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A pedigree file of 157,015 male and female Jersey cattle (born after 1955) from the Canadian herdbooks was investigated for the occurrence of inbreeding. A large proportion of Jersey bulls and cows were inbred (32.4 and 36.3% for bulls and cows, respectively). However, average inbreeding coefficients of these inbred cows and of all cows were low. First lactation milk, fat, and fat percentage records for 53,592 Jersey cows were analyzed. Inbreeding was included in the animal model as a linear covariate. The regression coefficients of milk, fat, and fat percentage on inbreeding were –9.84 kg, –.55 kg, and –0011% per 1% increase of inbreeding. Inbreeding depression was not enough to cause large reductions of milk and fat yield of a cow with average inbreeding. However, when the inbreeding coefficient was greater than 12.5%, the inbreeding depression was significantly higher than expected and such that …