Effect of the roughage/concentrate ratio on nitrogen entering the small intestine of dairy cows.

Abstract

The effect of different roughage:concentrate ratios on N entering the small intestine was studied in groups of 2 to 5 cows with a rumen fistula and re-entrant cannulae at the beginning of the small intestine. In 3 series there were 10 treatments in which DM intake ranged between 3.8 and 15.7 kg daily; the proportion of long roughage in the diet was from 29 to 81%. The apparent digestibility of organic matter (O), crude fibre (XF) and nitrogen-free extracts (XX) was 76, 69 and 81%, respectively. The proportion of the digestion taking place in the stomach was 59, 94 and 76%. A larger proportion of the apparently digestible organic matter tended to be digested in the stomach when there was a larger proportion of long roughage in the diet. The contribution of microbial N to the intestinal N was estimated. From this the efficiency of microbial protein synthesis was estimated and related to the amount of carbohydrates (XF + XX) fermented in the stomach. Per kg carbohydrates fermented, 32 plus or minus 1.5 g microbial N was produced. The percentage of dietary N not degraded in the stomach averaged 30 plus or minus 1.5. Varying the ratio between long roughage and pelleted concentrates seemed to have little effect on the degradation of dietary protein or on the efficiency of microbial protein synthesis. With long roughage in the diet between 29 and 81% there seems little effect of varying the roughage:concentrate ratio on the protein supply in dairy cows. (Abstract retrieved from CAB Abstracts by CABI’s permission