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Using a combination o f fistulation and the 1 5 N isotope technique, the f low rates o f endogenous N secreted postruminal into different sections o f the digestive tract were estimated in growing sheep by digesta exchange between 1 5 N-labelled and unlabelled animals. To investigate the influence o f fibre (CF) on these f low rates the estimations were made on two groups that received approximately isonitrogenous diets (16% CP in D M ) differing in crude fibre content (Group 1: 14.7%>, Group 2: 24.9% CF in D M ) . Supported by the Deutsche Forschungsgemeinschaft (DFG), the H . Wi lhe lm Schaumann Stiftung and by the Polish State Committee for Scientific Research 5 Corresponding author 606 N-METABOLISM ALONG THE GUT IN SHEEP The passage rates o f total N and 1 5 N at the duodenum and ileum and the excretion in faeces were measured directly. These data were used to calculate the passage o f postruminal N secreted in the different intestinal sections. The microbial N passage at the proximal duodenum and distal ileum was calculated as the product o f R N A passage at the duodenum and ileum, and the N : R N A ratio o f bacteria isolated from rumen f luid . The f low rates o f endogenous N into the duodenum and through the i leum and the excretion rate in faeces were estimated by measuring the appearance and disappearance rates o f 1 5 N in appropriate intestinal sections o f the unlabelled animals receiving 1 N-labelled digesta or in the labelled animals getting unlabelled digesta by digesta exchange. The microbial f low rate through the duodenum was in average 5.8 g N / d in Group 1 and 5.1 g N/d in Group 2 (50 and 46% o f N A N f lux) , and 0.9 g N / d (Group 1) to 1.3 g N / d (Group 2) through the ileum (19 and 22 % o f N A N flux) . The portion o f rumen undegraded protein-N o f N A N flow was in Group 1 47%) both in the duodenum and in the ileum. In Group 2 it was 42% . The endogenous N A N entering the duodenum was low for Group 1 (0.40 g N/d=3.4% o f N A N ) and approximately three-fold higher for Group 2 (1.3 g N/d=12% o f N A N ) . The corresponding values for endogenous N at the ileum were 1.6 g/d (34% o f N A N ) and 2.2 g/d (36 % o f N A N ) , indicating that in relative terms, endogenous N secretion in this section o f the digestive tract was independent o f the feeding regime. The excretion rate o f endogenous N in faeces varied between the individual experiments (1.84 to 2.39 g N / d for Group 1; 1.54 to 2.00 g N/d for Group 2). In relation to total faecal N excretion the endogenous fraction was higher for Group 1 (56%) than for Group 2 (40%). Dur ing ail o f the experiments, endogenous N as a proportion o f total N increased from the duodenum to the rectum. Despite some reservations about the , 5 N dilution method, its use together wi th the technique o f digesta exchange between labelled and unlabelled animals is a potentially useful way to quantify the passage o f postruminal secreted N in dependence o f nutritional factors. Further studies are needed to refine the methodology. K E Y W O R D S : sheep, digestive tract, digesta exchange, 1 5 N , endogenous N , microbial N , crude fibre

Studies on N-metabolism in different gastrointestinal sections of sheep using the digesta exchange technique. 2. Passage of endogenous nitrogen