Measures of protein quality for ruminant animals

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 synthesis at tissue level. The series of changes undergone by dietary protein between mouth and body tissue in the ruminant animal is illustrated schematically in Fig. 13.2. 

Satisfying the demands of the rumen microorganisms for readily available nitrogen is a major fimction 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 degradabUity 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 adds. The methods used to determine these components of the system are described next, after which their use in the systems will be illustrated. 

Nitrogen fractions within the diet will vary in their susceptibility to breakdown, from immediately degraded to undegradable, and from 0 to 1 in the extent to which they are degraded in the rumen and digested when they reach the small intestine (see Table 13.8). 

Degradability will be affected by such factors as the surface area available for microbial attack and the protective action of other constituents as well as the physical and chemical nature of the protein. Claims have been made that the solubility of a protein is correlated with ease of breakdown, but these do not survive critical examination. Thus, casein, which is readily degraded in the rumen, is not readily soluble whereas albumin, which is resistant to breakdown, is readily soluble. It has been suggested that a major factor affecting degradability is the amino acid sequence within the protein molecule. If this is so, then the nature of the microbially produced rumen peptidases is of considerable importance and it seems doubtful whether any simple laboratory test for degradability is possible. 

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