University of Missouri researchers have confirmed that tall waterhemp is the sixth glyphosate-resistant weed in the U.S. and the ninth such weed in the world.

Kevin Bradley, a University of Missouri Extension weed scientist, and graduate student Travis Legleiter found that tall waterhemp from a field near the Missouri River in Platte County could survive despite being treated with up to eight times the labeled rate of glyphosate.

Glyphosate is the active ingredient in Roundup herbicide. The fields where resistant waterhemp was found had been in continuous Roundup Ready soybean production since 1996.

Kevin Bradley, University of Missouri weed scientist, explains new waterhemp research trials at the MU Weed Resistance Management Field Day last month near Columbia.

While all resistant weeds are worrisome, Bradley says resistant tall waterhemp is especially troubling. "Waterhemp is one of Missouri's toughest weed problems," he says. "It has developed resistance to a number of other soybean herbicides." That resistance has been known to spread quickly. Waterhemp plants are either male or female, which means females rely on pollen shed from surrounding male plants.

"If the resistant trait is carried in the pollen, which we are fairly confident it is, then you have pollen traveling to fields all around the resistant plants," Bradley adds. Each female waterhemp plant produces hundreds of thousands of seeds, ensuring a ready supply of plants for the following season.

Positive news

Bradley and Legleiter have found good news in their field plots. The glyphosate-resistant waterhemp is killed by a number of popular pre-emergence soybean and corn herbicides. Bradley reported on this strategy at the Weed Resistance Management Field Day June 20 near Columbia, hosted by MU and Monsanto.

The pair plan at least two seasons of examining whether the resistant plants can be brought under control economically in continuous soybeans - using pre-emergence and post-emergence herbicides - or whether it is better for farmers facing resistant weeds to alternate plantings of corn and soybeans. The rotation opens up a wider array of herbicides labeled for use in corn.

The eight other confirmed glyphosate-resistant weeds throughout the world include common ragweed, buckhorn plantain, goosegrass, hairy fleabane, horseweed (a.k.a. marestail), Italian ryegrass, palmer amaranth and rigid ryegrass.