Cattle and Cowbirds in Cowling Arboretum

Carleton College’s Cowling Arboretum recently played host to a small herd of cattle. The eight animals grazed Hillside Prairie for six weeks this summer as part of an experiment in prairie management.

Historically, grazing has been an important ingredient in the health of local prairies, but Arboretum Director and Professor of Biology Mark McKone said the original grazers—bison—would now be an impractical addition to the Arboretum.

“Bison in the Arb would require large permanent fencing,” McKone said. “This would be highly intrusive, destroy the sense of continuity in our prairies and be very expensive.” Instead, the research team introduced domestic cattle into a temporary site.

Hillside Prairie is one of Carleton’s tallgrass prairies, so named for the dominant grass species which can grow to two or more meters in height. The problem, research assistant Brody Sandel ’04 said, is that the tall grasses can drown out sunlight, and smaller plant species falter. Eventually, the tall grasses take over.

“That’s where grazing comes in,” Sandel said. “Big grazers . . . prefer to eat these tall grasses. So, they do that, and [the grasses] get less tall, provide less shade, and the other plants do better. That increases diversity, which is good.”

The cattle were shipped out of the Arboretum on July 28, but research will continue throughout the summer. The project’s next task is to weigh the benefits of grazing against the impact of housing the livestock in the Arboretum. One potentially harmful side effect of the cattle’s visit has already been noted: the arrival of cowbirds.

The birds are brood parasites, nomadic nest-hunters who forage the grass seed stirred up by livestock and, along the way, lay their eggs in the nests of other bird species. The nest’s unsuspecting owner then cares for the baby cowbirds, often at the expense of her own offspring.

“Cowbirds tend to do quite a number on bird populations wherever they go. So, we’re kind of worried about that,” Sandel said. “Hopefully they will leave [now that the cattle are gone].”

One other possible drawback is concentrated, high-intensity grazing. “[Cattle] churn up the soil and generally crush things,” Sandel said. ” . . . they have the potential to pretty much reduce plant life on their pasture to nothing.” To minimize negative impact, the cattle were kept on the move, transferred frequently through a series of small paddocks.

Sandel and McKone will spend the rest of the summer determining recovery rates of the grazed plants and measuring light availability at different areas in the pasture. Newly exposed to the sunlight, the prairie’s smaller plants may respond by producing more flowers and seed, Sandel said. In the final stage of the project, the researchers will compare biomass in grazed and ungrazed plots.

“If this proves an effective way to maintain a high diversity of plant species, cattle grazing could be used on a larger scale in the Arb in future seasons,” McKone said.