SYMPOSIUM - PROTEIN SYSTEMS FOR FEEDING RUMINANT LIVESTOCK PROTEIN SYSTEMS FOR FEEDING RUMINANT LIVESTOCK - A EUROPEAN ASSESSMENT

The metabolizable protein system for ruminants, published recently in the United Kingdom (2), was based upon earlier proposals (3, 4). The s ystem includes a number of changes designed to represent the extent of protein degradation in the rumen and the synthesis of microbial prote in as variable functions. The system also provides a more rational des cription of the energy available for microbial growth (fermentable met abolizable energy) by discounting the energy content of dietary lipids and fermentation end products (silages), which are considered to make a relatively insignificant contribution or none to microbial metaboli sm. The assessment of availability of undegraded dietary protein in th e small intestine is revised also, and variable efficiencies of utiliz ation of absorbed AA in relation to the nature of the synthetic proces s (e.g., meat and milk) are proposed. This paper considers these chang es and assesses the adequacy of current laboratory techniques for dete rmining the protein value of feedstuffs for ruminants. A number of con ceptual and technical problems are identified, and ways of overcoming them are discussed. We conclude that such systems must be used in prac tice with guarded enthusiasm and suggest that mechanistic models, whic h provide a more appropriate representation of the biological processe s, be encouraged as the basis of the next generation of feeding stems.