Application of a structural model for genetic covariances in internationaldairy sire evaluations

A structural model for estimation of genetic covariances in international d airy sire evaluations was compared with a standard multiple-trait mixed mod el using milk yield data from 13 environments (Austria, Belgium, Czech Repu blic, Denmark, Estonia, Finland, Israel, Switzerland and five regions of th e United States). Data consisted of 2,885,117 first-lactation records from daughters of 17,867 Holstein sires. Parameters of the structural model for genetic covariances included an intercept and measures of genetic, manageme nt, and climate similarity. Genetic similarity was defined as the ratio bet ween the number of daughters of common bulls used in a given pair of region s and the total number of daughters of all bulls. The measure of management similarity was the ratio between the absolute value of the difference in a verage milk yield between two regions and the sum of these averages. Climat e similarity was a function of the difference in heat indices between pairs of regions. Results showed that the structural model for the genetic covar iances gave more precise estimates of the latter than the standard multivar iate analysis because the number of parameters to be estimated in the genet ic covariance matrix was reduced from 91 to 16. A comparison of the two mod els using a deviance information criterion (a measure of quality of fit) sh owed a slight superiority for the structural model. For the latter, the pos terior means of the genetic correlations between environments ranged betwee n 0.63 and 0.98.