Comparison of different protein evaluation systems for growing pigs: Digestible crude protein, and total, faecal digestible and ileal digestible amino acid intakes as performance response predictors

The validity of apparent faecal digestible crude protein and total, apparent faecal or ileal digestible amino acid intakes (lysine, threonine and methionine) as animal performance predictors was evaluated on the basis of digestibility coefficients obtained from the literature and performance and carcass parameter data from five different experiments on growing pigs. Correlations and regression equations were calculated between daily digestible crude protein or amino acid intakes and the performance and carcass parameters of the pigs. No connection was found between digestible crude protein intake and the performance of the pigs. The correlations were highest between different lysine intakes and daily gain (DG) (r = 0.808-0.867, p < 0.001). Ileal digestible lysine intake correlated with the performance of the pigs better than did intakes of total or faecal digestible lysine. The feed conversion ratio (FCR) (p < 0.05), and the thickness of back (BF) (p < 0.001) and side fat (SF) (p < 0.001) correlated highly with ileal digestible lysine intake. The correlations between these parameters and total or faecal digestible lysine intakes were lower but also significant (total lysine: BF p < 0.001, SF p < 0.01 and faecal digestible lysine: BF p < 0.01, SF p < 0.05). The regression equations agreed well with the coefficients of correlation. Ileal digestible lysine intake explained changes of performance and carcass parameters better than did intakes of total and faecal digestible lysine. The study confirms the advantage of using ileal digestibility coefficients of amino acids for detecting differences in the supply of amino acids from different feeds.