Evaluation of dairy bulls in Ontario for calving ease of offspring.

The first sire evaluations for calving ease for bulls used in Ontario were published in May, 1981. Evaluations used calving records collected by Ontario Dairy Herd Improvement Association from all supervised herds from May, 1980, to April, 1981. A total of 34,240 records including herd, breed, sire, and cow identification, date of calving, and information about cow size, parity, sex of calf, mortality, and dystocia score were collected. After editing, 28,947 records were available from 1,178 Holstein bulls. Dystocia scores were 1) unobserved or unassisted, 2) easy pull, 3) hard pull, and 4) surgery. Stillbirths represented 5.5% of births coded 1 and 2, 25.1% of births coded hard pull, and 50.7% of surgical births. Analysis of variance showed that cow size, sex of calf, parity, and season (May to September and October to April) affected dystocia scores. Variance components were estimated prior to evaluation by Henderson's new method and resulted in a heritability of .08. Sire evaluation was by best linear unbiased prediction and included the relationship matrix. The prediction model included herd-year-season, dam size-parity-sex, and sire effects. Ninety-one sires had evaluations with a Repeatability of at least 55% and records in at least 20 herd-year-seasons.