The effects of cattle grazing on optimal foraging in mule deer ( Odocoileus hemionus)

Abstract

A previous study of different cattle stocking rates on activity patterns of female mule deer ( Odocoileus hemionus) on summer range in California found that deer spent more time feeding and less time resting with increased cattle stocking rates (Kie et al., 1991). During a year of normal precipitation, deer spent more time feeding per day in late summer than in early summer in pastures grazed by cattle. In a drier year, deer spent less time feeding per day in late summer in grazed pastures. Deer increased their time spent feeding by including more feeding bouts each day, not by increasing the length of each foraging bout. Deer were also reluctant to forage at night, particularly when there was a full moon. Based on these results, we hypothesized that female mule deer act as time-minimizers when forage conditions are good, but shift to a energy-maximizing strategy when forage conditions are poor (Kie et al., 1991). Preliminary results from subsequent research on black-tailed deer ( O. h. columbianus) on Mediterranean-climate, foothill winter range found that deer acted as energy-maximizers and spent less time feeding with increasing cattle stocking rates during the fall and early winter when herbaceous forage was in limited supply. After mid-January when herbaceous plants began growing rapidly there appeared to be no competition for forage between deer and cattle, and increased cattle stocking rates had no effect on the time spent foraging by deer. These results were consistent with the original hypothesis.