1. College of Animal Science and Technology, Northeast Agricultural University, Harbin 150030, P. R. China
2. College of Animal Science and Technology, Southwest University, Chongqing 400715, P. R. China
3. Key Laboratory of Swine Facilities Engineering, Ministry of Agriculture and Rural Affairs, Harbin 150030, P. R. China
4. College of Life Science, Northeast Agricultural University, Harbin 150030, P. R. China