USE OF TEST DAY YIELDS FOR GENETIC EVALUATION OF DAIRY SIRES AND COWS

Genetic evaluations of dairy sires and cows for milk production based on 305-day lactation yields were compared to evaluations based on the corresponding test day yields from those lactations. First lactation data were from the Quebec Dairy Herd Analysis Service for 3094 cows calving from September 1983 to October 1989. There were 29462 test day yield records. Four animal models were studied for use with test day yields, and each model included four covariates to account for the shape of the lactation curve within eight age-season groups. Models contained either herd-year-season effects or herd-test date effects, and the residual variances were assumed to be either homogeneous or heterogeneous across days in milk. Herd-test date effects resulted in lower residual variances compared to models with herd-year-season effects. Correlations of test day evaluations with 305-day evaluations ranged from 0.87 to 0.97. Genetic evaluations based on test day yields offer many advantages over those based on 305-day lactations including better modeling of factors affecting yields, no need to extend records, and possibly greater accuracy of evaluations.