WINTER RANGE USE BY CATTLE OF DIFFERENT AGES IN SOUTHWESTERN MONTANA

During two winters, we compared the use of winter range by young (3-ye ar-old) cattle that had no experience on winter range with mature (7- to 8-year-old) cattle that had experience on winter range. Ten 3-year- old cows were alternated weekly with ten 7- to 8-year-old cows on the same 150 ha pasture in southwestern Montana from December to February. Before turning cattle onto the pasture, we measured standing crop (kg ha(-1)) within the pasture. Three times a day, 3 days each week, we r ecorded if the group was in an unprotected, moderately protected, or p rotected area relative to wind direction and topography. Microclimate variables were recorded at cattle locations and 12 weather stations in the pasture to assess preference or avoidance of certain microclimate s. We used standard operative temperature and lower critical temperatu res as indicators of cold stress. A Geographic Information System (GIS ) was used to map locations of thermal protection in the pasture relat ive to windspeed and topography. The 3-year-old cattle used unprotecte d areas more frequently than the 7- to 8-year-old cattle, and were in areas where the standard operative temperature was below their lower c ritical temperature more often than the older cattle. When grazing in unprotected areas, 7- to 8-year-old cattle used areas with higher stan ding crops than what was available, on average, in those areas (P < 0. 05). The 3-year-old cattle lost more backfat and weight than the older cattle. Apparently, 3-year-old cattle were less efficient at using th e pasture's forage and thermal resources than 7- to 8-year-old cattle, and were presumably cold-stressed more often.