The effect of test day models on the estimation of genetic parameters and breeding values for dairy yield traits.

The present study estimated genetic parameters for yields of milk, fat, and protein applying REML procedures under test day animal models. The data consisted of 155,494 test day records from 15,756 Friesian cows in first lactation from one region in northern Germany. The models applied included a traditional herd-year-season model for the analysis of single test days and 305-d records and two test day models differing by the definition of contemporary groups, either as herd-year-season of calving or as herd-test day. For single test days, heritabilities were highest for midlactation yields. For test day models, estimates of heritability varied with the number of test day records included for each cow. Estimates of .32, .19, and .20 for yields of milk, fat, and protein, respectively, were highest when only test d 3 to 7 were included; the corresponding estimates for 305-d records were .39, .32, and .30. Estimates of residual variances were reduced when test day records were converted to records of average yield in standardized intervals of 30 d. Breeding values were estimated for 305-d and test day models. A comparison of both sets of breeding values indicates only minor changes in sire rank, but more drastic reranking for individual cows.