ESTIMATES OF GENETIC TREND FOR YIELD TRAITS OF THE REGISTERED JERSEY POPULATION

Data for American Jersey cattle from 1960 to 1990 were used to study g enetic progress achieved by four paths of selection (sires of bulls, d ams of bulls, sires of cows, and dams of cows). Possible methods of ob taining optimal genetic gain for the breed were suggested by the analy sis. The generation interval by year of birth for parents of AI bulls was greater from 1975 to 1984 than for most recent 5 yr (1983 to 1987) because previously the use of old proven bulls was greater. Sires of bulls tended to have a longer average generation interval (9.3 yr) tha n the more nearly optimal interval reported in the literature for AI p opulations (7.0 yr). Selection differentials for the last 5 yr were la rger than the average over all years for milk yield, fat yield, protei n yield, and fat percentage. More intense selection for sires of bulls gave a larger increase in PTA over time than did other paths of selec tion. When all paths of selection were combined, estimated annual gene tic change for the Jersey breed was greater from 1983 to 1987 than fro m 1960 to 1987 for milk yield, fat yield, protein yield, and fat perce ntage. Potential genetic response from the sires and darns that were a vailable to produce sons was greater than the response from parents ac tually used. Therefore, better sires and dams were available for genet ic enhancement of their progeny than those actually used to produce yo ung sires and replacement females.