PROTEIN AND ENERGY-UTILIZATION BY RUMINANTS AT PASTURE

Low live weight gain of cattle in the wet season of tropical areas was identified as a major limitation to achieving annual growth rates fro m tropical pasture systems sufficient to meet new market specification s of young animals of high carcass weight. Both protein and energy are limiting nutrients for growth. Net transfer of feed protein to the in testines is often not complete, and losses occur with grasses and legu mes when CP content exceeds 210 g of CP/kg of digestible OM. This prot ein loss is important because a collation of experimental data indicat ed that cattle consuming low- and high-quality pasture and silage-base d diets all responded to extra protein. The response was less for the higher-quality forage. The role of legumes in supplying this protein w as investigated and, unless legumes can increase total DMI by at least 30%, they will not supply sufficient intestinal protein to increase l ive weight gain by about 300 g/d. The problem with legumes and some gr asses is the loss of protein from the rumen, and increasing energy sup ply to the rumen, either through improved digestibility or energy supp lements, is a strategy that could be used to reduce this. Strategies t o increase the proportion of escape protein would be successful, but i ncorporation of lowly degradable protein fractions into legumes may be more difficult because of the level of expression of these protein fr actions required for a significant live weight gain response. Cattle e ntering the wet season usually exhibit compensatory growth and are exp osed to high ambient temperatures and often to high humidity. Intestin al protein above that stipulated in feeding standards may be beneficia l in these circumstances, and more emphasis should be placed on the ab ility of legumes to supply protein postruminally. At present the prote in delivery capacity of agronomically competitive legumes seems to be inadequate for the higher growth rates required in production systems, and supplements of energy and protein will be needed to achieve these higher targets until new cultivars appear.