The act of grazing animals isn’t as simple as letting them loose on a patch of land and encouraging them to eat their fill. When the health of the landscape is also a priority — whether to reduce wildfire risk or fight invasive species — monitoring the many-bellied is necessary to ensure what can be helpful does not tumble into destructive: overgrazing and introducing unwanted species.
In Boulder, Andy Pelster heads this monitoring. As the city’s agriculture and water senior manager, he leads efforts to use cattle in ways that can be beneficial to all parties involved. Pelster said that with proper and consistent management, grazing can be used to “to shift vegetation communities from one set of species to another”: lessening the dominance of one species to encourage growth of another.
On Shanahan Ridge, for example, the city is hoping cattle will help them combat tall oatgrass, an invasive species that has been encroaching on the territory of native big blue stem, switchgrass and yellow Indiangrasses.
Tall oatgrass matures faster than those other species, allowing it to commandeer territory and resources (water, soil nutrients, sunlight) that would otherwise be utilized by natives. But if cattle are put on the land when oatgrass is sending up shoots and the native species are still germinating, the invaders will be chewed back, providing an opening to slower growers.
The second half of the equation is getting cattle off the land when the native grasses start to come up in early summer. This allows these natives to enjoy a full lifecycle undisturbed, and have “good reserves going into the winter” so they’re “ready to grow again the next spring.”
According to Pelster, it seems to be working.
“We’ve been able to reduce the cover of the tall oatgrass and we’re starting to see some nice response from the native grass,” he said.
Pelster and his team use different animals for different purposes. Where there aren’t natural water sources, water must be supplied to the grazing animals. On South Shanahan, one can see water drums dotting the landscape. But providing water to more remote areas can prove problematic, especially for cattle who are notoriously thirsty. In such situations, goats are a better option than cows, as they’re much more efficient in terms of water consumption. They also have different dietary preferences. So depending on what plant Pelster is targeting, goats might be the ticket.
“If we’re looking to manage chicory, which is an invasive herb we have in some of our sites, goats will do a fairly nice job of grazing it when it’s mature,” Pelster said.
Grazing also driving spread of highly flammable cheatgrass, a wildfire hazard
Grazing is not without its drawbacks, however. Often, rather than fighting invasive species, cattle have been pivotal in their proliferation.
Adam Mahood, a former researcher with CU’s Earth Lab who currently works for the USDA Agricultural Research Service in Fort Collins, spoke of the spread of cheatgrass in Nevada, where much of his research takes place.
The invasive species, native to Eurasia, grows densely and matures early: turning brown and dying by late summer. By the time fire season rolls around, cheatgrass provides abundant dry fuels to whatever flames enter its abode.
Grazing is one of the main drivers that allowed cheatgrass to dominate so many ecosystems in Nevada and other western states — including Colorado, and more locally Boulder — after being introduced to the continent in the 1800s.
“Cattle break up the biological soil crust,” Mahood said. “And cheatgrass seeds stick to everything, including the fur on cow’s legs. That’s its dispersal mechanism. So if you put cattle on an intact stand of sagebrush, they will trample the biological soil crust and deposit a bunch of cheatgrass seeds.”
Soil crusts, found in arid and semi-arid ecosystems (Boulder is semi-arid), would normally prevent the germination of non-native seeds. Native seeds, Mahood explained, have evolved to dig into the crust to reach the nutrients lying below. But cattle disturb that crust, creating loose soils into which cheatgrass seeds fall and establish themselves.
Due to the flammability of cheatgrass, if brought by cattle into new areas, the grazing animals do not reduce fire risk but heighten it (contrary to one of Boulder’s goals with their grazing program). And cheatgrass recovers quickly after fire. So once it is entrenched and welcoming flames into its residence, every fire enables it to further crowd out native species — and further increase fire risk.
“Much of this land is now dominated by cheatgrass,” reads the introduction of a paper published by Mahood in 2019 about Nevada sagebrush ecosystems and the reduction of plant diversity with each subsequent fire. “This in turn is initiating a positive feedback, wherein invading plants increase the probability of fire, and increased fire activity stimulates more annual grass invasion.”
Large swathes of Colorado already deal with cheatgrass, and Boulder County, in partnership with Colorado State University, has been testing the efficacy of different herbicides to combat the grass. In their Weed Management – Policy and Procedures document, the City of Boulder lists cheatgrass under Troublesome Weeds Not On the Boulder County Weed List: they’re not on the list because of their “high frequency of occurrence and subsequent difficulty of enforcement.”
In the document, the city cites efforts to use herbicide to manage cheatgrass in addition to prescribed burns (though those date back to 2004 on Rabbit Mountain). This raises questions, and concerns, that perhaps in using cattle to quell the dominance of oatgrass the city might be introducing cheatgrass to novel landscapes.
In response to the cheatgrass risk, Pelster said livestock grazing can have negative impacts on the ecosystem – if not properly managed.
“Grasslands in the western U.S. were fire/grazing-driven systems,” Pelster said. “So they are tolerant to fire and they are tolerant to grazing if it’s managed appropriately.”