1. Nitrogen kinetics were studied in six sheep (45–55 kg live weight) consuming either a high-N grass silage or a low-N dried grass made from swards of perennial ryegrass (Lolium perenne). The diets were fed hourly at a level of 600 g dry matter/d and supplied 19.5 and 11.0 g N/d respectively.
2. The amounts of organic matter (OM) consumed and flowing at the duodenum and ileum and excreted in the faeces were similar (P < 0.05) with both diets. Each diet supplied 23 g digestible OM/d per kg live eight0.75, which was sufficient to maintain body-weight.
3. There were no differences (P < 0.05) between diets in rumen fluid volume, fractional outflow rate of fluid from the rumen, total concentration of volatile fatty acids or molar proportion of acetate in the rumen. The pH and molar proportion of propionate in rumen fluid were higher (P < 0.01), and molar proportion of butyrate lower (P < 0.001) when the silage was given.
4. There was a net loss of N (4.0 g/d) between mouth and duodenum when the silage was consumed but a net gain (5.5 g/d) when the dried grass was consumed. As a result, total non-ammonia-N (NAN) flow at the duodenum did not differ (P / 0.05) between diets. Rumen microbial NAN flow at the duodenum, based on 15N as the marker, also did not differ (P < 0.05) between diets but the efficiency of microbial N synthesis in the rumen (g/kg OM apparently digested) was higher (P < 0.05) with the dried grass.
5. When the sheep were consuming silage they had a higher concentration of ammonia in rumen fluid (P < 0.01), a higher rate of irreversible loss of ammonia from the rumen (P < 0.05) and a higher rate of absorption of ammonia across the rumen wall (P < 0.01). The rate of absorption was found to be more closely related to the unionized ammonia concentration in rumen fluid (r2 0.85) than to the total ammonia concentration (r2 0.36).
6. Endogenous N entry into the forestomachs was calculated to be 5.5 g/d when the silage was given and 9.4 g/d when the dried grass was given, of which 1.7 and 3.5 g/d respectively were in the form of urea. Thus, approximately 4–6 g N/d were derived from non-urea materials.
7. Within the small intestine the apparent absorption coefficient of rumen microbial NAN (0.72) did not differ (P < 0.05) between diets but the apparent absorption coefficient of total NAN was lower (P < 0.05) when the I silage was given, owing to a lower (P < 0.01) absorption coefficient of the non-microbial NAN fraction (undegraded feed and endogenous).
8. Within the large intestine, diet had no effect (P < 0 05) on the apparent absorption coefficients of total N (0.22) and rumen microbial NAN (0.63).
9. Plasma urea concentration, the rate of urea synthesis in the body and urinary urea excretion were higher (P < 0.001) when the silage was consumed. However, the transfer of urea to the whole digestive tract and to the post-ruminal part of the tract did not differ (P < 0.05) between diets; urea transfer to the rumen was higher (P < 0.01) when the dried grass was given.
10. The results were used to construct a whole-animal model of N flows between the digestive tract and the tissues.