Post-ruminal digestibility of crude protein from grass and grass silages in cows

Abstract

Grass samples were grown on a clay or sandy soil, fertilised with 150 or 300 kg N/ha per year, and harvested on different days during two consecutive growing seasons. The grass samples were stored frozen or ensiled after wilting to approximately 250 or 450 g DM/kg. The recoveries of crude protein (CP) after passing the rumen, the intestines and the total gastro-intestinal (GI) tract in grass and grass silages were determined with the two-step mobile nylon bag technique and a two-step in vitro technique, mimicking the rumen and intestine. Mobile nylon bag experiments were performed with 9 grass samples and 18 silages of the same grasses, pre-wilted to 250 or 450 g DM/kg. The in vitro experiments were performed with a series of 112 samples (54 grass and 58 grass silage samples). Both the in situ and in vitro results showed that the intestinal recoveries of CP were largely influenced by the amount of rumen-undegraded protein (RUP). A high value of RUP was compensated by an enhanced digestibility in the small and large intestine, resulting in a rather constant recovery of CP after passage through the total gastro-intestinal tract. It is suggested that residual CP after total tract passage is protein that is interwoven in the cell wall construction, cell wall bound or of microbial origin.