Effects of diet quality on urea fates in sheep as assessed by refined, non-invasive [15N15N]urea kinetics

The effect of diet quality on urea production, entry into the gastrointestinal tract (GIT) and subsequent diversion to anabolic or catabolic fates was examined in four sheep (mean live weight 49.5 kg). The animals received, in a crossover design, each of two rations, hay-grass pellets (1:1 HG) and a mixed concentrate-forage (CF). Measurements were made of N balance and urea kinetics based on a 4 d continuous intravascular infusion of [(NN)-N-15-N-15]urea. Enrichments of [(NN)-N-15-N-15]- and [(NN)-N-14-N-15]urea in the urine, and faecal N-15 content were determined each day. After 24 h of infusion, urinary [(NN)-N-15-N-15]urea enrichments reached constant enrichment but a further 24 h was required before [(NN)-N-14-N-15]urea enrichment was at plateau. The latter is derived from hydrolysis of urea to (NH3)-N-15 in the digestive tract with subsequent absorption and re-conversion to urea. The diets were not isonitrogenous (14.3 v. 17.1 g N supplied daily for HG and CF respectively) but showed no difference in N balance. Urea-N production was much greater (16.3 v. 11.1 g/d; P=0.011) for CF compared with HG and more urea-N entered the GIT (9.9 v. 7.7; P=0.07). A larger proportion of GIT entry was returned to ureagenesis (51 v. 42 %; P=0.047) for the CF diet but a smaller fraction was lost in the faeces (3.3 % v. 7.1 %; P=0.013). In consequence, most of the additional urea-N which entered the GIT on the CF diet was returned to the ornithine cycle (probably as NH3) and the absolute amount available for anabolic purposes was similar between the rations (3.9 v. 4.5 g N/d).