Controlling Weeds with Cover Crops

University of Idaho graduate student Adam Kennedy sees promise in using volunteer wheat as a cover crop to rid sugarbeet fields of herbicide-resistant weeds.

Similarly, U of I graduate student Prayusha Bhattarai has had success with planting three different cereal species as cover crops for weed control in dry beans.

Cover crops are a cornerstone of sustainable agriculture — planted primarily to improve soil health, prevent erosion and boost organic matter in soil, among a host of other agronomic benefits, rather than for commercial sale. Through their separate research trials, Kennedy, of Washington, and Bhattarai, of Nepal, aim to demonstrate yet another economically important benefit of raising cover crops — outcompeting troublesome weeds.

Some staple crops in Idaho such as sugarbeets, alfalfa and corn have been bred using biotechnology to withstand applications of glyphosate herbicide, sold under the trade name Roundup. Heavy use of glyphosate has led to the onset of herbicide resistance in kochia and other common weeds, making weed control far more challenging in Roundup-ready crops. Idaho sugarbeet farmers are especially concerned about the recent arrival of two pigweeds — Palmer amaranth and water hemp — that resist glyphosate and several other herbicides. The weeds were likely imported into Idaho from the South and Midwest in birdfeed and cottonseed meal used in livestock rations.

“We are very worried about controlling these resistant weeds. Relying solely on chemicals won’t be a sustainable way of managing these weeds,” Bhattarai said. “It is advisable to combine chemical management with other weed-control measures, and cover crops could be one of those measures because there are many benefits that farmers can get when they are using cover crops.”

Both students are working under advisor Albert Adjesiwor, a UI Extension specialist and assistant professor of weed science.

Bhattarai’s master’s thesis was funded with a $101,583 Idaho State Department of Agriculture specialty crop block grant through the Idaho Bean Commission, in addition to a $30,000 student grant through Western Sustainable Agriculture Research and Education. She conducted trials in 2023 and 2024 at both the U of I’s Kimberly Research and Extension Center and Parma Research and Extension Center, planting cover crops of barley, triticale or wheat in the fall and terminating them in the spring.

Her cereal cover crops were either harvested as forage, which would provide another commercial crop for farmers, or terminated with glyphosate herbicide. At the Parma site, the first forage cutting yielded 2,793 pounds per acre of triticale, 2,320 pounds per acre of wheat and 2,402 pounds per acres of barley. Bhattarai cut regrowth two weeks later just before she planted dry beans, harvesting another 528 pounds per acre of triticale, 778 pounds per acre of wheat and 1,019 pounds per acre of barley.

In plots in Parma where cover crops were planted without any supplemental herbicide applications, Bhattarai had up to 79% control of weeds and effectively suppressed glyphosate-resistant kochia. A cold spring disrupted cover crop growth in Kimberly during 2023.

Results from 2024 are not yet available, but Adjesiwor anticipates this season’s forage yields in Kimberly will be triple last year’s totals. In fact, this season’s Kimberly trials have yielded so much forage biomass, growth of the following bean crop was stymied, leading Adjesiwor to recommend that strip tillage be used in the future after swathing forage and before planting beans to improve bean seed contact with soil.

In other treatments in the experiment, Bhattarai used a combination of cover crops and herbicide applications to provide additional weed control. She’s also taken soil samples to evaluate how cover crops affected soil microorganisms and organic matter, and UI Extension economist Patrick Hatzenbuehler will help her conduct an economic analysis of cover crop use in her trials.

“This combination of cover crops and herbicides could be an amazing strategy for managing resistant weeds in dry beans,” Bhattarai said.

For his master’s thesis, Kennedy watered plots within a fall wheat field in Kimberly following harvest to raise a cover crop from the seeds that fell to the ground. The goal of his project is to help farmers cut costs of seed, fuel and other inputs by raising a volunteer cover crop. Watering volunteer wheat also saves time that would otherwise be spent preparing ground and planting cover crops.

“You don’t spend any money with volunteer wheat, but you get the same stand density as planting cover crops, and you also have much better biomass just because you get a head start,” Adjesiwor said.

Adjesiwor plans to use data from Kennedy’s experiment to apply for a USDA National Institute of Food and Agriculture grant next year.

In some treatments, Kennedy added collard greens and peas to the volunteer wheat to lend additional biomass. He plans to terminate his cover crops with herbicide in April and plant sugarbeets into the standing biomass. He intends to evaluate various combinations of cover crops with volunteer wheat and herbicide rates for weed control.

“The main problem is a lot of these weeds are Roundup resistant, and since sugarbeets are Roundup ready, that’s the go-to herbicide for weed control. You can’t blame anyone for doing that because Roundup is about the only effective herbicide option, but we all know that spraying more Roundup is going to create more resistance,” Kennedy said. “So are there alternative ways to reduce weeds?”

Though Kennedy has no official data yet, he’s been impressed by the blanket of thick foliage resulting from volunteer wheat alone, which he anticipates will result in effective weed control.

“Visually, it is a carpet of green grass out there,” Kennedy said. “I was surprised at how thick it was. It looks like the weeds are suppressed by the wheat and cover crop mixture, but it also looks like the wheat alone suppressed the weeds very well.”

Using cover crops to outcompete weeds, thereby reducing reliance on herbicides, would also help farmers comply with a new Endangered Species Act strategy the U.S. Environmental Protection Agency approved seeking to reduce herbicide use for the protection of more than 900 federally endangered and threatened species.

Adjesiwor said, “We know herbicide resistance is a numbers game. The fewer weeds you expose to an herbicide, the less likely you are to select for resistance.”

Published in Catching Up with CALS