MEETING THE PROTEIN-REQUIREMENTS OF RUMINANT LIVESTOCK

Ruminant animals acquire their amino acids from the small intestinal d igestion of ruminally derived microbial protein and dietary protein wh ich has escaped ruminal degradation. The UK metabolizable protein syst em provides a framework with which the net absorption of amino acids f rom the small intestine is computed in relation to the animal's requir ements and is based largely upon a set of criteria by which feedstuffs are evaluated. While conceptually the system has many positive featur es, a number of serious weaknesses have been identified, particularly with respect to the techniques used to evaluate feedstuffs. Some of th ese issues are considered, particularly the over-reliance which is pla ced upon in vitro methodologies which have not been adequately validat ed against in vivo observations and have been shown to give unacceptab le variation. Attention is also drawn to the inadequate representation of host tissue metabolism within such systems, for example, the impor tance of splanchnic metabolism with respect to overall nutrient utiliz ation. It is concluded that improved representation of energy (i.e. ca rbohydrate) and protein interactions within the whole animal and not s imply within the gastro-intestinal tract is urgently required; this in evitably will lead to a change in research direction. Equally, support for a mechanistic understanding rather than an empirical representati on of protein metabolism in ruminant livestock is presented, given tha t the demands upon the ruminant industry, particularly with respect to predictability of animal response, is likely to increase as a consequ ence of increased consumer impact on the marketing of animal products.