Glyphosate-resistant Tumbleweed Poses Problem for Oregon Farmers

An advocate of direct seeding and no-till farming hopes Northeastern Oregon wheat growers don’t give up the practice in wake of news that patches of Russian thistle, or tumbleweed, have developed resistance to glyphosate, the herbicide commonly used to control weeds in wheat fields.

Judit Barroso, a weed scientist at Oregon State University, recently published her research that confirmed what some growers have been worried about since they first reported trouble controlling Russian thistle with glyphosate in 2015. Barroso collected thistle samples from 10 locations in Morrow, Sherman and Umatilla counties; three from Morrow County turned out to be glyphosate resistant.

Barroso said those populations probably were treated much more frequently than others sampled, and had developed tolerance to the herbicide. Glyphosate is the active ingredient in Monsanto’s widely-used Roundup weed killer. Farmers who grow on a summer fallow rotation typically spray their fields after harvest and while the field lies fallow. The practice kills weeds without tillage, which can cause erosion.

Russian thistle competes with wheat plants for water and nutrients, and can reduce yield. When it dries, breaks off the stem and tumbles with the wind, it can spread seeds across wide areas, meaning glyphosate-resistance could spread as well.

Barroso advises growers to delay the onset of glyphosate resistance by rotating the use of different herbicides or using other weed control methods.

Blake Rowe, CEO of the Oregon Wheat Commission, said dryland growers in Oregon and Washington are closely following Barroso’s work and are trying to figure out the next step in research. Possibilities may include revised chemical strategies or timing, or planting cover crops that would compete with Russian thistle and perhaps weaken it. A return to cultivation is possible, he said.

“We’re looking at this one pretty hard,” Rowe said.

The Pacific Northwest Direct Seed Association, based in Colton, Wash., has been monitoring the findings as well. The organization is a non-profit that helps growers transition to no-till farming and direct seeding practices, in which seeds and fertilizer are planted into the stubble of the previous crop with minimal disturbance of the soil.

Executive Director Kay Meyer said a couple strategies have emerged to cope with glyphosate resistant Russian thistle. There’s no “silver bullet,” she said, but some farmers may be able to break the weed cycle by rotating in other cash crops rather than follow the grain-fallow-grain pattern year after year. Austrian peas may be an option for some, she said.

Cover crops may break disease cycles and build up soil, but some producers are worried they would take too much moisture from land that otherwise would lie fallow, she said.

Technology may hold an answer as well, Meyer said. New spot spray systems such as WeedIt and WeedSeeker can optically identify and spray only growing weeds, not bare ground. Such systems can reduce chemical use by 80 percent, Meyer said, and the savings might allow growers to use more expensive chemicals other than glyphosate.

The technology is expensive, but in some cases farmers might jointly purchase and share the system, she said.

Source: Eric Mortenson, Capital Press

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