MONSANTO and the University of Missouri are investigating a potential case of glyphosate-resistant waterhemp. The waterhemp was identified in two soybean fields in northwest Missouri.

“Waterhemp grown from seed collected from suspect fields in 2004 shows high tolerance to glyphosate in two greenhouse dose-response trials this summer,” reports Kevin Bradley, MU Extension weed specialist. “The weeds were found in fields planted to Roundup Ready soybeans continuously since the new varieties were introduced in 1996.” Field studies of the waterhemp are planned for 2006.

USDA reports that 89% of Missouri’s 2005 soybean acreage was planted to genetically modified soybean varieties that tolerate glyphosate.

Herbicide resistance develops in weeds when a plant in a weed population has natural resistance to a herbicide, Bradley explains. The plant is not killed and produces seeds. Over time, resistant plants dominate the field.

“Monsanto takes product stewardship very seriously, and we are working with the University of Missouri to follow up with these growers and provide effective recommendations,” says Jennifer Ralson, Monsanto’s Roundup technical manager. “In Roundup Ready soybeans, we recommend that growers who have dense stands of weeds, such as waterhemp, use a preemergence residual product such as Intrro or Valor. Additionally, in this situation the growers should consider the agronomic benefits of rotating to Roundup Ready corn.”