Whey protein digestion in the distal ileum of the preruminant calf

Abstract

Ileal digestion of three milk replacers containing either total milk protein or a mixture (50 : 50 on a crude protein basis) of total milk protein and native or heated whey protein concentrate was studied in six preruminant calves fitted with ileo-caecal re-entrant cannulas. Ileal digesta were collected for 4 days. Apparent nitrogen digestibility was significantly lower for the diets containing native or heated whey proteins than for the control (0.89, 0.90 and 0.92, respectively). Digestibility was also lower for most amino acids (AA), and especially for glutamic acid, proline and glycine, but tended to be higher for cystine. Replacement of the control diet by the whey diets resulted in slight but significant changes in the proportions of aspartic acid, leucine, tyrosine, histidine and arginine in the digesta. In contrast, treatment of whey protein did not alter the AA composition of the digesta. Iterative comparisons of AA profiles suggested that the major part of the protein escaping digestion in the small intestine for all diets was of endogenous origin. However, for the whey diets, a small peptide resulting from whey protein hydrolysis could constitute an important part of the undigested protein present in the whey digesta in comparison with the control digesta. This suggested that some whey fractions were probably not totally digested in the intestine. In conclusion, moderate heat treatment had no pronounced effect on whey protein digestion, and whey protein seemed to be satisfactorily digested in the small intestine of the preruminant calf.