Studies on the effect of dietary crude protein supply on the composition of ileal endogenous crude protein loss in growing pigs

Two experiments were carried out to investigate endogenous crude protein (N × 6.25) (CP) losses in growing pigs. In experiment I, 42 cross-bred gilts [live weight (LW) 35-40 kg] were fitted with a simple T-cannula at the terminal ileum. Seven experimental diets (four feed-stuffs: barley, wheat, soyabean meal and rapeseed meal, and three mixtures with varying compositions of the same feedstuff batches) were investigated. Each diet was fed at six different CP levels, obtained after dilution with a 1:1 mixture of sucrose and dextrose. In total, 42 different diets were used and on each pig three different diets were tested. In experiment II, 12 (LW ≃ 18 kg) plus eight (LW ≃ 88 kg) cross-bred castrates were fitted with a simple T-cannula at the terminal ileum. Four experimental diets based on casein as the sole protein source and with CP dietary levels from 6 to 25% were used. By extrapolation to zero CP intake the basal endogenous protein loss in experiment I was 15.7 g kg^-1 dry matter (DM) intake, when including all diets, and 14.3 g kg^-1 DM intake after excluding the barley diets. In experiment II the basal endogenous CP loss was 11.3 g kg^-1 DM intake. In experiment I a relative increase in almost all amino acids in the endogenous CP loss was found when the dietary CP level in the diets increased from zero to 17%. Exceptions were glycine, which was constant, and proline, which decreased significantly from 176 to 58 g 160 g^-1 CP. However, the sum of proline and glutamic acid in the endogenous protein loss was almost constant. Similar results were obtained after correction for undigested dietary CP and amino acids, based on in vitro analyses of CP digestibility. In a supplementary study, ileal samples, selected for unusually high levels of proline were analysed for free amino acids. For proline, 36-64% was free, whereas insignificant proportions of the other amino acids were free. It is concluded that the composition of endogenous CP is generally relatively constant. However, experimental conditions with low or inappropriate dietary CP supply may influence the composition and occasionally induce unexpected high levels of proline from secretion of free proline into the lumen of the digestive tract.