Comparison of nitrogen utilization and urea kinetics between yaks (Bos grunniens) and indigenous cattle (Bos taurus).

Under traditional management on the Qinghai-Tibetan Plateau, yaks () graze only on natural pasture without supplements and are forced to cope with sparse forage of low N content, especially in winter. In contrast, indigenous Tibetan yellow cattle () require supplements during the cold season. We hypothesized that, in response to harsh conditions, yaks cope with low N intakes better than cattle. To test this hypothesis, a study of whole-body N retention and urea kinetics was conducted in 2 concurrent 4 × 4 Latin squares, with 1 square using yaks and 1 square using cattle. Four isocaloric forage-concentrate diets differing in N concentrations (10.3, 19.5, 28.5, and 37.6 g N/kg DM) were formulated, and by design, DMI were similar between species and across diets. Urea kinetics were determined with continuous intravenous infusion of NN urea for 104 h, and total urine and feces were concomitantly collected. Urea production, urea recycling to the gut, and ruminal microbial protein synthesis all linearly increased ( < 0.001) with increasing dietary N in both yaks and cattle. Urinary N excretion was less ( = 0.04) and N retention was greater ( = 0.01) in yaks than in cattle. Urea production was greater in yaks than in cattle at the 3 lowest N diets but greater in cattle than in yaks at the highest N diet (species × diet, < 0.02). Urea N recycled to the gut ( < 0.001), recycled urea N captured by ruminal bacteria ( < 0.001), and ruminal microbial protein production ( = 0.05) were greater in yaks than in cattle. No more than 12% of urea recycling was through saliva, with no difference between species ( = 0.61). Glomerular filtration rate was lower ( = 0.05) in yaks than in cattle. The higher urea recycling and greater capture of recycled urea by ruminal microbes in yaks than in cattle suggest that yaks use mechanisms to utilize dietary N more efficiently than cattle, which may partially explain the better survival of yaks than cattle when fed low-N diets.