This work was supported by Key Project of Scientific Research Plan of Hunan province (2017NK2151; 2018NK2081) and Changsha City (kq1801014), and also supported by the Hundred-talent Plan of Hunan Province (XZ2016-08) and Hunan Co-Innovation Center of Animal Production Safety. We thank the following: Mr. Yan Gong, Mrs. Yu Xiao and Mrs. Shishu Ying in College of Animal Science and Technology, Hunan Agricultural University, for their selfless help and valuable suggestions during the experiment and manuscript writing.