Date of this Version
Estimated genetic values from an animal model based on first lactation milk records for 6OOO AI Holstein sires and 1,074,971 Holstein cows born in 1981 or before were used to estimate average genetic selection differentials and generation intervals for the four paths of selection for each year of birth. Selection differentials for paths of sires of bulls, dams of bulls, sires of cows, and dams of cows averaged over all years were 405, 395, 239, and 42 kg and for the most recent 5 yr 884, 598, 235, and 28 kg. Generation intervals averaged for all years were by path 10.2, 22.214.171.124, and 5.1 yr and for the most recent 5 yr 11.0, 6.4, 8.9, and 4.9 yr. Genetic trend based on the average selection differentials and generation intervals would be 34.9 kg/yr, but based on the latest 5-yr periods considering parents of grade cows genetic trend would be 57.2 kg/yr. Estimates of annual trend are considerably less than the potential rate of 96 kg/yr because of longer than necessary generation intervals and smaller selection differentials than theoretically possible.