When thin shoots of cheatgrass are the first plants to green up in the spring, mule deer might nibble the tops like any of us might idly crunch last week’s pretzels still sitting on our office desks. 

Come early summer, though, that same cheatgrass turns into brown, leggy strands. At that point, mule deer want to eat it as much as we want to consume the cardboard box containing those stale pretzels. They just won’t do it. 

In fact, they dislike cheatgrass so much that mule deer will avoid an area completely once it contains about 20% of the annual invasive grass, according to a study published in early September in the journal Rangeland Ecology and Management. 

The study’s authors, all from the University of Wyoming, compared movement patterns from 115 GPS-collared mule deer with range maps showing variation in plant cover. They found that when cheatgrass covers less than 10% of an area, deer will still browse. When it covers 10-16%, they will begin to avoid an area. Anything above 20% is utterly unappealing. 

Cheatgrass, an invasive plant that’s overtaking western landscapes. (Jennifer Strickland/ U.S. Fish and Wildlife Service/FlickrCC)

Even more alarmingly, the study shows that as cheatgrass continues to spread in northeast Wyoming over the next couple decades, up to 50% of current good habitat could be rendered useless to mule deer. 

The spread of cheatgrass likely won’t be the final nail in mule deer’s coffin, but it will be one of many contributing to their continued decline. Unlike many studies that focus exclusively on declines, however, this one carries a rather large kernel of hope. When treating cheatgrass with herbicides in a targeted and strategic way, land managers can start to win the fight and mule deer will also return. 

A perpetual plague

Cheatgrass has been the scourge of western landscapes for well over a century, when it first arrived from Europe and Asia nestled in bags of seed and bales of straw. Like most nonnative, invasive grasses, it didn’t evolve here and quickly found a way to outcompete its native neighbors. It “cheats,” germinating over winter to be the first grass to grow in the spring, using water and resources before native grasses, plants and shrubs are awake enough from their winter slumber to start looking for nourishment again. 

But by early summer, cheatgrass cures, losing any trace amounts of nutritional value. It becomes a fire hazard, another part of its evolutionary strategy. As fires whip across a landscape, aided by dry cheatgrass, the blazes kill native competitors, giving cheatgrass even more opportunities to spread. 

It’s a problem that has plagued ranchers, wildlife biologists and land managers for decades, but until recently, few publications had explicitly shown at what threshold mule deer will simply avoid cheatgrass-covered areas. 

A mule deer at Mammoth Hot Springs in Yellowstone National Park. (Neal Herbert/National Park Service)

UW professors Jerod Merkle and Brian Mealor along with research scientist Kurt Smith used data showing how mule deer acted in much of the northeast corner of the state, provided by Western EcoSystems Technology biologist Hall Sawyer. The researchers could see deer collar points move into and out of areas with good food and habitat and avoid those areas with increasing amounts of cheatgrass. 

Jill Randall, Wyoming Game and Fish Department big game migration coordinator, who was not involved in the study, said the results make sense. 

“Deer are super selective foragers, and if they can choose between native and non-native, they will go where there’s something better,” she said. “If cheatgrass is scattered in and among things, they will nibble, but 20% or above is pretty dominant. If they can go elsewhere and eat other native species, they will.”

The problem then becomes when deer start to run out of other places to go. 

Hope for conservation

Losing half their potential habitat in the next 20 years sounds dramatic. And it is, especially for a herd already suffering from energy development, highways, fences and drought.

But solutions exist, the authors say, particularly in an herbicide called Rejuvra, which was approved by the EPA in 2020 and is being used on rangelands across the West. 

The herbicide works by killing annual seeds and leaving perennial grasses and plants relatively untouched. It’s expensive, Randall said, and requires repeated treatments. But she and many other experts say it’s critical for maintaining sagebrush ecosystems upon which countless mule deer, sagegrouse, songbirds and even lizards depend. 

A mule deer buck and doe peer through the grass and weeds near Boxelder Creek in northern Johnson County. (Mike Koshmrl/WyoFile)

A study published in August used camera traps to show that mule deer will return to areas with native plants after cheatgrass removal. 

That’s also what Smith, Merkle and Mealor’s models showed. In fact, mule deer will return in abundance following treatment of cheatgrass-covered acres, potentially reversing much of that 50% of habitat that was otherwise lost. 

The authors say the paper isn’t meant to cause alarm but to explain the direness of the situation and hopefulness of potential solutions. 

Gov. Mark Gordon and state lawmakers agree, and earlier this year, provided tens of millions of dollars in grants and loans to fight invasive grasses. 

Bottom line, Smith said: “If we do nothing, we’re in trouble. But there is hope.”