Prediction of the apparent ileal digestibility of protein and amino acids in feedstuffs and feed mixtures for pigs by in vitro analyses

An in vitro method for the prediction of protein and amino acid digestibility at ileal level is described. Values of in vitro digestibility of protein in 15 common feedstuffs were all higher than corresponding values of apparent ileal digestibility. The difference is suggested to correspond to endogenous protein losses (EPL). A close relationship between EPL and in vitro undigested dry matter (UDM) was demonstrated: EPL, g kg−1 DM intake = 13.2 + 0.066 × UDM, g kg−1 DM (n = 15;r2 = 0.61; RSD = 4.5; CV = 17.1). The prediction of apparent ileal digestibility of protein (pdNileal) by using the equation: pdNileal, %=dNin vitro, % − 100 × (13.2+0.066×UDM, g kg−1 DM)/Proteinfeed, g kg−1 DM, was highly related to the in vivo determined ileal digestibility of protein (r2 = 0.92; RSD = 3.5; CV = 4.9) in the same 15 feedstuffs. The method was validated with 48 feed mixtures with known in vivo digestibility. The relationship was considerably lower (r2 = 0.57), which was partly due to the narrow variation range in protein digestibility in vivo. The in vitro digestibility of amino acids (measured in nine feedstuffs) was in general closely related to that of protein. Exceptions were cystine, arginine, aspartic acid, glutamic acid and proline. The endogenous losses of the individual amino acids were calculated in a similar way as described for EPL. The resulting amino acid composition of the endogenous protein was very close to reported values in the literature based on direct measurements in vivo. Apparent ileal digestibility of the individual amino acids was predicted in a similar way as described for protein. The relationship was generally higher for essential amino acids, and generally lower for non-essential amino acids, than for protein (e.g. r2 values of 0.65, 0.71 and 0.77 was obtained for lysine, leucine and isoleucine, respectively, but 0.34 and 0.14 for glutamic acid and proline, respectively). The described in vitro approach for protein evaluation seems to have a significant potential for practical purposes.