Nitrogen secretion and absorption in different segments of the digestive tract estimated by digesta exchange between N-15-labelled and unlabelled sheep

Three male sheep of about 25 kg body weight fitted with a cannula into the rumen and reentrant cannula in the proximal duodenum and in the distal ileu m were used for this study. Sheep No. 1 was infused intraruminally with N-1 5 urea (1g/d, 95 atom% N-15) for labelling with N-15, Nos. 2 and 3 with unl abelled urea. All animals were fed 800 g DM/d of a hay and concentrate diet (40 : 60) containing 15.4% crude protein and 15.3% crude fibre, divided in to 6 feeding portions per day. After reaching a quasi-steady state in the N -15 level of animal No. 1, passage of total N and N-15 throughout the duode num and the ileum, N excretion in faeces and urine were estimated. On day 7 and 8 of the experiment the duodenal and ileal digesta were exchanged betw een the labelled animal (No. 1) and the unlabelled ones (No. 2, 3) accordin g to the respective scheme. Digesta flow was measured directly during 48 h and 3% aliquots were sampled for analysis of N and N-15 content. The conten ts of N and N-15 were also estimated in faeces, urine and blood. The obtain ed data were used for calculation of the passage and absorption of exogenou s N and the secretion, absorption and reabsorption of endogenous N in three digestive sections from mouth to proximal duodenum, from duodenum to ileum , and from ileum to rectum. The flow of total N, exogenous N and endogenous N decreased from duodenum t o ileum and faeces, but the relative content of endogenous N increased from 36 to 54%. N-secretion was highest in the first section of the digestive t ract (9.9 g N/d = 60% of the secretion in total digestive tract) and absorp tion was highest in the second section (14.1 g N/d = 46% of total absorptio n). Endogenous origin N was about 54% of faecal N. Faecal loss, 2.0 g endog enous N/d, was only about 12% of the total endogenous N (16.5 g N) secreted into the lumen during the intestinal passage of digesta. Total reabsorptio n of endogenous N amounted to 88%. The results demonstrate the high dynamic s of N metabolism during digestion in ruminants.