Nitrogen metabolism in sheep. I. The effect of protein and energy intake on the flow of digesta into the duodenum and on the digestion and absorption of nutrients
The digestion of organic matter, nitrogen, and sulphur was measured in sheep fitted with re-entrant duodenal cannulae. The sheep were given 1.4, about 16, or 30 g of nitrogen per 24 hr from barley roughage with or without a nitrogen supplement of lucerne, wheat gluten, or casein. A further diet supplied 16.1 g of nitrogen per day but consisted of 83 % low-quality roughage compared with 50% in the other diets. The flow of digesta to the duodenum varied from 4.4 to 6.8 1. per 24 hr with dry matter intakes of 480–800 g per 24 hr. The sheep were fed hourly and watered continuously, and there was a more than 10-fold variation in the flow of digesta to the duodenum over 2-hr collection periods. The digestibility of organic matter was lower for the diet supplying 1.4 g of nitrogen per day than for the other diets. Between 77 and 96 % of the apparent digestion of organic matter occurred before the duodenum. In the sheep given 1.4 g of nitrogen per 24 hr, the flow of nitrogen to the duodenum was 4.8 g per 24 hr, while in the sheep given about 16 g of nitrogen daily the flow of nitrogen to the duodenum was 8.4–11.8 g per 24 hr. Only 35 % of the dietary intake of nitrogen reached the duodenum in the sheep given 30 g of nitrogen daily. The sheep given the diet containing 83 % low-quality roughage showed a lower retention of nitrogen than those given the other diets of similar nitrogen content. Between I and 9 % of the flow of nitrogen to the duodenum was in the form of ammonia. The flow of sulphur to the duodenum was between 33 and 55% of the dietary intake, while 72-99% of the apparent digestion of sulphur occurred before the duodenum. The digestibility of dietary sulphate sulphur was greater than that of the sulphur found in lucerne.