Intestinal digestion of dietary and endogenous proteins along the small intestine of calves fed soybean or potato.

This study was conducted to investigate the quantitative and qualitative changes in dietary and endogenous protein components along the small intestine of calves. They were fed milk replacers in which protein was provided either by skim milk powder alone, as control diet, or by skim milk powder and a soy concentrate, a partially hydrolyzed soy protein isolate, or a potato protein concentrate (1:1 on a CP basis). The calves were continuously infused with the liquid milk substitute diets into the abomasum. Duodenal, jejunal, and ileal digesta were collected through T-piece cannulas. Digestibility (corrected for total endogenous protein) of dietary protein and the concentration of dietary, host endogenous, and bacterial proteins were estimated from the AA composition of digesta using multiple linear regression. The apparent digestibility of feed components increased linearly between the duodenum and the ileum. It was lower with the diets containing plant protein than with the control, especially at the ileum (85% for CP with soy concentrate, 73% with soy isolate, and 81% with potato concentrate vs 91% with skim milk powder). The real digestibility of dietary protein at the ileum was estimated to be 96, 95, 94, and 99%, respectively, for each of the dietary components. The partition of protein components in digesta between dietary, nonspecific endogenous, specific host endogenous, and(or) specific bacterial proteins varied along the small intestine and between diets at a given site. Intestinal apparent reabsorption of host endogenous protein was estimated to be equivalent to at least 86% of the jejunum flow. To conclude, the lower apparent digestibility of the plant protein sources studied resulted more from an enhanced loss of host and bacterial endogenous proteins than from decreased hydrolysis of dietary protein and absorption of their AA.