Application of a structural model for genetic covariances in international dairy sire evaluations

A structural model for estimation of genetic covariances in international dairy sire evaluations was compared with a standard multiple-trait mixed model using milk yield data from 13 environments (Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, Israel, Switzerland and five regions of the 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, management, and climate similarity. Genetic similarity was defined as the ratio between the number of daughters of common bulls used in a given pair of regions 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 average milk yield between two regions and the sum of these averages. Climate similarity was a function of the difference in heat indices between pairs of regions. Results showed that the structural model for the genetic covariances gave more precise estimates of the latter than the standard multivariate analysis because the number of parameters to be estimated in the genetic covariance matrix was reduced from 91 to 16. A comparison of the two models using a deviance information criterion (a measure of quality of fit) showed a slight superiority for the structural model. For the latter, the posterior means of the genetic correlations between environments ranged between 0.63 and 0.98.