1. Animal Breeding and Reproduction Research Division, Institute of Livestock and Grassland Science, National Agriculture and Food Research Organization, Ibaraki 305-0901, Japan
2. Division of Animal Sciences, Institute of Agrobiological Sciences, National Agriculture and Food Research Organization, Ibaraki 305-8602, Japan
3. The United Graduate School of Veterinary Science, Yamaguchi University, Yamaguchi City, Yamaguchi 753-8515, Japan