The authors wish to thank Qiang Yu, Jiwei, Peng, Dian and Yang for their assistance in the experiment. This study was supported by the National Natural Science Foundation of China (grant no. 31672666, 31302195), the Natural Science Foundation of Hubei Province of China (grant no. 2016CFB389), Educational Commission of Hubei Province (grant no. D20151705) and Hubei Collaborative Innovation Center for Animal Nutrition and Feed Safety. Thanks are also given to the anonymous reviewers for their helpful suggestions.