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Sagebrush-grass range normally grazed in the spring and fall can be grazed in the summer to provide a maintenance ration for ewes if their lambs are weaned early. Moderate grazing (57-99 sheep days/ha) in early or late summer did not change vegetative composition or yields. Heavy grazing (185-198 sheep days/ha) in the early summer decreased yields of grasses and the cushion-forb Hoods phlox. Late summer grazing did not change the grass or forb yields. Sagebrush yields increased in the sagebrush subtype where balsamroot was abundant under early summer grazing. Each decade since the early 1900's, sheep numbers grazing traditional high mountain summer ranges have been declining. The major reasons for decline have been to lessen watershed damage and to avoid losses caused by predators or poor herders. Because of these and sheepmen's needs to maximize wool and lamb production, we decided to investigate the use of traditional spring-fall sagebrush-grass range in the summer to maintain ewes that had their lambs weaned early. In this paper we are reporting the effects of summer grazing on two sagebrush community subtypes and ewe weight responses during the summer season. Doyle et al. (in press) reported on breed weights, longevity, reproductivity, wool production, and lamb production between comparable groups of sheep sent to sagebrush-grass and high mountain summer ranges. Study Area and Methods The study was conducted on the U.S. Sheep Experiment Station at Dubois, Idaho, at an elevation of 1,646 m. Average annual precipitation is 28 cm with peaks in January due to snow and May and June due to rain. Topography is level to undulating with varying amounts of lava outcrops in all of the study pastures. The vegetation consists of threetip sagebrush (Artemisia tripartita) with an understory mixture of grasses and forbs. Two subtypes of the sagebrush-grass community are identifiable by the amount of arrowleaf balsamroot (Balsamorhiza sagittata) present. The subtype with balsamroot (balsamroot subtype) is generally positioned in the swales. The subtype without balsamroot (pricklygilia subtype) generally has more granite pricklygilia (Leptodactylon pungens) and is generally positioned on the lava outcrops. Three treatments (early summer grazing, late summer grazing, and no grazing) were applied in each of the two subtypes on 0.4-ha pastures. Four sheep grazed each subtype at a heavy rate (185 to 198 sheep days/ ha) to permit rapid assessment of the retrogressive effects on the plant community. Early summer grazing occurred during a 20-day period starting about July 7; late summer grazing occurred during a 20-day period starting about August 16 each year. Estimates of total ground cover, yields, and frequencies by species were obtained from 10 randomly located 0.89-m2 plots in The authors are range scientist, Intermountain Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Ogden, Utah, and Horn professor, Department of Range and Wildlife Management, Texas Tech University, Lubbock, Texas. The support and cooperation of the SEA-Agricultural Research personnel at the U.S. Sheep Experiment Station, Dubois, Idaho, is acknowledged and appreciated. Manuscript received March 10, 1980. each of five 0.04-ha permanent plots in each pasture in 1965, 1967, 1969, 1971, 1972, and 1974. The weight-estimate method was used to estimate species yields (Pechanec and Pickford 1937). Utilization was estimated by plant species each year on 30, 0.89-m2 plots in each 0.4-ha pasture. Photographs were taken in the pastures in the same years that production was measured. To ascertain the animal response, four 16.2-ha pastures in the pricklygilia subtype were grazed by 87 to 155 mature sheep in early and late summer at a moderate rate. A group of inbred Columbia, Rambouillet, and Targhee ewes, ranging from 210 years of age was split randomly and weighed on and off two pastures each period. Ewes were weighed after an overnight fasting period of about 15 hours. The early and late periods began each year at the same time as on the smaller 0.4-ha pastures. The length of the grazing periods was set to maintain a moderate grazing rate between 86 to 99 sheep days/ ha. Vegetation on a 0.8-ha portion of each 16.2-ha pasture, selected for comparable topography, vegetation, and distance to water, was sampled similarly to the 0.4-ha pasture in 1965, 1972, and 1974. Differences in ground cover and herbage weight of species and species categories on the 0.04-ha permanent plots in each pasture between 1965 and subsequent years were used in an analysis of variance to distinguish vegetation changes between the three treatments in a subtype. The study is not replicated in the true statistical design sense, but because site differences between pastures of the same subtype are believed to be negligible, the analysis of variance design is assumed to be correct. Relative values were calculated to illustrate the vegetation trends because of the natural variation in herbage production between the grazed and ungrazed pastures and between years (Fig. 1). First the air-dry production of a species (P) for each year (y) was expressed as a ratio (V) of the grazed (g) to the ungrazed (u) pastures; i.e., Vgy = Pgy/ Puy. Using the ungrazed pasture as a base accounts for yearly differences in production caused by normal fluctuation in temperature and precipitation. The resulting ratios for a grazed pasture for the years 1967, 1969, 1971, 1972, and 1974 were then expressed as a percentage (RV) using 1965 ratios on the grazed pastures as the base; i.e., RVgy = (Vgy/ Vglg&5)*100. This latter step accounts for pasture differences by scaling the data to a

Summer Grazing of Sagebrush-Grass Range by Sheep