Protein evaluation for ruminant animals

Traditionally, proteins in foods for ruminant animals have been evaluated in terms of crude protein (CP) or digestible crude protein (DCP). Realization that the crude protein fraction contained variable amounts of non-protein nitrogen led to the use of true protein instead of crude protein but this was unsatisfactory since no allowance was made for the nutritive value of the non-protein nitrogen fraction. The concept of protein equivalent (PE), introduced in 1925 but now no longer used in this context, was an attempt to overcome this difficulty by allowing the non-protein nitrogen fraction half the nutritive value of the true protein. The term `protein equivalent' is currently used in connection with foods containing urea. Such foods must by law be sold with a statement of their content of protein equivalent of urea. This means the amount of urea nitrogen multiplied by 6.25.
The use of DCP for evaluating food proteins for ruminants has been largely abandoned. This resulted from a growing awareness of the extensive degradative and synthetic activities of the microorganisms of the rumen. Rumen microorganisms are responsible for providing the major part of the energy requirements of the host animal by transforming dietary carbohydrates to acetate, propionate and butyrate. In order to do this and to exploit the energy potential of the food fully, they must grow and multiply and this involves large-scale synthesis of microbial protein. The nitrogen for this is obtained, in the form of amino acids, peptides and ammonia, by breakdown of the nitrogen fraction of the food. Bacteria acting on the structural carbohydrate (SC) fraction of the diet use only ammonia, whereas those acting on the non-structural fraction (NSC) derive about 65 per cent of their nitrogen from amino acids and peptides, and the remainder from ammonia.
The microbial protein passes from the rumen, is digested in the small intestine, and so makes a contribution to satisfying the nitrogen require¬ments of the host animal. The magnitude of this contribution depends upon the speed and extent of microbial breakdown of the dietary nitrogen fraction, upon the efficiency of the transformation of the degraded material into microbial protein (nitrogenous compounds), the digestibility of the microbial protein and the biological value of the latter.
The degradative and synthetic processes taking place in the rumen are of major importance in the nitrogen economy of the host animal since they determine the nature of the amino acid mix made available for protein syn¬thesis at tissue level. Satisfying the demands of the rumen microorganisms for readily available nitrogen is a major function of the diet and to this end a certain proportion of the nitrogen fraction must be degradable by the rumen microorganisms.
Current systems for the evaluation of food protein for ruminant animals involve determinations of the degradability of protein in the rumen, the synthesis of microbial protein, the digestion in the lower gut of both food and microbial proteins, and the efficiency of utilisation of absorbed amino acids. The methods used to determine these components of the system are described next, after which their use in the systems will be illustrated.