S. Zhuang*1, R. Yan*, W.C. Dong*, T. Zhang*, G.J. Chang** and X. Z. Shen**
*College of Animal Sciences and Technology, Nanjing Agricultural University, Nanjing, Jiangsu 210095, P.R. China
**College of Veterinary Medicine, Nanjing Agricultural University, Nanjing, Jiangsu 210095, P.R. China