lumax® herbicide which offers residual control. A simple guideline for corn and soybean growers is to follow 2-1-2: or no more than two applications of glyphosate in one field over a two-year period.

Other proactive measures include using effective herbicides with multiple modes of action, applying herbicides when weeds are small and controlling weed escapes manually.

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