Estimation of the true ileal digestibility of amino acids and nitrogen from their apparent values for growing pigs

Abstract

An equation was calculated to estimate the true ileal digestibility for individual amino acids from their apparent digestibility; true digestibility = apparent digestibility + endogenous ileal output of individual amino acids (g kg −1 dry matter (DM) intake) / dietary individual amino acid content (g kg −1 in DM). In Experiment 1, endogenous ileal levels of amino acids and nitrogen were determined by regression analysis with pigs fed on diets containing casein as the sole protein source or on a proteinfree diet. The two methods gave similar estimates. Threonine, leucine and arginine were the most prominent indispensable amino acids, while glycine was the most prominent dispensable amino acid. In Experiment 2, apparent and true ileal digestibility were determined at 80 g or 160 g kg −1 crude protein (CP) in diets with either soya-bean meal or cotton-seed meal as the sole protein source. For all the amino acids and nitrogen studied, the apparent ileal digestibility measured with 160 g CP kg −1 tended to be higher than that measured with 80 g CP kg −1. The apparent values for 80 g and 160 g CP kg −1 for the average of all the amino acids were 0.78 and 0.82, respectively, in soya-bean meal, and 0.68 and 0.76, respectively, in cotton-seed meal. Of the indispensable amino acids, differences in digestibility measured at different CP levels were largest for methionine and threonine, at 0.06 in soya-bean meal, and for threonine at 0.10 in cotton-seed meal. In contrast to the apparent digestibility values, the true values were similar, irrespective of dietary CP levels. It is concluded that the true ileal digestibility of amino acids and nitrogen is a more accurate measure than the apparent ileal digestibility and that the true digestibility can be estimated from the apparent digestibility with the equation given above.