Nitrogen digestion in sheep given poor-quality indigenous hill herbages

1. In two experiments, the sites of digestion of non-ammonia nitrogen (NAN) and the amounts of urea N recycled to the rumen were measured in mature wether sheep given diets of indigenous hill herbage (Agrostis-Festuca and heather). 2. Duodenal and ileal flow values were obtained using 103Ru-phenanthroline and 51Cr-EDTA markers in animals prepared with simple (T-shaped) cannulas. Amounts of urea N recycled to the rumen were estimated from measurements of the transfer of plasma urea carbon into rumen bicarbonate and the production rate of rumen bicarbonate using 14C-labelled urea and bicarbonate respectively. 3. The flows of NAN at the duodenum and ileum were linearly related to the intake of herbage (P less than 0.001). There was a net gain of non-ammonia N anterior to the duodenum on both diets (at an intake of 460 g organic matter (OM)/d, 3.7 g NAN/d on Agrostis-Festuca and 3.3 g NAN/d on heather). 4. Net digestibility of NAN entering the small intestine was within a normal range on the Agrostis-Festuca (0.58 at 460 g OM intake) diet but low on the heather diet (0.43 at 460 g OM intake). 5. It was calculated that at 460 g OM intake only 0.9 and 1.1 g/d respectively of the duodenal NAN on the Agrostis-Festuca and heather diets could have been derived from urea-N recycled to the rumen. Thus 2.8 g and 2.2 g/d had to be accounted for as non-urea endogenous NAN.