1. College of Animal Husbandry and Veterinary Medicine, Southwest Minzu University, Chengdu, P. R. China
2. Key Laboratory of Ministry of Education and Sichuan Province for Qinghai-Tibetan Plateau Animal Genetic Resource Reservation and Utilization, Chengdu, P. R. China
3. Sichuan Industrial Institute of Antibiotics, Chengdu University, Chengdu, P. R. China
4. Department of Veterinary Medicine, Southwest University, Rongchang, P. R. China