This research was jointly supported by the National Natural Science Foundation of China (Grant No. 30871801), National Natural Science Foundation of China (No. 30972213), Wuhan Disciplines Leaders Project (Grant No. 200951830554), the Program for Innovative Research Groups of Hubei Provincial Natural Science Foundation (Grant No. 2007ABC009) and National Research Initiative Competitive Grants from the Animal Growth and Nutrient Utilization Program (2008-35206-18764) of the USDA National Institute of Food and Agriculture and Texas AgriLife Research (H-82000).