Journal of Animal Science, Vol 73, Issue 9 2760-2773, Copyright © 1995 by American Society of Animal Science

JOURNAL ARTICLE

Desirable characteristics of forage legumes for improving protein utilization in ruminants

G. A. Broderick
U.S. Dairy Forage Research Center, ARS, USDA, Madison, WI 53706, USA.

Forages help meet the protein requirements of ruminants by providing degraded CP for microbial protein synthesis plus protein that escapes ruminal degradation. Evidence from numerous feeding studies with lactating dairy cows indicates that excessive ruminal protein degradation may be the most limiting nutritional factor in higher-quality temperature legume forages. Hence, there is interest in identifying factors that influence the rate and extent of ruminal degradation of forage proteins. Condensed tannins found in legumes are known to decrease protein degradation, either by altering the forage proteins or by inhibiting microbial proteases. 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. 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 detected among alfalfa germplasm. Protein in alfalfa harvested as hay, rather than as silage, was used more efficiently for milk protein synthesis when fed to lactating cows; degraded CP from hay was captured more efficiently by ruminal microbes for protein synthesis in vitro. A ruminal escape of approximately 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%.

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