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Research at U of I ranch finds promise in long-lasting herbicide

By John O’Connell

University of Idaho

The 7 acres of hilly pasture along Little Rock Creek were lush with bluebunch wheatgrass, needlegrass and an abundance of native forbs.

Georgia Harrison, a University of Idaho plant sciences graduate student, was encouraged by the healthy native plant communities in her experimental plot, which had been aerially treated nearly two years earlier with the long-lasting herbicide Rejuvra.

Just as important to the researcher were the plants that weren’t present -- the hillside was devoid of cheatgrass, Japanese brome and other invasive annual grasses, which outcompete native species, degrade forage quality and increase wildfire risk.

While showcasing her research to a tour group at U of I’s Rinker Rock Creek Ranch in central Idaho’s Wood River Valley in the spring of 2022, Harrison also surveyed an 8-acre untreated check plot nearby.

Both areas were part of the same grazing unit, but annual grasses were getting a foothold in the untreated pasture.

Harrison, of Kalamazoo, Mich., is involved in one of three ongoing U of I studies at the research ranch evaluating the efficacy of Rejuvra, as well as how it should be applied and its effects on nontarget plant or animal species.

The herbicide could provide land managers with an important new tool.

The research projects

Harrison’s three-year study, funded with $90,000 from Bayer Crop Science, wraps up this season, analyzing how treatments affect annual grasses, nontarget species and wildfires.

“We saw reduced annual grass cover and minimal impacts on native forbs and grasses,” Harrison said. “It looks promising.”

Another project wrapping up this season, funded with $30,000 from Bayer, will quantify how the broad and even distribution of Rejuvra droplets improves the herbicide’s efficacy.

The study is also evaluating a treatment of Rejuvra applied in combination with the herbicide imazapic, which moves farther laterally in soil but doesn’t last as long. 

A two-year study launching this spring will be funded with $110,000 from a U.S. Forest Service competitive grant, building upon the prior research.

The project seeks to inform federal land managers how Rejuvra’s effects on nontarget vegetation may impact small mammals and sage grouse.

The Forest Service will use the results to guide the possible approval of Rejuvra, which contains the active ingredient indaziflam, for use on its terrain.

The federal Bureau of Land Management may also look to the study for its own Rejuvra approval process.

Studies show Rejuvra controls cheatgrass for at least 18 months following spring application, compared with about one year with imazapic, which is the agencies’ current go-to herbicide.

Other researchers involved in the Rejuvra studies include Tim Prather, a College of Agricultural and Life Sciences professor and senior associate director of the U of I Rangeland Center; Tracey Johnson, an assistant professor in the College of Natural Resources and director of research at Rinker Rock Creek Ranch; and master’s student Kirby Lau, of Hillsborough, N.C., who is studying natural resources.

“The idea is when you make changes in plant communities, that’s basically the habitat in which small mammals and sage grouse live,” Prather said. “When you make changes, is that to their benefit or detriment?”

A powerful tool

Rejuvra would help federal land managers remove highly flammable annual grasses from fuel breaks on public lands. The herbicide could also provide a tool for restoring largely intact plant communities where annual invasive grasses are beginning to take hold.

Lau will evaluate treatment responses of forbs of importance to sage grouse, in addition to trapping small mammals to assess their responses. The researchers will also evaluate how annual grass density affects fire patterns and behavior.

“If we understand something about how animal communities or wildlife habitat are affected by herbicides, that’s going to inform when and how to use these herbicides,” Johnson said.

Small mammals of interest in the study area include deer mice, Great Basin pocket mice and sagebrush voles, which are a species of conservation concern.

The rodents are essential cogs in the food web of their ecosystem and seeds from annual grasses may provide them an important food source. Furthermore, the researchers will be interested to see how treatments affect certain shallow-rooted native annual grasses.

To understand how the herbicide affects sage grouse habitat, the team will monitor bird droppings, the time sage grouse spend in treated areas versus untreated areas and how well forbs of importance to chicks fare.

“What we have found is we’re getting forb recruitment. Forbs are increasing,” Prather said.

Within hill pasture at the research ranch, where Rejuvra has been applied aerially, Prather has also observed encouraging signs for sage grouse.

“There is a lek there,” Prather said, referring to locations where sage grouse gather for courtship displays, “and when we go there to sample plants for a study looking at differences in the plant community, pretty much every time we flush at least one group of sage grouse. We haven’t lost those sage grouse.”