Postruminal degradation of crude protein, neutral detergent fibre and starch of maize and grass silages in dairy cows

The Dutch feed evaluation system for ruminants uses assumptions and regression equations to estimate the intestinal digestibility of crude protein (CP), neutral detergent fibre (NDF) and starch. These assumptions and equations are based on many different studies, obtained over a very long period. The objective of this study was to develop a unique dataset on the ruminal degradability and the postruminal digestibility of CP, NDF (aNDFom, amylase neutral detergent fibre organic matter basis) and starch in maize and grass silages, using the mobile nylon bag technique. Twenty samples of maize silage and 20 samples of grass silage were used. The samples were selected to represent a broad range in digestibility and chemical composition. Prior to the intestinal incubations, samples were incubated in the rumen for 6 h (starch), 12 h (CP) or 24 h (aNDFom) using the rumen nylon bag technique. Residues from the rumen incubations were transferred to mobile nylon bags and inserted in the duodenum through a cannula. Half of the bags for CP and starch were collected from the ileal cannula and the remaining half of the bags from the faeces. For aNDFom, all the bags were collected from faeces. There was a large variation in the rumen degradability and the intestinal digestibility (small and/or large intestine) of CP, aNDFom and starch. The rumen degradable fractions, the intestinal digestible fractions and the total tract undigested fractions of CP, aNDFom and starch were influenced by their proportions in the maize and grass silages. The results proved the assumption of the Dutch feed evaluation system that the rumen undegraded starch is completely digested in the small intestine of dairy cows. Regression showed that the rumen degradability, the intestinal digestibility and the total tract undigested contents were influenced by the chemical composition of the maize and grass silages.