Efficiency of Selection on Milk Yield To a Fixed Age

Abstract

Selection on records in early life was compared for a) milk produced in a fixed lactating period, 305 days or complete lactation, regardless of age and b) for milk produced to a fixed age of 41 mo regardless of the lactational periods. The expected efficiency of selection for genetic gain in total milk produced up to 48 mo, 72 mo, and for the lifetime was the basis of comparison. Data consisted of monthly Dairy Herd Improvement lactation records on 1806 Holstein cows from 38 California herds. The heritability and genetic correlations indicated selection on total milk produced to 41 mo of age was more efficient than selection on milk produced to 305 days in first lactation by 16, 11, and 11% for genetic gain in total milk produced up to 48 mo, 72 mo, or for the lifetime, respectively. Genetic correlations between age at first calving and total milk produced to 48 mo, 72 mo, and lifetime became less negative as ages advanced, −.57, −.43, −.37. Positive phenotypic correlations between the length of the first calving interval and total milk up to 48 mo, 72 mo, and lifetime declined as age advanced .42, .29, and .18. Evaluating cows on total milk to a fixed age will lead to efforts in management and breeding which will affect tradeoffs between the rate of daily milk production and reproductive performance necessary to maximize total milk production for age. © 1978, American Dairy Science Association. All rights reserved.