Long-term effects of black-tailed prairie dogs on livestock grazing distribution and mass gain

Authors: Augustine, David; Derner, Justin

Submitted to: Journal of Wildlife Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/26/2021
Publication Date: 7/28/2021
Citation: Augustine, D.J., Derner, J.D. 2021. Long-term effects of black-tailed prairie dogs on livestock grazing distribution and mass gain. Journal of Wildlife Management. 85(7):1332-1343. https://doi.org/10.1002/jwmg.22103.
DOI: https://doi.org/10.1002/jwmg.22103

Interpretive Summary: Rangeland managers in the western Great Plains are often concerned when prairie dog colonies establish and begin to expand in their pastures, because they can substantially reduce the amount of forage present on the colonies, and hence are perceived to compete with livestock. However, there have been surprisingly few experimental studies of prairie dog - livestock competition, and the severity of such competition is rarely measured. We conducted a 12-years study where we compared the weight gains of yearling steers grazing shortgrass rangeland during the growing season in 10 pastures where prairie dogs were controlled using rodenticides to 4 pastures where prairie dogs were not controlled. We found that when prairie dog colonies expanded in occupancy from 0% to 60% of a pasture, yearling steer weight gain declined by 8%, compared to pastures without prairie dogs. However, prairie dogs rarely reached such high occupancy in pastures where they were not contolled, due to periodic outbreaks of plague which decimated the prairie dog populations. As a result, averaged over the entire 12-year study, cattle gained an average of 2.13 lbs per steer per day (equivalent to a gain of 299 lbs over a 140-day grazing season) where prairie dogs were controlled by rodenticide, and 2.09 lbs per steer per day (equivalent to a gain of 293 lbs over a 140-day grazing season). However, during our study, years when prairie dogs reached peak abundance (in between plague outbreaks) did not coincide with a drought. Thus, our findings suggest that under conditions of near-average rainfall, prairie dogs can have only minor effects of cattle production. Measures of how abundant prairie dogs affect livestock during drought years are still needed.

Technical Abstract: The conservation and management of black-tailed prairie dogs (Cynomys ludovicianus) have been contentious issues in grasslands of central North America for much of the past century, primarily because of the perception that they compete with livestock for forage. Studies quantifying the magnitude of competition between prairie dogs and cattle are difficult to conduct because of the large spatial and long temporal scales needed to quantify how competition varies in response to interannual variation in precipitation and prairie dog abundance. We examined variation in weight gains of yearling steers in shortgrass steppe of northeastern Colorado with and without prairie dogs over a 12-year period that encompassed a full cycle in prairie dog abundance from a nadir in 2008 following plague-induced population collapse, to peak abundance during 2013 – 2015 following population recovery, to plague-induced population lows again during 2014 – 2017. Analyses of cattle grazing distribution with GPS collars revealed preferential grazing on colonies following a period of unusually high vegetation production, and preferential grazing off colonies following a period of rapid vegetation senescence, but these patterns were not clearly related to cattle weight gains. Across all 12 years of the study, average daily weight gain (ADG) during the growing season did not differ between pastures with prairie dogs absent (0.97 kg steer-1 day-1) and present (0.95 kg steer-1 day-1). Analyses of generalized linear mixed models showed ADG was a quadradic function of precipitation and a linear function of prairie dog occupancy within a pasture, with the model predicting an 8.0% decrease in ADG as prairie dog occupancy increased from 0 to 60% of a pasture with average growing-season precipitation. One limitation of our study is that the only drought year (2012) occurred when prairie dogs occupied low percentages (10 – 25%) of the study pastures. Our findings show prairie dogs have a relatively small but detectable negative effect on cattle weight gains during the growing season in shortgrass steppe. Key remaining unanswered questions include the impact of prairie dogs if they occur at >25% occupancy of a pasture during a drought, and effects of prairie dogs on cattle performance during dormant-season grazing.