The Mount Haggin Wildlife Management Area in Montana is managed under a three-pasture grazing system incorporating about 18,000 acres. Cattle graze these pastures (all similar in size and grazing capacity) from June 15th to October 15th every year; each receiving one of three grazing treatments annually, and all three treatments within a three year time period: Treatment A is available to livestock throughout growing season, B is for grazing after seed-ripe, and C is rested from livestock grazing. The goal of this study was to examine how the grazing system effects the potential supply of mice as prey for raptors. The population densities of deer mice and montane voles were very low in 1985, lower still in 1986, then increased in 1987 to almost twice what they were at the beginning of the study. As cover was reduced voles became vulnerable to predation and densities were low. However, deer mouse populations were higher in areas with less cover. The biomass of mice available to raptors in the grazing system was smaller than that available in the ungrazed control. The rested treatment had the highest populations of voles and mice in the grazing system but produced the smallest biomass of mice available for raptors. Over time the resting process allows the rodent populations to recover and produce more prey during the grazing periods than would be produced under continuous grazing.
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