Date of this Version
Selection procedures which weight equally each test day sample may not be optimum for selection on test days.
Monthly test day data from 63,300 records of artificially sired Holsteins with 305-day lactations were from the New York Dairy Records Processing Laboratory. Heritabilities (h 2) were estimated intraherd-year- season from sire components of variance for monthly test day production within lactations 1, 2, 3, 4, and 5 or greater. Heritability of lactation fat percentage decreased from .60 to .40 from first to fifth lactations whereas heritability for 305-day milk yield increased slightly from .25 to .37 with succeeding lactations. Heritabilities for both fat percentage and milk production of monthly test days were lower for early and later tests than for middle monthly tests. Genetic and phenotypic correlations for early and late months with total lactation were less than for middle months.
Estimates of genetic gain by selecting on bimonthly or tri-monthly test days were almost as great as by selecting on the completed 305-day lactation record. Quadra-monthly records showed only a slight decrease in relative response as compared to bi- or tri-monthly testing. Selecting on part records would not result in any measurable loss of genetic gain in total milk yield or lactation fat test as compared to monthly testing.