Heterogenost varijance i kovarijance težine mladih simentalskih bikova u različitoj dobi

Genetic and environmental variances and covariances and associated genetic parameters were estimated for weight of young Simmental bulls expressed over time. Heterogeneity over time was analyzed using covariance functions. Data utilized in the study consisted of weight records of 779 young bulls measured at the age of 6, 9 and 12 months. Accounting for the pedigree there were 2215 animals. Variances and covariances were estimated by REML from a multivariate animal model with interaction season x year defined as a fixed effect. Variances and covariances were also explained using covariance functions. Heritability estimates were 0.08, 0.21 and 0.23 for the ages of 6, 9 and 12 months, respectively. Curvilinear-changes in additive genetic and environmental variances and covariances were detected and they are explained by quadratic functions. Between any two age points, additive genetic correlations are larger then environmental correlations. This explains that the animal with high capacity of growth shows its potential during the whole growth period. Detected variance and covariance heterogeneity leads to conclusion that a multivariate model or a covariance function must be used to explain variability of trajectory variables in order to correctly estimate genetic parameters.