During a symposium on glyphosate-resistant weeds held during the annual early winter meeting of the North Central Weed Science Society in St. Louis, Mo., university researchers confirmed the development of glyphosate-resistant waterhemp.

No-tillers and researchers alike consider waterhemp to be one of the top problem weeds growing throughout the Midwest. It developed overwhelming levels of resistance to ALS-inhibiting herbicides such as Pursuit in the late 1980s, and it is now showing signs of developing resistance to other herbicide modes of action, as well. Its control is critical as no-tillers rely heavily on glyphosate for control of waterhemp in Roundup Ready soybeans, which accounted for 75 percent of the 2002 soybean acreage in the United States.

Tough To Identify.

Genetically, waterhemp has an inherent variable response to glyphosate, according to Mike Owen, an Iowa State University weed management specialist. “We have identified individual plants within populations with sufficient glyphosate resistance to raise concern,” he says.

His research has observed waterhemp plants that survive 3 1/2 times the labeled rate of glyphosate. Other studies in Missouri have found waterhemp plants that survive up to eight times the labeled rate of glyphosate while still producing viable seed.

“In our research, the fourth generation selected against a lethal rate of glyphosate still resulted in survivors. We have resistance to glyphosate,” says Reid Smeda, weed physiologist at the University of Missouri and a scheduled speaker at the 11th annual National No-Tillage Conference held in early January in Indianapolis, Ind.

“It is low-level resistance and the…