1. College of Animal Science and Technology, Yangzhou University, Yangzhou 225009, P. R. China
2. College of Agriculture and Animal Husbandry, Qinghai University, No.251 Ningda Road, Xining, Qinghai 810016, P. R. China
3. Mammalian Nutrition Physiology Genomics, Department of Animal Sciences and Division of Nutritional Sciences, University of Illinois, Urbana, Illinois 61801, United States