1. Division of Animal Sciences, Institute of Agrobiological Sciences, National Agriculture and Food Research Organization, Ibaraki 305-8602, Japan
2. Division of Animal Breeding and Reproduction, Institute of Livestock and Grassland Science, National Agriculture and Food Research Organization, Ibaraki 305-0901, Japan
3. Department of Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Ghent University, B-9820 Merelbeke, Belgium