ANALYSIS OF LEVELS OF INBREEDING AND INBREEDING DEPRESSION IN JERSEY CATTLE

A pedigree file of 157,015 male and female Jersey cattle (born after 1 955) from the Canadian herdbooks was investigated for the occurrence o f inbreeding. A large proportion of Jersey bulls and cows were inbred (32.4 and 36.3% for bulls and cows, respectively). However, average in breeding coefficients of these inbred cows and of all cows were low. F irst 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 pe rcentage on inbreeding were -9.84 kg, -.55 kg, and -.0011% per 1% incr ease of inbreeding. Inbreeding depression was not enough to cause larg e reductions of milk and fat yield of a cow with average inbreeding. H owever, when the inbreeding coefficient was greater than 12.5%, the in breeding depression was significantly higher than expected and such th at intentional inbreeding is not justified unless the mating is to an animal with exceptionally high breeding value.