Nitrogen and carbon flows between the caecum, blood and rumen in sheep given chopped lucerne (Medicago sativa) hay

1. Experiments involving 15N and 14C tracers were made in sheep consuming 800 g air-dry chopped lucerne (Merlicugo sativa) hay/d and providing 20.4 g N/d to study N and C flows within the caecal digesta and between the caecum, blood and rumen. 2. Continuous infusions of 15N tracers were made into the caecal ammonia, blood urea and rumen NH3 pools. The concentration and enrichment of caecal digesta NH3-N, caecal microbial N, caecal digesta non-urea. non-ammonia-N (NU-NAN), faecal NU-NAN, blood urea-N, rumen digesta NH3-N and rumen bacterial N were estimated at intervals during the infusions. A three-pool open-compartment model was solved to estimate N flows between the caecal digesta NH3-N, blood urea-N and rumen digesta NH3-N pools. 3. The rate of irreversible loss from the caecal digesta NH3-N pool was 2.17 (SE 0.623) g N/d. On average 0.9 (SE 0.56) g N/d of caecal digesta NH3-N was derived from blood urea and 0.1 (SE 0.08) g caecal digesta NH3-N/d was apparently derived from the fermentation of undigested rumen microbes in the caecum. The amount of NH3-N produced by proteolysis and deamination of dietary and endogenous N was 1.1 (SE 0.13) g/d. 4. There was net incorporation of 0.56 (SE 0.306) g caecal digesta NH3-N/d into caecal microbes. The microbial U synthesized de novo in the caecum was not determined, but 2.9 (SE 0.52) g microbial N/d of both rumen and caecal origin flowed out of the caecum and constituted 0.48 of the NU-NAN flow. The majority (mean 0.83 (SE 0.044)) of this microbial N was excreted in faeces. 5. On average 1.8 (SE 0.80) g caecal digesta NH3-N/d were absorbed. Of this NH3-N, 0.92 (SE 0.054) was converted to blood urea, contributing 0.10 (SE 0.031) of blood urea-N. Only 0.012 (SE 0.0041) of rumen digesta NH3-N and 0.005 (SE 0, 0009) of rumen bacterial N were derived from caecal digesta NH3-N. 6. Infusions of 14C tracers were made into the caecal digesta bicarbonate, blood bicarbonate, rumen digesta bicarbonate and blood urea pools, and samples were obtained at intervals to determine the specific radioactivity of each pool. A four-pool open-compartment model was solved to estimate C flows between these pools. 7. The rate of irreversible loss of blood urea estimated with [14C]urea (17.1 (SE 1.18) g N/d) was greater (P < 0.01) than that estimated with [15N]urea (14.0 (SE 0.87) g N/d). 8. Transfer of blood urea to the caecal digesta estimated with 14C tracers (1.4 (SE 0.61) g N/d) was greater (P < 0.01) than that estimated with I5N tracers (0.9 (SE 0.56) g N/d). The estimate of transfer of blood urea to the rumen digesta was also greater with 14C tracers (P < 0.05; 1.7 (SE 0.15) and 1.2 (SE 0.19) g N/d respectively). The urea hydrolysed in the gastrointestinal tract other than in the rumen digesta pool and the caecal digesta pool was 0.56 of total urea hydrolysis when estimated with 14C tracers, or 0.69 when estimated with 15N tracers. Results from previous acute experiments suggested that with three of the four observations made in three sheep in the present experiment the transfer of blood urea to the caecal digesta could have occurred entirely via ileal digesta. Similarly, urea transfer to the rumen digesta could have occurred entirely via saliva.