Protein degradation of warm-season grass and the effect of forage availability on intake, selectivity, and protein degradation of cattle grazing smooth bromegrass

Supplement needs of growing beef cattle grazing warm-season grass were determined in a two-year grazing study. Cattle were supplemented with escape protein, rumen degradable protein, or a combination of the two protein types while grazing a mixed stand of primarily switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii (Nash) Fern.). Forage protein degradation was determined at 16 hours in situ. Pulse and continuous dosing of ytterbium or erbium were compared in two trials. Continuous dosing of ytterbium was used to estimate fecal output in a one-year study of the effect of forage availability on intake, selectivity, and rumen protein degradation of smooth bromegrass (Bromus inermis Leyss.). Cattle grazing warm-season grass produced more gain when supplemented with rumen degradable protein or the combination of protein types when compared to the controls. Escape protein in the forage was approximately 50% of the crude protein. Rumen ammonia was often less than 5 mg/100 ml in unsupplemented animals. Intake of smooth bromegrass reached a maximum at a forage availability between 500 and 1500 kg/ha. As forage availability increased, selectivity increased for crude protein and in vitro dry matter digestibility. However, selectivity was not able to fully compensate for reduced whole-plant crude protein at high forage availabilities. Selectivity of neutral detergent fiber was approximately 10 percentage units lower than the standing forage and did not seem to be affected by forage availability. Cattle selected a diet that averaged 63% in vitro dry matter digestibility for all forage availabilities. Only 17 to 25% of the crude protein passed intact to the small intestine