Urinary purine derivatives excretion, rumen microbial nitrogen synthesis and the efficiency of utilization of recycled urea in Tibetan and fine-wool sheep

Urinary purine derivatives (PD) excretion, rumen bacterial purine bases and nitrogen (N) variations, estimated microbial N synthesis and the efficiency of utilization of recycled urea-N were determined in Tibetan and Gansu Alpine fine-wool sheep offered 4 diets with different N concentrations [11.0, 16.7, 23.1 and 29.2 g N/kg dry matter (DM)] in 2 concurrent 4 × 4 Latin square designs. Urine output increased linearly with increasing dietary N concentration (P < 0.001), and was lower in Tibetan than in fine-wool sheep (P < 0.05). With an increase in dietary N, urinary PD and the components increased linearly (P < 0.05), except for uric acid (P > 0.10); however, the purine nitrogen index (PNI) decreased (P < 0.001). For the lowest N diet, allantoin, total PD and PNI were greater in Tibetan sheep than in fine-wool sheep (linear dietary N × breed, P < 0.05). As dietary N increased, rumen bacterial purine bases [ribonucleic acid (RNA) equivalent] decreased linearly (P < 0.05), whereas total N content increased (P < 0.05). Compared to fine-wool sheep, rumen bacteria of Tibetan sheep contained higher RNA and lower N content (P < 0.01). Total microbial N production and microbial N from recycled urea-N increased linearly with dietary N but did not differ between breeds at low N diets (P > 0.05). As a proportion of total microbial N synthesis or hepatic urea-N production, recycled urea-N was utilized more efficiently in Tibetan than in fine-wool sheep (P < 0.05). These results suggest that Tibetan sheep are better able to cope with a low protein diet, which is characteristic of the Qinghai-Tibetan Plateau during the long-cold season, than are Gansu Alpine fine-wool sheep.