Influence of Feeding Level on Digestion and Protein Passage to the Duodenum in Cows Fed High Concentrate Diets

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Abstract
Dairy cows equipped with rumen and duodenal T-piece cannulae were given 7 diets each fed at two feeding levels, viz. approximately 7 and 14 kg DM per day, equivalent to 1.2 to 1.5 and 2.4 to 3.0 times energy maintenance requirement respectively. The diets consisted of app. 25% barley straw plus concentrates. The concentrates were soybean meal, beet-pulp-molasses, cottonseed cakes, fish meal and grass pellets with 18 or 12% protein. Cromic oxide and polyethylene glycol were given as indigestible markers. Twelve rumen, duodenal and faecal samples were taken to represent a 24-hour period. As a mean for the 7 diets, level of feed intake did not significantly influence neither the passage to the duodenum of non-ammonia-N (NAN) or amino acid-N (AAN) per kg dry matter intake, nor the calculated feed protein degradability in the rumen. Doubling of the feeding level resulted in a significant change in rumen pH from 6.59 to 6.47, in the digestibility of organic matter in the rumen from 37.1% to 39.8% and in the passage of starch + sugar to the duodenum from 11.8 g to 15.4 g per kg dry matter intake. The digestibility of crude fibre in the rumen, and the total digestibility of crude fibre and organic matter was as a mean unaffected by feeding level. Significant interactions between diet and feeding level were found for the passage of NAN and AAN to the duodenum, the calculated degradability of feed protein, the ruminal digestion of organic matter and crude fibre and the total digestion of crude fibre as well as for the passage of starch + sugar to the duodenum per kg dry matter intake.