In vivo milk digestion in the calf abomasum. III. Amino acid compositions of the digesta leaving the abomasum.

Calves were fed five different test meals: whole milk, skim milk, 3% whole casein solution, 3% whole casein in simulated milk ultrafiltrate, and whey. The digesta leaving the abomasum before feeding and during the first 7 postprandial hours were collected by fractions. After precipitation with 12% TCA, the amino acid compositions of the sediments and the supernatants were determined and compared by multivariate analysis. The composition of prefeeding digesta was similar to that of gastric juice. When the calves were fed the two casein diets, the amino acid composition of the sediments changed little with time. In contrast, the changes observed in the composition of the supernatants suggested rapid abomasal emptying of caseino-macro peptide. With the whey diet, it was not possible to evidence more rapid hydrolysis or abomasal emptying of any particular whey protein. During the first 10 min following the ingestion of whole or skim milk, the amino acid composition of the sediment was close to that of milk protein. Immediately afterwards, the composition of the sediment was similar to that of whey protein. Thereafter, the composition of the sediment became more like that of casein and almost reached that of casein during the 7th hour. The amino acid composition of the supernatant was similar to that obtained with the casein diets; this fact suggests that the small peptides produced by proteolysis in the abomasum originated more from casein than from whey proteins.