In ruminants, both absorption of amino acid-N (AA-N) and hepatic urea-N synthesis can exceed apparently digestible-N. This illustrates the importance of N recycling, particularly urea-N, to the ruminant gut and which may add between 16 and 61\% to apparent digestible-N. Thus, urea inflows must be included in calculations of digestive tract N balance for, while 24 to 278\% of apparently digested-N is converted to net absorbed AA-N with conserved and dried diets, this 'efficiency' is only 17 to 68\% when urea-N inflows are included.
Through use of two major techniques, arterio-venous transfers and isotopomer analysis following labelled urea injection, entry of urea to the gut (GER) has been monitored in order to identify factors that improve transfer of urea-N to microbial protein. This conversion occurs primarily in the rumen and a new model, based on [$^{15}$N$^{15}$N] urea, indicates that between 40-60\% of GER is used for anabolic purposes (AA-N), 35-55\% is absorbed as ammonia and returned to the ornithine cycle in the liver and less than 10\% is excreted in feces. Multiple entries into the gut of N atoms from urea enhance the potential conservation towards anabolic purposes. Factors that alter this anabolic fate include: diet quality; diet quantity; supply of specific and general energy substrates; plus the extent of intra-ruminal recycling of bacterial-N. Optimisation of these factors can ensure that nutrient supply to the rumen microflora is not a limitation in feeding systems.