Field trials confirm waterhemp resistant to glyphosate herbicide
Herbicide tests in a western Missouri soybean field have confirmed that tall waterhemp is the sixth glyphosate-resistant weed in the U.S. and the ninth such weed in the world.
"Our field trials follow full greenhouse tests in which this tall waterhemp survived higher-than-recommended rates of glyphosate and produced seed that grew into plants that also were resistant," said Kevin Bradley, University of Missouri weed scientist.
Bradley and MU graduate student Travis Legleiter are testing several soybean herbicides on the resistant weeds found in a field near the Missouri River in Platte County.
"In the field we used up to eight times the labeled rate," Legleiter said. Notepad in hand, he kneels to count weeds in one high-rate section of the field plot.
"You can see the waterhemp was injured initially but recovered," he said. Plants are light green to yellow in the center, a symptom of glyphosate treatment. But the bulk of each plantâ€™s leaves are green and healthy.
"They are regrowing and will likely survive to produce resistant seed," Bradley said.
Their confirmation this week placed tall waterhemp on the international herbicide-resistant weeds Web site, www.weedscience.org. The site, dedicated to information on weed resistance, lists 183 weed species that have been proven to be resistant to one herbicide or another.
Glyphosate is the active ingredient in Roundup and other herbicides. Bradley and other scientists have been concerned about weeds developing resistance to the herbicide due to its popularity when used with Roundup Ready soybean, corn and cotton seed.
Bradley said the field history reads like a recipe for developing resistant weeds: back-to-back seasons of one crop -- soybeans -- and continuous use of the same herbicide -- glyphosate -- since 1996.
MU weed scientist Reid Smeda has been working on another field, in central Missouri, containing glyphosate-resistant common ragweed for several seasons. That field has been in a soybean-soybean-wheat rotation with almost sole use of glyphosate herbicide for weed control.
While all resistant weeds are worrisome, Bradley said resistant tall waterhemp is especially troubling.
"Waterhemp is one of Missouriâ€™s toughest weed problems. 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." Each female waterhemp plant produces hundreds of thousands of seeds, ensuring a ready supply of plants for the following season.
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. 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.