W.F. BROWN and W.D. PITMAN

Agricultural Research and Education Center, University of Florida, Ona, Florida, U.S.A.

Abstract

Newer systems of protein evaluation partition feedstuff N into the proportion degraded in the rumen and that which escapes ruminal degradation. Protein and fibre degradation characteristics were measured in the grasses Pensacola bahiagrass (Paspalum notatum) and Bigalta limpograss (Hemarthria altissima), and the legumes aeschynomene (Aeschynomene americana) and hairy indigo (Indigofera hirsuta). Nitrogen concentration of legumes (30–39 g/kg) was greater than that of grasses (7–13 g/kg). Legumes contained greater absolute amounts of N soluble in a buffer solution, and potentially ruminally degradable N than grasses. Ruminal degradation rate of the potentially degradable N fraction was greater in legumes (24–44%/hr) than in grasses (6–18%/hr), leading to a greater estimated escape N as a percentage of total N in grasses (12.8–25.0%) than in legumes (6.6–11.8%). Major differences in cell wall structure between grasses and legumes occurred in hemicellulose (HQ concentration, with legumes containing much less HC than grasses. Nitrogen in these tropical grasses and legumes appears to undergo rapid and extensive ruminal degradation. Although ruminal degradation of N in these grasses was extensive, low absolute quantities of ruminally soluble and degradable N may limit microbial protein synthesis in ruminants fed tropical grass diets. Legume addition to tropical grass diets may enhance digestion by providing N for rumen function, but a combination of ruminally degradable and escape proteins may be required for optimal animal performance.