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 system includes a number of changes designed to represent the extent of protein degradation in the rumen and the synthesis of microbial protein as variable functions. The system also provides a more rational description of the energy available for microbial growth (fermentable metabolizable 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 metabolism. The assessment of availability of undegraded dietary protein in the small intestine is revised also, and variable efficiencies of utilization of absorbed AA in relation to the nature of the synthetic process (e.g., meat and milk) are proposed. This paper considers these changes and assesses the adequacy of current laboratory techniques for determining the protein value of feedstuffs for ruminants. A number of conceptual and technical problems are identified, and ways of overcoming them are discussed. We conclude that such systems must be used in practice with guarded enthusiasm and suggest that mechanistic models, which provide a more appropriate representation of the biological processes, be encouraged as the basis of the next generation of feeding systems.