Effect of Grazing Horses Managed as Manipulators of Big Game Winter Range

Abstract

A sagebrush-grass range in northern Utah, a critical winter area for deer and elk, was grazed by domestic horses in order to evaluate their potential to reduce selectively herbaceous vegetation which competes with bitterbrush (Purshia tridentata), an important big game winter forage. The diets of horses were recorded during spring and summer grazing under two stocking levels on small pastures. Horses consumed largely grass species throughout the study period in this high seral community. Forbs were important forage only under heavy stocking. No use of bitterbrush was recorded. Rate of forage disappearance, expressed on a peranimal-weight basis, was found to be lower on heavily stocked pastures. All pastures grazed by horses responded with increased seasonal twig production of bitterbrush over the non-grazed state. Seasonal twig production was greatest in response to heavy stocking during early July. Herds of migratory mule deer (Odocoileus hemionus hemionus) and elk (Cervus elaphus nelsoni) in the Rocky Mountains and Great Basin often rely on foothill rangelands dominated by big sagebrush (Artemisia tridentata) and other more palatable shrubs for winter browse. Winter deer mortality in Utah appears to be inversely related to the amount of palatable browse species available (Robinette et al 1952). Bitterbrush supplies important browse on many big game winter ranges in this area. Much of present bitterbrush range in our western states is considered to be seral communities created and maintained by perturbations of some sort (Ellison 1960). Extensive livestock grazing on palatable grasses and forbs in the late 1800’s and early 1900’s led, in part, to widespread successional changes favoring bitterbrush, which was frequently subordinate in the pristine vegetation (Hull and Hull 1974). In response to an increase in palatable shrubs and intensive management, elk and deer prospered by mid-century. In recent years, increasing urbanization near many foothill winter range areas has promoted a trend towards protection of big game winter range from livestock use because of water quality and soil erosion concerns. On many protected rangelands, succession is now favoring grasses since they receive little use during the growing season to balance heavy winter use of shrubs by big game (Smith 1949, Urness 1976). As palatable shrubs disappear from the community, productivity of these lands as deer winter habitat decreases. Declines in the Intermountain Region of deer herd productivity in the mid-1970’s (Connolly 1981) may have been partially due to these plant community changes. It is well recognized that controlled livestock grazing can be used in a manipulative sense to direct plant community succession (Anderson and Scherzinger 1975, Lewis 1969). Research with catAt the time of the research authors were graduate research assistant and associate professor, Range Science Dept., UMC-52, Logan, Utah 84322. Reinerisnow research assistant Range and Wildlife Manage. Dep., Texas Tech Univ. Lubbock, 79409. The authors wish to acknowledge Charles H. Jensen for his advice on the study design and layout. Statistical analysis was assisted by Dr. Ronald V. Sisson and Mr. David L. Turner. Thanks are also due to John Moody and Deborah Bowdenfortheir help in collecting the field data, and to the Starwood Ranch, Aspen, Cola., for generously loaning the study animals. The study was supported in part by the Utah Division of Wildlife Resources, Project W-105-R. Manuscript received August 20, I98 I. JOURNAL OF RANGE MANAGEMENT 35(5). September 1982 tle and sheep has shown that the productivity of bitterbrush can be increased by springtime grazing of competing herbaceous vegetation (Smith and Doe11 1968, Jensen et al. 1972, Smith et al. 1979). One problem encountered when using sheep or cattle for this purpose is that both animals consume bitterbrush. Smith and Doe11 (1968) found that on moderately stocked pastures bitterbrush utilization by cattle increased dramatically in July, in some cases exceeding utilization of grasses. Jensen et al. (I 972), in developing guidelines for sheep grazing on big game winter range, also reported heavy use of bitterbrush beginning in July. Both investigations suggested that stocking and season ofhvestock use must be precisely controlled to avoid excessive utilization of bitterbrush. The use of horse grazing as a “biological tool” to enhance bitterbrush on big game winter range was investigated in this study because horses were presumed to select for grass species (Stoddart et al. 1975, p. 257, Wagner 1978). The objectives of this study were twofold: (1) to investigate the use of spring-summer horse grazing as a means of increasing the seasonal production of bitterbrush, and (2) to determine the effect of grazing season and stocking intensity on the diets of horses grazing sagebrush-grass rangeland.