DIFFERENT EXPRESSIONS OF DIETARY-PROTEIN AND AMINO-ACID DIGESTIBILITYIN PIG FEEDS AND THEIR APPLICATION IN PROTEIN EVALUATION - A THEORETICAL APPROACH

There is a distinction between the terms apparent, true, real, and ''n et'' ileal protein and amino acid digestibility. For low-protein feeds tuffs, ''apparent'' values relate directly to the feedstuff and are ac curate estimates of net digestibility. For diluted protein-rich feedst uffs, apparent values underestimate the net digestibility, which, howe ver, can be obtained from apparent digestibility by mathematical extra polation. ''True'' digestibility values have traditionally been obtain ed by correcting ileal digesta flows for endogenous protein determined after feeding an N-free mixture. Such values are not affected by the protein content of the test diet (degree of dilution) but are generall y underestimated and influenced by the amount of endogenous protein lo ss induced by the N-free mixture. True digestibility determined using enzyme hydrolysed casein includes the minimal or ''basal'' endogenous loss induced by protein-containing feedstuffs and is, by definition, n ot influenced by different N-free mixtures used to dilute the experime ntal mixtures. Real digestibilities (including in vitro measures) are direct measures of the digestibility of a feed protein, and are not in fluenced by endogenous loss. Values for apparent and real protein and amino acid digestibility are not directly suitable for use in practica l protein evaluation systems but those for net or true digestibility c an be used. Both sets of values can be calculated from determinations of either real or apparent digestibility. For net digestibility, total endogenous protein loss is included in the digestibility coefficients . For true digestibility, only the extra endogenous loss is included i n the digestibility coefficients while the basal endogenous loss needs to be standardized and included in the estimate of amino acid require ments.