1. College of Animal Science and Technology, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China;
2. College of Veterinary Medicine, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China;
3. State Key Laboratory of Plateau Ecology and Agriculture, Qinghai University, Xining, Qinghai 810016, P. R. China