Farmers in Northeast Oregon have discovered three infestations of glyphosate-resistant Russian thistle, also known as tumbleweed, Oregon State University researchers have confirmed.
Multiple growers in several counties reported instances glyphosate failing to kill tumbleweed last summer, which led OSU researchers to collect samples, germinate seeds and spray the offspring with the herbicide.
Last week, Judit Barroso, an OSU weed scientist, confirmed that three of the tumbleweed populations actually were glyphosate-resistant.
Tumbleweed, an iconic Western weed, spreads seeds prolifically when it dries out and literally tumbles across the landscape. Weeds develop resistance when individual plants survive spraying and then multiply.
“The resistance is going to spread really fast, so we need to convince growers to control these weeds in a different way,” Barroso told members of the Oregon Board of Agriculture during a Sept. 12 meeting in Pendleton, Ore.
However, alternatives to glyphosate have serious drawbacks.
Tillage is one option, but it can cause erosion, Barroso said.
Herbicides other than glyphosate are often more expensive, while paraquat — which growers have recently begun using on the weed — is more toxic to humans, she said.
“The wheat grower doesn’t have a lot of room (financially) to spend on weed control,” she said.
While unfortunate, “herbicide resistance is a matter of time,” Barroso said.
Glyphosate usage is common in the region partly due to the popularity of no-till farming, which involves seeding wheat directly into the earth without first plowing it.
While the system greatly reduces erosion, growers rely on glyphosate to suppress weeds that would otherwise compete with their crop.
Conventional farmers also use glyphosate to control weeds in their fields, said Gregg Goad, a retired farmer near Pendleton who attended the meeting.
“It’s the frequency that you use the compound that really raises the likelihood of resistance,” he said. “It was a good compound for an awful lot of things, so it got used frequently. But with that frequency, it became ubiquitous in the environment and that selected for glyphosate resistance.”
Barroso said she intially hoped that the tumbleweed infestations reported by farmers were not reacting to glyphosate because dust and drought had impeded the herbicide’s function.
While that may have been the case for other instances of tumbleweed surviving glyphosate sprays reported to OSU last year, the offspring from three locations in Morrow County clearly were resistant, she said.
Tumbleweed doesn’t tolerate competition and will be crowded out by a vigorous crop, but given the region’s arid climate, that kind of vigor generally isn’t realistic, said Barroso.
“It’s robust, and it’s hard to control,” said Goad.