Digestibility and amino acid composition of digesta at the end of the ileum in preruminant calves fed soyabean protein.

The ileal digestion of 3 milk substitutes in which skim milk powder was either the only protein source (control diet) or was partially replaced (50%) by a heated soybean flour (SF diet) or an alcohol-treated soyabean protein concentrate (SC diet) was studied in 6 preruminant calves which were fitted with an ileo-caecal re-entrant cannula. The apparent digestibility of total nitrogen and amino acids was lower with the SF and SC diets than with the control diet (0.89, 0.89 and 0.94, respectively, for amino acid nitrogen). Assuming that true digestibility was complete with the control diet, the values were lower with the SF and SC diets, especially for cystine, threonine, valine, isoleucine, leucine and histidine. With the SF and SC diets, digesta contained more aspartic acid and glutamic acid but less threonine, sulfur amino acids, lysine, serine and alanine than with the control diet. The additional undigested fractions obtained with the SF and SC diets compared to the control diet were rich in aspartic and glutamic acids, and poor in arginine, suggesting that partially degraded dietary fractions different from the whole soyabean escaped digestion in the small intestine; these fractions probably originated mainly from glycinin. No significant differences were observed between the 2 soyabean diets.