College of sciences, Northwest A&F University, Yangling, 712100, Peoples Republic of China.|College of sciences, Northwest A&F University, Yangling, 712100, Peoples Republic of China.|College of sciences, Northwest A&F University, Yangling, 712100, Peoples Republic of China.|College of sciences, Northwest A&F University, Yangling, 712100, Peoples Republic of China.|Department of Animal and Avian Sciences, University of Maryland, College Park, MD, 20742, USA.|College of sciences, Northwest A&F University, Yangling, 712100, Peoples Republic of China.|College of Animal Science and Technology, Northwest A&F University, Yangling, 712100, Peoples Republic of China. liml75@126.com.