Determination of nitrogen requirement for microbial growth from the effect of urea supplementation of a low N diet on abomasal N flow and N recycling in wethers and lambs.

1. Plasma urea entry rate, urinary area excretion and, by difference, urea recycling in the body, together with the flow of non-ammonia N through the abomasum and digestion of dry matter (DM) before the abomasum were determined in both wethers and lambs receiving cereal-starch diets supplemented with urea to give 60-120 g crude protein (N X 6-25)/kg DM. 2. Lambs excreted less urea in urine than wethers given the same diet. 3. Relationships between plasma urea entry rate or urine urea excretion rate and plasma urea concentration were different for lambs compared to wethers suggesting greater conservation of body N by renal control in lambs. 4. Recycling of urea was not related to plasma urea concentration in wethers but was related exponentially in lambs, suggesting recycling is controlled rather than the result of simple diffusion from the blood to the gastro-intestinal tract. 5. Abomasal non-ammonia-N flow was similar for wethers and lambs and increased linearly with urea supplementation. 6. DM digestion prior to the abomasum was not significantly altered, although there was a tendency for decreased digestion of the basal diet given to lambs. 7. Maximum microbial N flow to the abomasum was estimated as 30 g N/kg organic matter (OM) fermented in the rumen. 8. This work and the literature reviewed suggested maximum net microbial production can be obtained when the diet supplies an amount of fermentable N equal to the microbial N output. It is calculated the diet should supply approximately 26 g fermentable N/kg digestible OM or 1-8 g fermentable N/MJ metabolizable energy. This corresponds to a fermentable crude protein supply varying from 65 to 130 g/kg DM as digestible OM content increases from 400 to 800 g/kg DM.