Metabolic Mechanism Of Urea Nitrogen For Adapting To Alpine Nutritional Stress In Yak

The experiment was conducted to investigate effects of different levels of dietary nitrogen (1.43,1.97,2.45 and 2.90% N of dietary DM) on the rumen fermentation characters, the nitrogen metabolism and the kinetics of urea nitrogen recycling in yaks. Four 3-year-old castrate yaks with similar body weight (148.5±9.2 kg) were used in 4×4 Latin square design. To clarify urea nitrogen flux in yaks, the 15N15N-urea stable isotope was used in this experiment.With the increase of dietary nitrogen, there was no influence on the amount of N excreted in the fece, but the amount of N excreted in the urine increased remarkably (P<0.05). The proportion of urinary nitrogen to the apparently digestible nitrogen in the 1.97% N diet was the lowest for 52.88% and the ratio of nitrogen deposition to the apparently digestible nitrogen was highest for 47.36% among the four diets, which indicated that the highest nitrogen use efficiency was obtained in yaks fed with 1.97% N diet. In the meawhile, the amount of microbal protein sythesis was the highest in the 1.97% N diet compared to other levels of dietary nitrogen. A reasonable ratio of energy and nitrogen can improve the efficiency of nitrogen utilization. The ratio 1.36 of NFC to DCP in the 1.97% N diet provided a theoretical basis for feeding yaks.According to the results of urea nitrogen dynamics calculations, the highest gut entry rate of urea synthesized in the liver was observed in the lowest N diet, which was 0.87 and significantly higher than the other three groups and other ruminant species. The proportion of urea returned to the ornithine recycling in the digestive tract was also the highest in the lowest N diet, which is 0.45. All these results suggested that yak could use dietary N more efficiently in the case of low N intake as most of the urea synthesized in the liver was returned to the gut for anabolic use and ornithine recycling instead of the excretion in the urine. This probably provided the potential adaption mechanism of yak under the long time of nutritional stress in a year at high altitude of Qinghai-Tibetan Plateau.