By Chris Branam
Researchers have confirmed what Oregon farmers have feared for nearly two years – some populations of Russian thistle in the northeastern part of the state have developed resistance to glyphosate, one of the most widely used herbicides in the United States.
“This is not good news for growers,” said Judit Barroso, a weed scientist at Oregon State University.
“This species is a serious threat to the sustainability of the wheat-summer fallow cropping systems of the inland Pacific Northwest. It is very often the predominant broad-leaved weed in many of the fields, and farmers rely extensively on glyphosate to control it.”
The finding is published online in the journal Pest Management Science.
Russian thistle, also known as a tumbleweed, causes serious crop production problems in dryland small-grain producing areas in the United States, costing farmers more than $50 million annually in control measures. Farmers in the arid region of northeastern Oregon rely on repeated applications of herbicides such as glyphosate to control Russian thistle.
Glyphosate is the herbicide of choice for growers in the Pacific Northwest to control Russian thistle after harvest and in summer fallow.
Ten years ago, a similar plant – kochia – became resistant to glyphosate in Kansas. Glyphosate-resistant kochia populations are new found in many other states.
Now, Barroso is working with growers to delay the glyphosate resistance in Oregon by rotating different herbicides and using other weed control practices.
“There needs to be an immediate transition to a more diversified approach for control of this troublesome weed species,” she said.
Some of the reasons for poor post-harvest glyphosate effectiveness include dust, water stress, or generally poor growing conditions during application. However, reduced control may be the result of the evolution of glyphosate resistance in the species as well, Barroso said. Glyphosate is cheaper than other herbicide options and that has encouraged its repetitive use, she said, raising the likelihood of resistance.
In the fall of 2015, farmers in northeast Oregon reported difficulties in controlling Russian thistle with glyphosate. The following February, OSU researchers randomly collected 10 Russian thistle populations on fallow fields in Umatilla, Morrow and Sherman counties.
Lab testing of the samples showed that three of the collected populations in Morrow County were glyphosate-resistant. Those three populations were likely treated with glyphosate much more often than the plants that were susceptible to the herbicide, Barroso said.
Russian thistle breaks off the stem when it dies and moves with the wind. As a tumbleweed, it can spread seeds over long distances, which may allow the glyphosate resistance to spread very quickly. Each plant, growing without competition, produces more than 50,000 seeds.
Barroso conducts research at the Columbia Basin Agricultural Research Center in Pendleton. The Agricultural Research Foundation of Oregon funded the research. The foundation is a private, non-profit corporation and an affiliate of OSU’s College of Agricultural Sciences. The board of directors is made up of representatives of numerous segments of Oregon's agriculture industry.