Heritability of Age at First Calving

Abstract High heritabilities of age at first calving reported by a number of researchers may be due to confounding of service sires and time heifers were bred. Heritabilities from paternal half-sister analyses of variance models which included the effects of period of birth were less than from models where these were not included. Heritabilities were .22 and .23 from data on Holstein cows in institutional herds in Ohio and herds in California.