Submitted to: Journal of Animal Science
Publication Type: Review Article
Publication Acceptance Date: 3/31/1996
Publication Date: N/A
Citation: Interpretive Summary:
Technical Abstract: Forages help meet the protein needs of ruminants by providing degraded CP for microbial protein synthesis plus rumen undegraded protein. Evidence from numerous feeding studies with lactating cows indicates that excessive ruminal protein degradation is the most limiting nutritional factor in high quality temperate legume forages. Hence, it is important to identify the factors influencing the rate and extent of ruminal degradation of forage proteins. Condensed tannins in some legumes decrease protein degradation; quadratic regressions of degradation rate and estimated protein escape on tannin concentration reached minimal rate (.048/h) and maximal escape (56%) at 27 g of tannic acid equivalents/kg of DM, respectively. Although most tannin containing forages are not well adapted to growing conditions in North America, biotechnology has been used to inject genes for tannins into adapted germplasm. The CP in red clover, which has no detectable tannins, was found to be less degradable than that in alfalfa, both in the silo and in the rumen. Small differences in protein degradability also were found among alfalfa germplasm. Protein in alfalfa hay was used more efficiently than that in alfalfa silage for milk protein synthesis in lactating cows; ruminal microbes captured degraded hay CP more efficiently for protein synthesis in vitro. A ruminal escape of 35% for total dietary CP is recommended by the NRC for lactating dairy cows fed mixed diets with 1.6 to 1.7 Mcal of NE1/kg of DM. Ruminal degradation of CP from the forage portion of the diet can exceed 65% when forages are the major source of degradable protein. When ruminants obtain most or all of their nutrients from forage, the ruminal escape for forage protein should approximate 35%.