Researchers are warning Pacific Northwest farmers to avoid using Group 1 herbicides year after year or risk losing their effectiveness against weeds such as downy brome.

Quizalofop, a Group 1 herbicide, is used with CoAXium wheat, which is resistant to it. Albaugh LLC, the Colorado Wheat Research Foundation Inc. and Limagrain Cereal Seeds market the CoAXium wheat system. Albaugh markets quizalofop as Aggressor.

Another company, AMVAC, markets quizalofop as Assure II for use with pulse crops and canola.

Drew Lyon, a weed researcher at Washington State University, recommends farmers not use CoAXium wheat more than two out of every six years to avoid developing resistance in downy brome.

Growers should consider diversified crop rotations in which winter wheat is grown once every three or four years, he said.

Lyon’s recommendations include:

• Consider rotating the CoAXium wheat production system with the Clearfield wheat production system, in which the herbicide imazamox is used.

• Avoid the use of Assure II in broadleaf crops grown in rotation with CoAXium wheat.

• Always remove downy brome plants that survive herbicide treatments.

“If used wisely and sparingly, it would be a good tool for growers, but history suggests that it will be used too heavily and will not last long, especially for downy brome control, which is the main driver for the system,” Lyon said.

Chad Shelton, global director of innovative technology for Albaugh in Rosalia, Wash., said the company requires farmers to sign a CoAXium stewardship agreement each year.

The company recommends using the wheat no more than three out of every six years.

Albaugh produces an annual report for the Environmental Protection Agency, including the amount of seed planted per state and the amount of Aggressor used per state, any resistance concerns and how those situations were resolved, Shelton said.

Quizalofop is the primary herbicide for controlling grass weeds in broadleaf crops after they germinate.

“So, regardless of broadleaf crop, fall or spring seeding, whatever, farmers use a Group 1 herbicide post-emergence to control the grasses,” said Ian Burke, a weed science professor at WSU.

Resistance to Group 1 herbicide is a “major concern,” Burke said, adding that other herbicides that work like the Group 1 herbicides aren’t available.

Grass weeds evolve resistance to Group 1 herbicides “really fast,” Burke said. “So fast, in fact, that the CoAXium system is already obsolete in most of the areas we have Italian ryegrass.”

Now in its second year, CoAXium wheat is grown on 30,000 to 50,000 acres in the Pacific Northwest, and on more than 600,000 acres across the U.S., Shelton said.

It is only available as hard red winter wheat, so adoption is limited in the region, Shelton said.

The company is working with partners to breed the trait into soft white wheat, the class of wheat primarily grown in Washington, Oregon and Idaho.

Burke calls the situation “complicated.”

“In many ways it’s the end of the road for post-emergence grass control in the inland PNW,” he said. “I sincerely hope farmers use it wisely and sparingly.”