Data from a closed Hariana herd were analyzed to assess the genetic gains in milk production through selection over 20 years and also to evaluate the genetic changes made in age at first calving and in first calving interval, toward which no attention had been directed. Genetic progress in milk production, estimated as -2b(S,P)T (twice the pooled intrasire regressions of sire progeny on time, the records being deviated from contemporary average), was 10 kg, or 1.5% of the average first lactation yield per year. The expected progress from direct selection was 16 kg, or 2.5% of the herd average, per year. In spite of some qualifications, these estimates of genetic progress indicate a large improvement in milk production in the herd since its establishment. There was little selection on age at first calving or on first calving interval in this herd. The small culling differential for cows retained for additional lactations supports this for age at first calving. The cows retained for each additional lactation did have a slightly shorter first calving interval. This tendency showed a steady increase in later lactations. The annual genetic change in age at first calving and in first calving interval, estimated as -2b(S-P)T, was -1.48 and -.23 months, respectively. These genetic changes might represent mostly a correlated response to selection for milk production. They indicate that age at first calving and first calving interval tend to decrease as milk production increases. The first calving interval and the age at first calving were thus both positively correlated with milk production, on the scale of desirability. © 1968, American Dairy Science Association. All rights reserved.
