COMPARISON OF POSSIBLE COVARIATES FOR USE IN A RANDOM REGRESSION-MODEL FOR ANALYSES OF TEST DAY YIELDS

Random regression models have been proposed for the genetic evaluation of dairy cattle using test day records. Random regression models cont ain linear functions of fixed and random coefficients and a set of cov ariates to describe the shapes of lactation curves for groups of cows and for individual cows. Previous work has used a linear function of f ive covariates to describe lactation shape. This study compared the fu nction of five covariates with a function of only three covariates in three random regression models. Comparisons of estimates of components of variances and covariances, as well as comparisons of EBV and their prediction errors for milk yield, were made among models. Small pract ical differences existed between models in all respects. The model usi ng regressions with five covariates had a slight advantage for compari son of prediction error variances of daily yields.