1. Key Laboratory of Agro-Ecological Processes in Subtropical Region, National Engineering Laboratory for Pollution Control and Waste Utilization in Livestock and Poultry Production, Institute of Subtropical Agriculture, Chinese Academy of Sciences, Changsha, Hunan 410125, China
2. College of Animal Science, South China Agricultural University, Guangzhou, Guangdong 510642, China
3. Department of Animal Science, Texas A&M University, College Station 77843-2471