Workshop explores benefits of combo approach to invasive annual grasses

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Mountain side

Corteva Agriscience, land managers, government agency representatives and other stakeholders share knowledge, research and goals aimed at supporting rangeland resiliency.

The complexities around restoring, enhancing and preserving the vast diversity across the western rangeland region took center stage this summer.

Corteva Agriscience helped bring together land managers, government agency representatives and other stakeholders to share knowledge, research and goals in Salmon, Idaho. The focus: rangeland resilience.

“We all know rangeland is incredibly resilient,” says Trent Brusseau, an Idaho-based specialist with Corteva. “We also know the ecosystem is incredibly fragile, and invasive plants pose a serious threat.”

Presenters representing the Salmon/Challis National Forest, United States Forest Service, the Idaho Department of Fish & Game, and Corteva Agriscience discussed results and learnings from several multiyear projects aimed at managing invasive annual grasses, such as cheatgrass, medusahead, ventenata and other species.

These threats first popped up more than 120 years ago. Increasingly arid conditions across the West, along with limited management tools, have aided their expansion.

A double threat

Invasive annual grasses complete their life cycle by midsummer. The remaining dry hardened-off grasses serve as tinder and, once burning, fuel fast-moving, dangerous wildfires. Postfire landscapes are extremely vulnerable to reestablishment by these grasses, furthering the spread.

And it doesn’t take much. Even a small — 1% to 5% — infestation of cheatgrass has an impact. When cheatgrass groundcover reaches 15%, it doubles the wildfire risk and increases the likelihood of multiple wildfires in a 15-year period by 4X. The impact of the wildfire/invasive annual grass combo is far-reaching:

  • Threatens human safety
  • Destroys ecosystem diversity and habitat
  • Causes economic hardship — lost property, agricultural production, tourism, etc.

These changes in the fire cycle hit wildlife the hardest.

“Workshop presenters hammered home how a diversity of wildlife requires a diversity of vegetation,” Brusseau says. “More frequent fires take important shrubs, forbs and browse species and then fire again before they can reestablish.”

The area around Salmon provides critical habitat for mule deer, elk and bighorn sheep. Each species requires a different mix of plants — during different times of year. As the availability of those species declines, so do wildlife populations.

Shifting from killing weeds to restoration

Workshop attendees learned about a new approach to invasive annual grass management. Focusing only on removing invasive annual grasses opens the land. The quickest way for nature to fill the void is with another invasive species, such as knapweeds, thistles and other species, thus, in effect, swapping one invasive monoculture for another. Invasive broadleaf species do similar harm by creating ecosystem monocultures, destroying habitat and opening the soil to erosion.

“Management programs that exclusively address invasive annual grasses clear the way for other invasive species,” Brusseau says. “Much of the ongoing research in the Salmon/Challis National Forest focuses on an approach that addresses both of these challenges for more-desirable outcomes.”

Tools from mechanical to biological to cultural and chemical can bring success against invasive annual grasses. The latest work has focused on a bigger picture — rangeland restoration — by controlling secondary invasive species, such as knapweeds, thistles and other threats.

This approach provides resources and habitat, including:

  • Diverse bunchgrass communities and perennial wildflowers
  • Healthy shrub components
  • Annual wildflower components

It also provides important ecosystem services:

  • Slope stability
  • Nutrient cycling
  • Water dissipation

Herbicides that contain indaziflam and imazapic offer control of invasive annual grasses. Recent work has found broader success and more desirable outcomes when aminopyralid — the active ingredient in Milestone® herbicide — is included in the tank mix.

“Including Milestone herbicide adds the broadleaf control component that keeps knapweeds and other threats at bay, allowing desirable species time to reestablish,” Brusseau says. “Customers have observed improved consistency in cheatgrass control when including Milestone herbicide in commercial applications.”

Existing data shows Milestone herbicide is gentle in nature and goes easy on desirable plants. Research presented during the rangeland resilience workshop provided confirmation. When aminopyralid was included with cheatgrass applications, annual wildflowers recovered and “took off” after the pressure from cheatgrass was removed. That’s led to a return of pollinator species. Thriving insect populations benefit as well as sage grouse and other wildlife and bring greater balance to the fragile ecosystem.

This treatment program — a tank mix of Milestone herbicide at 5 to 7 ounces per acre plus imazapic at 5 ounces per acre and methylated seed oil, according to label directions — has been successfully applied across several thousand acres. These results continue driving interest in restoring more land.

“Fall is an excellent time to make these herbicide applications because the cooler soils will allow extended residual activity to help manage invasive annual grasses and noxious weeds into the next spring,” Brusseau says. “Broadleaf weeds, such as knapweed, rush skeletonweed, perennial Canada thistle and biennial scotch, bull and musk thistle are especially susceptible to fall treatment.”

Your Corteva Agriscience specialist can answer questions and help plan these applications. You can find your local Corteva Vegetation Management Specialist here.

Meanwhile, watch for additional reports from this workshop — including specific data and additional findings from ongoing research — at


Milestone® has no grazing or haying restrictions for any class of livestock, including lactating dairy cows, horses (including lactating mares) and meat animals prior to slaughter. Label precautions apply to forage treated with Milestone and to manure and urine from animals that have consumed treated forage. Consult the label for full details. Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Always read and follow label directions.

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