Roundup-resistant tumbleweeds found on Hi-Line

A Montana university researcher has warned agricultural producers that a strain of tumbleweed growing on the Hi-Line has developed resistance to a common herbicide, and could spread rapidly if not controlled.

"(This weed) is the most troublesome weed in wheat-fallow cropping systems because it spreads so quickly and has developed a resistance to glyphosate — commonly known as Roundup," Prashant Jha, associate professor and weed scientist at the Montana Agricultural Experiment Station's Southern Agricultural Research Center in Huntley, said in a release.

The Roundup-resistant weed, kochia, is the first identified in Montana. Jha said in the release that he has identified the glyphosate-resistant kochia populations in fields north of Gildford and Hingham and suspects the tumbleweed also is growing near Rudyard, Inverness, and Joplin.

Roundup-resistant weeds are raising more of a concern around the world, especially as they put a damper on Roundup-ready crops. Farmers finding weeds resistant to glyphosate herbicides are starting to wonder if they should pay extra for crops engineered to resist Roundup when the herbicide won't take care of weeds.

Jha said the problem arises with the weeds developing a resistance to the herbicide where farmers relied solely on glyphosate for weed control under chem-fallow cropping systems.

In chem-fallow systems, farmers use herbicides to kill weeds and any other plants on sections of land intended to lie fallow for a year or more.

Canadian researchers confirmed in January that they had found Roundup-resistant kochia in southern Alberta, joining other glyphosate-resistant weeds previously found in Ontario.

Jha said controlling kochia has been a major concern for Montana ag producers previously, but this year the weed has exploded with infestations of the tumbleweed in some fields north of Hingham and Gildford as high as 70 percent.

He said the highest infestations were in fields where producers were using high concentrations of Roundup. Producers using blends, including mixes of Roundup, 2-4-D and Banvel, had less severe infestations but the kochia still was evident.

Jha said the problem will grow if not quickly managed, adding that there are alternative herbicide programs that can control kochia.

He said growers should avoid using Roundup in their burndown program to kill plants.

The use of high rates may aggravate the problem even more as plants develop higher levels of resistance, Jha said in the release. Rotating herbicides and using tank-mixes with multiple modes of action should manage the problem, he said.

Jha said using alternative burndown herbicides like paraquat — commonly known by the brand-name Gramoxone — to clean up their chem-fallow fields before the kochia plants produce seeds this year could help control the problem.

The release said if able, farmers using shovel plows before planting winter wheat could help get rid of the glyphosate-resistant kochia.

"Managing the resistant seedbank is critical to managing this problem," Jha said.

Jha also said an integrated weed-management program could help control the problem.

In the release, he advised growers to apply herbicides at the rate suggested by the manufacturer and to use a tank mixture which includes multiple herbicide products.

He also said that using soil-applied residual herbicides at, or prior to, planting can potentially reduce weed bank recruitment and reduce weed interference, especially early in the season.

Diversified crop rotations also are critical to successful integrated weed management, and the release recommended rotating in pulse crops such as peas and lentils, especially where growers have traditionally relied on wheat fallow rotations. Jha also advised high seeding rates for crops as a long-term weed management strategy.