Genetic evaluation of dairy cattle for conformation traits using random regression models

Random regression models were applied to eight conformation traits (i.e. st ature, rump angle, thurl width, rear leg set, rear udder width rear udder h eight, udder depth, and fore udder attachment) of Holstein cows from the no rtheastern United States. Covariates for fixed and random regressions inclu ded age and age-squared for six of the traits, and two additional covariate s were included for rear udder width and rear udder height. Other effects i n the model were herd-year-classifier and months in milk. Fixed covariates were nested within year of birth of the cow. Variance components were estim ated using Bayesian theory and Gibbs sampling procedure. Estimated breeding values from the random regression models were compared to two single trait models. The first model utilized only the first classification record of t he cow in first lactation, and the second model utilized all classification s of the cow in a simple repeatability model. Additive genetic merit for co nformation traits changed with the age of the animal. Some traits were affe cted by age more than others. The single tot, single record model and the s imple repeatability model were nor appropriate in predicting breeding value s at mature ages for rear udder width and rear udder height.