• Cattle Grazing Blue Grama Rangeland. I. Seasonal Diets and Rumen Fermentation

      McCollum, F. T.; Galyean, M. L.; Krysl, L. J.; Wallace, J. D. (Society for Range Management, 1985-11-01)
      Four field trials were conducted from early August to late October, 1982, on blue grama (Bouteloua gracilis) rangeland in south-central New Mexico to examine relationships among grazing season, diet botanical and chemical composition, and rumen fermentation in beef steers (Bos taurus). Diets contained an average of 83% grasses and 17% forbs from early August through late September and 77% forbs in late October. Cell wall content of the diet decreased from the early growing season (74.9%) through the onset of dormancy (64.9%) while acid detergent fiber and lignin increased (41.9 to 52.9% and 5.2 to 12.7%, respectively) and crude protein content declined from 18.4 to 11.7%. Soluble and insoluble nitrogen (N) fractions of the diet reflected crude protein; from 13 to 36% of N was in unavailable forms. The extent of in vitro organic matter digestion declined from the early growing season (66.5%) through onset of dormancy (47.9%). Ruminal ammonia concentrations declined as season progressed: 6.0 mg/100ml was the lowest concentration observed. Declining diet quality was accompanied by an upward shift in digesta pH and altered proportions of volatile fatty acids (VFA) in rumen contents. Total VFA concentration was highest in late August (106.3 mmoles/liter). Ruminal measures generally reflected changes in dietary protein and digestibility but concentrations could also reflect changes in digesta flow rates. Finally, data suggest that crude protein may not be a good measure of protein supply to livestock grazing on ranges with diverse forage types.
    • Cattle Grazing Blue Grama Rangeland. II. Seasonal Forage Intake and Digesta Kinetics

      McCollum, F. T.; Galyean, M. L. (Society for Range Management, 1985-11-01)
      Four field trials were conducted from early August to late October, 1982, on blue grama (Bouteloua gracilis) rangeland in south-central New Mexico, to examine relationships among grazing season, forage intake and digestive function in beef steers (Bos taurus). Organic matter intake declined from 24.0 g/kg body weight (BW) in early August to 17.4 g/kg BW in late September and then increased to 20.9 g/kg BW in the early dormant season (late October). From early growing season to early dormancy, diet digestibility and passage rates decreased while retention time of digesta in the rumen increased. Rate and extent of in vitro organic matter disappearance were 66.5, 6.7; 63.1, 6.7; 51.6, 4.8; and 47.9%, 5.6%/hour in early August, late August, late September and late October, respectively. Fluid and particulate passage rates (%/hour) were 24.9, 4.6; 12.7, 3.9; 11.1, 3.7; and 10.5, 3.5, respectively, for the same periods. Retention of particulate digesta varied from 26.1 hours in the early growing season to 34.3 hours in early dormancy. Gastrointestinal fill gradually increased as season progressed. Results suggest that maintenance of a diverse plant community, containing not only desirable grasses but also palatable forbs, may allow cattle to maintain a higher level of nutrient intake during periods of grass dormancy.