Records of test-daymilk yields of the first three lactations of 25,500 Holstein cows were usedto estimate genetic parameters for milk yield by using two alternatives of definitionof fixed regression of the random regression models (RRM). Legendre polynomialsof fourth and fifth orders were used to model regression of fixed curve (definedbased on averages of the populations or multiple sub-populations formed by groupinganimals which calved at the same age and in the same season of the year) orrandom lactation curves (additive genetic and permanent enviroment). Akaikeinformation criterion (AIC) and Bayesian information criterion (BIC) indicatedthat the models which used multiple regression of fixed lactation curves oflactation multiple regression model with fixed lactation curves had the bestfit for the first lactation test-day milk yields and the models which used asingle regression of fixed curve had the best fit for the second and third lactations.Heritability for milk yield during lactation estimates did not vary among modelsbut ranged from 0.22 to 0.34, from 0.11 to 0.21, and from 0.10 to 0.20, respectively,in the first three lactations. Similarly to heridability estimates of geneticcorrelations did not vary among models. The use of single or multiple fixedregressions for fixed lactation curves by RRM does not influence the estimatesof genetic parameters for test-day milk yield across lactations.
