The cattle-cheatgrass connection


Can grazing help control cheatgrass? That’s one of those questions that in some places doesn’t mean a thing, but in the Great Basin is likely to elicit a range of answers, from a decisive ‘yes’ to a forceful ‘absolutely not.’ The answer, as usual, lies somewhere in between: It depends on how dominant cheatgrass already is on the land, and when and how heavily it’s being grazed.

A new study in the Journal of Applied Ecology adds more nuance to the debate. Researchers studying intact sagebrush and native bunchgrass ecosystems in eastern Oregon found that grazing indirectly aids cheatgrass invasion by stressing out the system, making it more vulnerable to the invasive grass. They suggest reducing grazing intensity in these areas, by having fewer cows on the land for less time, could help intact Great Basin ecosystems ward off cheatgrass.

If you are one of those people for whom “can grazing help control cheatgrass” is an innocent question, let’s pause here and explain why cheatgrass is so bad.

Cheatgrass in Nevada. Courtesy Barry Perryman.

Cheatgrass, an annual which came to the West from Asia in the late 1800s, is super flammable, and areas of the Great Basin where the grass dominates burn every three to 10 years, instead of the normal 30 to 70. The year after a fire, cheatgrass outcompetes any native grasses that survive, forever changing the area’s fire cycle. That’s problematic for ranchers, and for wildlife. As Stephanie Paige Ogburn reported in her 2012 HCN story, “Weed Whackers,”

The bare soil left after fire can blow away in spring and land on snow in far-away mountains, causing quicker melt-off. With nowhere to hide and no shrubs to browse or nest in, wildlife -- including mule deer and the imperiled sage grouse -- quickly move out.

Cheatgrass also grows faster than native bunchgrasses, using up water and soil nutrients before natives can establish themselves, and dries out earlier in the summer, which makes it less palatable to cattle. And once it’s established, it opens the door to a whole slew of other invaders, like medusahead wildrye and thistles.

But once cheatgrass takes over a native bunchgrass and sagebrush ecosystem, it's too late to do much to combat the invasion. Mike Reisner, a co-author of the recent Journal of Applied Ecology study, wanted to figure out how that takeover happens in the first place, and find a reliable  early warning sign that an ecosystem is on the cusp of cheatgrass domination.

He discovered that ecosystem stress is a good indicator of an impending cheatgrass invasion, and that stress level can be measured by looking at the spaces between bunches of native grasses.

Drought is a major factor in causing grasses to be spaced more widely, and so are cattle. They can shrink the size of the bunchgrasses through repeated munchings, and as they tromp around season after season, they crush the moss-like biological soil crust that protects bare soil. That means there’s more space available for cheatgrass to establish itself.

Because of the way cattle can open the door for cheatgrass to invade an intact sagebrush and bunchgrass ecosystem, Reisner and his co-authors have “serious concerns” about using cattle to control cheatgrass in these places, as some previous studies have proposed.

But that doesn’t mean cattle can’t help combat cheatgrass once it’s already there. The Nature Conservancy is studying just that in a test plot in eastern Washington by enlisting cattle to spread native grass seeds and a native soil bacteria that kills cheatgrass. Basically, the cattle will trample overly dense shrubs that are shading out native grasses and push seeds into the ground to help them grow. From their website:

The project team thinks that the combination of all three treatments—bacteria, seed and cattle—will work best, aridlands ecologist Sonia Hall said. Why? The bacteria will inhibit the growth of cheatgrass, but the seed and cattle will help the native grasses become established.

And researcher Barry Perryman, a professor of agricultural science at University of Nevada-Reno, found that when cattle eat cheatgrass for two autumns in a row (it’s more palatable then), the following spring there is less cheatgrass and more of other perennials. That can help control rangeland fires, which often spread from cheatgrass monocultures into intact sagebrush ecosystems, converting more land to the invasive grass.

The conclusion seems to be that overgrazing in healthy Great Basin ecosystems can help cheatgrass spread. But once it’s there, cattle may also help control it and the wildfires it causes.

Emily Guerin is the assistant online editor at High Country News.

Photo courtesy Barry Perryman.

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