Graduate Thesis Or Dissertation
 

Responses of vegetation and sheep to three grazing pressures

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  • A study was conducted over a two year period, 1973 through 1974, on perennial ryegrass-subclover pastures with stocking rates of 7.4 (moderate), 9. 9 (heavy), and 12.4 (overstocked) ewes per hectare to establish guidelines on stocking intensity for dryland improved pastures in western Oregon. Climatic conditions varied annually and caused variation in growth of the vegetation and differences in stocking dates among treatments. Grazing significantly depressed total forage yields in each year. However, there was no significant difference noted between years for total forage yields. Ryegrass yields remained the same for both years in all treatments. The yield of other grasses (primarily annuals) increased in heavily stocked and overstocked treatments during the period of this study, but not in the moderately stocked pastures. Moderately stocked pastures had more ryegrass available per head per day than the other treatments in all seasons. Nutritive value of the herbage followed normal seasonal trends. Herbage in 1974 was of lower nutritive value than in 1973. Animals in moderately stocked pastures were able to obtain sufficient forage of good quality to meet or exceed dry matter intake, protein and energy requirements throughout both years. The data indicated that during the fall (1973) and summers (1973, 1974) diets of animals from overstocked pastures were deficient in dry matter intake, crude protein and digestible energy. Digestibility of dry matter in the diets increased from 77 percent both years in early spring to 55 percent during the summers; also, there was no difference among treatments. Annually, during early spring ewes on all treatments gained weight at the same rate. Ewes from moderately stocked pastures lost the least weight in the summer; ewes on the overstocked treatment lost the most. Weight gains of ewes and lambs were higher in 1973 than in 1974. There were no significant differences among treatments for the rate of gain of lambs. Likewise, no noticeable differences were observed in the total number of lambs born on each treatment. The results from the heavily stocked treatment were highly variable and sometimes they were similar to either the moderate or the overstocked treatment. Therefore, data indicate that 7.4 ewes per hectare may be the proper spring through fall stocking rate for these pastures.
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