1. Key Laboratory of Agro-ecological Processes in Subtropical Region, The Chinese Academy of Sciences, Changsha PO Box 10, Hunan 410125, P. R. China
2. College of Animal Sciences and Technology, Southwest University, Chongqing 400715, P. R. China
3. College of Animal Sciences and Technology, Northwest A and F University, Yangling 712100, P. R. China