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