Studies of the large intestine of sheep

Abstract

1. A study was made of nitrogen kinetics in the large intestine of sheep given 800 g chopped lucerne (Medicago sativa) hay/d. Four sheepwere continuously infused with (15NH4)2SO4into the caecum and three other sheep were infused intravenously with [15N]urea. A digesta marker,51Cr complexed with EDTA (51Cr-EDTA), was infused into the rumen of each sheep to allow estimation of the rates of flow of digesta constituents. Infusions were continued until tracer concentrations reached plateaux in digesta and blood pools, after which the sheep were anaesthetized and slaughtered.2. Pre-infusion samples and samples on plateau were obtained before slaughter for subsequent analysis to give plasma urea and rumen ammonia-N concentration and enrichment. At slaughter, digesta were obtained from the ileum and segments of the large intestine. These were analysed for51Cr-EDTA content and concentration and enrichment of ammonia-N, microbial N and non-urea non-ammonia-N (NU-NAN).3. N flows in segments of the large intestine were calculated and represented in a quantitative eight-pool model.4. Transfer of plasma urea across the wall of the caecum and proximal colon was negligible but there was an input of 0·8 g endogenous NU-NAN/d.5. Flow of urea plus ammonia-N in digesta from the ileum into the caecum contributed 1·0 g N/d to the caecal ammonia pool.6. Proteolysis and deamination produced a further 3·0 g ammonia-N/d in the caecum and proximal colon.7. The net absorption of N between the ileum and the rectum was 2·8 g N/d but 3·0 g ammonia-N/d was absorbed from the caecum and proximal colon and, in addition, at least 0·9 g ammonia-N/d from the distal colon and rectum.8. Ammonia-N was incorporated into caecal microbes (0·6 g N/d) and approximately 57% of the NU-NAN in caecal digesta was microbial N. The majority of the microbial N flowing from the caecum was excreted in faeces.9. The rate of irreversible loss of urea-N from plasma, measured by intravenous infusion of [15N]urea, was 13·6 g/d. On average 83 (SE 6·8)% of the15NH3apparently absorbed from the caecum was incorporated into plasma urea; caecal ammonia contributed 9–19% of the N in plasma urea and 0·2–3·1% of the N in rumen ammonia.