A graduate student at Colorado State University has found that cattle pay little attention to whether or not they’re grazing amid prairie dog towns. Debra Guenther, a recent master’s graduate from the Department of Biology, found that cattle graze at the same rate on prairie dog towns as they do elsewhere on the shortgrass prairie, or steppe. And while the study did not address the nutrient quality of grass on prairie dog towns, the results suggest cattle find the forage at least adequate. Guenther, a research associate at Colorado State’s Natural Resource Ecology Laboratory, an interdisciplinary research group, will present her findings today (Aug. 6) at the annual meeting of the Ecological Society of America in Snowbird, Utah.
At the heart of the argument is the knowledge that black-tailed prairie dogs eat the same grass that larger herbivores do and therefore leave an area with less forage. On the mixed-grass prairie, a separate ecosystem, bison prefer the grasses found on prairie dog towns because of their high nutritional content, even though there is less vegetation.
"The theory is that bison like prairie dog towns because the plants on the towns have higher protein concentrations," Guenther said. "They’re more digestible and, because of prairie dog grazing, there is more live plant growth than old or dead plant material. Animals grazing in these areas would get more nutritional bang for their buck."
Guenther noted that prairie dog waste adds nitrogen to the soil around their towns and that grazing effects may account for a higher nutritional capacity.
However, what works for bison on the mixed-grass prairie of Wyoming and the Dakotas doesn’t necessarily hold true for cattle and the shortgrass steppe of eastern Colorado and northern New Mexico. To find out if beef cattle sought out or avoided grazing on prairie dog towns, Guenther spent the summer of 1999 observing cattle grazing behavior at the Pawnee National Grasslands in northeastern Colorado. Her 4 a.m. to 10 p.m. workdays allowed her to observe three pastures intensively at dawn and dusk, when grazing activity was most active. She conducted bite-and-step counts, checking the number of times a cow took a bite of grass before moving on.
"In a highly nutritious habitat, an animal will take more bites before they take another step," she said. In addition to the close observations of individual cattle, she observed herd use of prairie dog towns on a dozen other pastures.
She found that prairie dog towns made up roughly 3 percent of the total area in the various pastures and that cattle spent roughly 3 percent of their time grazing among those towns.
"What the study shows is that cattle don’t avoid prairie dog towns," said Jim Detling, professor of biology and Guenther’s advisor. What hasn’t been established is the relative nutritional quality of blue grama, sedges and forbs in the two ecosystems.
"On the shortgrass steppe, prairie dogs are removing biomass and the protein in that biomass, so there’s less for cattle to eat," Detling said. "However, what’s left may be a higher quality forage."
Detling thinks the difference lies in the two ecosystems and not between bison and cattle. In other words, he suspects cattle would preferentially graze prairie dog colonies on the mixed-grass prairie, and bison would neither avoid nor seek out colonies on the shortgrass steppe, just as cattle do.
"I think cattle, just like bison, still prefer to graze on prairie dog towns in those grasslands that are more productive," he said.