Protein degradation of smooth bromegrass, switchgrass, and big bluestem in grazing cattle1

The objective of this 2-yr study was to estimate the influence of plant maturity on protein escaping ruminal degradation in steers grazing a cool-season grass, smooth bromegrass (Bromus inermis Leyss.; SB), and 2 warm-season grasses, switchgrass (Panicum virgatum L.; SG) and big bluestem (Andropogon gerardii Vitman; BB). Three ruminally fistulated steers strip grazed monocultures of SB, SG, and BB at 3 stages of plant development (vegetative, stem elongation, and reproductive). Omasal samples were collected after total rumen evacuation to evaluate escape protein. Diet samples were also collected after rumen evacuation and a 45-min grazing period. Purine-N was used to quantify microbial contributions to total omasal-N. Total diet CP was greater (P < 0.01) for SB than for SG and BB. Total diet CP was least (P < 0.01) at the reproductive stage for each grass. Escape protein, calculated as a percentage of DM (EPDM), was similar in SB, SG, and BB, with an average of 5.0% across maturities. The EPDM average of SG and BB decreased (P < 0.05) from vegetative to reproductive stages, whereas EPDM of SB remained constant, even slightly increasing at the reproductive stage. Escape protein as a percentage of CP (EPCP) in SG and BB was greater (P < 0.05) than that in SB (43.2 vs. 24.1%). Maturity had little influence on EPCP. Microbial contributions to escape protein averaged 36.4% with this technique, with variation due to species and maturity.