Submitted to: Professional Animal Scientist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/3/2011
Publication Date: 10/1/2011
Citation: Kirch, B.H., Moser, L.E., Waller, S.S., Klopfenstein, T.J., Klotz, J.L. 2011. Protein degradation of smooth bromegrass switchgrass and big bluestem in grazing cattle. The Professional Animal Scientist. 27:422-427. Interpretive Summary: Animal performance from warm-season grass pastures is often higher than would be predicted from common whole-plant forage quality parameters, such as IVDMD, CP, or fiber fractions. One explanation for this discrepancy in performance may be escape protein (EP), which seems to be prevalent at higher levels in warm-season grasses than in cool-season grasses. The objective of this study was to evaluate dietary EP levels in cool- and warm-season grasses selected by grazing ruminally cannulated beef animals at several plant maturities. Results indicated that selection ability of grazing livestock must be considered when evaluating production situations. Cool- and warm-season grasses seem to differ little in escape protein on a dry matter basis, thus supplementation for grazing warm season grasses should be directed at rumen degradable proteins to increase performance. It is essential to ascertain the stage of plant development that the animal is offered to maximize performance of supplement management. Late season grazing of regrowth pastures provides diets similar to diets of animals grazing pasture early in the growing season.
Technical Abstract: The objective of this two-year 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 two 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 three stages of plant development (vegetative, elongation, and reproductive) and one regrowth stage. Omasal samples were collected following total rumen evacuation to evaluate EP. Diet samples were also collected following rumen evacuation and a 45- min grazing period. Intake of grazing animals was estimated using indigestible ADF (IADF). Purine-N was used to quantify microbial contributions to total omasal-N. Total diet CP was higher (P<0.01) for SB than for SG and BB. Total diet CP was lowest (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 from vegetative to reproductive stages, while SB remained constant, even slightly increasing at reproductive stage. The EPDM of regrowth was comparable to elongation and reproductive stages. Escape protein as percentage of CP (EPCP) in SG and BB was higher (P<0.05) than in SB (43.2 vs. 24.1%). Maturity had little influence on EPCP. Microbial contributions to EP averaged 36.4% with this technique, with variation due to species and maturity.