A recent study refutes the idea that cattle manure helps spread invasive cheatgrass seeds, which grazing advocates hope will weaken arguments against livestock on public lands.

However, a leading critic of public lands grazing argues the study doesn’t address the more common ways that livestock are thought to disperse cheatgrass seeds.

In the study, rangeland researchers from the University of Nevada-Reno found that cheatgrass seeds are generally too degraded by cattle digestion to viably germinate in manure.

“By the time the cow would be pooping out the seeds in the range, most of the seed would be dead,” said William Richardson, an ecologist who helped write the study, which was published in the most recent edition of the Society of Range Management’s journal.

Ranchers, public land managers and environmental advocates frequently debate whether livestock grazing is more likely to aggravate or suppress the expansion of cheatgrass, an undesirable invasive species in the West.

Environmental lawsuits often cite cheatgrass dispersal as an adverse consequence of livestock on federal property, while ranchers and government officials defend grazing as a method for controlling the weed.

Claim leads to study

The claim that cheatgrass was spreading through fecal material prompted Tamzen Stringham, a rangeland and riparian ecology professor, to examine the matter more closely.

As part of the study, Stringham and the other authors evaluated the germination potential of cheatgrass seeds that spent time in a cow’s rumen, its largest stomach compartment, and were deposited in manure.

After enduring 48 hours in the rumen, which would be common in autumn, the viability of seeds fell to roughly zero, according to the study. After 36 hours, which would be more likely in spring, about 10% or less of the seeds sprouted.

Given the high acidity of cow manure, even those seeds that remained viable after 36 hours in the rumen would be unlikely to germinate, especially since they’d also have to survive chewing, Stringham said.

“The seed dies in the rumen and the small percentage that might make it through can’t tolerate the fecal environment,” she said.

Stringham hopes to pair the findings with other research in which native and beneficial grass seeds are incorporated into protein licks and consumed by cattle grazing on cheatgrass.

The desirable seeds would be coated to protect them against degradation, allowing them to germinate in manure and replace the cheatgrass, thus improving rangeland health and reducing wildfire fuels.

“If we’re going to do that, we need to put beneficial species back in,” Stringham said.

If the strategy is opposed in litigation against the U.S. Bureau of Land Management, for example, the study helps rebut claims that suppressing cheatgrass with grazing disperses seeds through manure, she said. “This provides BLM a great counterargument.”

Critics, supporters

But the Western Watersheds Project, an environmental nonprofit that often goes to court against grazing, doesn’t consider seed dispersal through manure as the main problem, said Erik Molvar, its executive director.

“The real method of cheatgrass transport is external,” Molvar said. “This new study doesn’t change that analysis.”

Cheatgrass seeds lodge in the fur of cattle and sheep or the mud that sticks to them, distributing the invasive weed as livestock move along the range, he said.

“That’s consistent in all the science that’s been done previously,” Molvar said. “This body of science is pretty definitive.”

The Public Lands Council, an organization that supports livestock grazing, believes the new study sheds light on managing cheatgrass and negates a common misperception, said Kaitlynn Glover, its executive director.

“It allows us to focus on the way that cheatgrass does spread,” Glover said. “Grazing in the late season can be incredibly effective in preventing regrowth that next growing season.”

Such research helps ranchers and public land managers use grazing to fight cheatgrass without contributing to the weed’s spread, she said.

“We’re very encouraged we can eliminate this vector, but that doesn’t mean there aren’t other ways cheatgrass spreads,” Glover said. “It’s not reasonable or common sense to believe we can eliminate all risk in an ecosystem.”

Stringham, the researcher, acknowledges that seeds can be transported in the fur of an animal but says study would be required on how many of those seeds survive, for how long, and whether they remain viable and actually produce seedlings.

“Those items would need to be quantified in a scientifically valid manner,” she said.