On assessing contrasts between groups of animals

The main objective of assessing connectedness or linkage is to obtain an indication of the accuracy of comparisons between EBVs estimated in different herds or flocks. Several methods have been proposed to evaluate connectedness. Definition of connectedness is not always the same in those methods, and some of those methods can be very demanding computationally, which makes routine application difficult. The objective of this paper is to describe a straightforward method that assesses linkage across groups of animals, and to investigate the effect of heritability and the number of offspring from across flock sires on the prediction error (co)variance of flock means. The method is illustrated using a small example and Merino sheep data extracted from the Sheep Genetics Australia database.