INTERACTION OF AGE AND MONTH OF CALVING WITH YEAR OF CALVING FOR PRODUCTION TRAITS OF ONTARIO HOLSTEINS

Production records of 835,789 Ontario Holsteins calving between 1958 and 1990 were used to determine the presence of an interaction of the age and month of calving of cows with the year of calving for production of milk and fat and fat percentage. An animal model was employed including the effects of herd-year-season of calving, interaction of age and month of calving with year of calving, animal additive genetic effects, and permanent environmental effects. Tests for interactions of age with year within months of calving were significant for production of milk and fat, but not for fat percentage. Tests for interactions of month with year within age groups were significant for only a few age groups in the second and third parities. The implication of these findings on genetic evaluation is that an interaction of age and month of calving with year of calving should be included in the animal model for genetic evaluations rather than preadjusting production for age and month of calving.