Change in Variance Components Associated with Milk Records with Time and Increase in Mean Production

First-lactation milk records of artificially sired Holstein cows in New York were analyzed by a sire-by-herd variance component analysis separately for each year from 1954 to 1962. The two variables analyzed were 305-day, 2 ×, mature equivalent records and the mature equivalent records expressed as deviations from the average of their herd-mates. The total variance steadily increased with change in time. Most of the increase (r = .97) was accounted for by increase in mean production. The relative increase in the sire component of variance was slightly faster than the increase in residual variance. Thus, heritability estimated from the paternal half-sib correlation tended to increase slightly. The difference in variance with time could bias the heritability estimate from daughter-data regression upward by about 10% in these data. This bias does not account for all the difference in heritability estimates from paternal half-sib correlation (.25) and daughter-dam regression (.40) for the records analyzed as deviations. For mature equivalent records the estimates from both methods were .36