Digestion of plant protein sources in the preruminant calf. Quantitation of endogenous protein and importance of mucin

Abstract

Improving the digestive utilisation of alternative protein sources by the calf demands a better understanding of their interactions with the gut. The recent concepts of digestibility differentiate between the basal losses and the specific losses that depend on the diet. The aim of this work was to characterise the basal losses and those specific to plant proteins, and to specify the contribution of mucin. The flow of basal endogenous protein measured with a protein-free diet increased between the duodenum and the mid-jejunum. Endogenous protein was then reabsorbed at 70 % before the ileum where its flow was 20 g/kg dry matter intake. Mucin contributed to 20, 40 and 25 % of basal losses of nitrogen, lysine and threonine, respectively. Proteins from plant sources (a soya concentrate, a soya isolate, and a potato concentrate) were apparently less well-digested than milk protein at the ileum (0.85, 0.73 and 0.81 vs 0.91). However, plant protein sources had a similar real digestibility that was still lower than that of milk protein (0.95 vs 0.99 for milk). This was accounted for by differences in the amount of specific endogenous protein, and that of mucin in particular. Interestingly, the contribution of bacterial protein to the protein losses was more sensitive to dietary changes than the contribution of the host. To conclude, differences in apparent digestibility between the plant proteins studied were determined mainly by specific protein losses at the ileum they generated along the gut.