Genetic and Environmental Portions of the Variation among Herds in Butterfat Production

The genetic part of the variation between herds has been estimated. In Method A the intra-sire regression of heifer production on herd average is used; whereas, in Method B, the variance components for herd differences between related and between unrelated cows in different herds are compared. Both methods imply that the genetic part of the differences between herd averages is caused entirely by additive gene effects. Evidence from the literature, though scanty, indicates that this is a good approximation. Method A was applied to records of 1,072 Jersey HIR cows and of 880 Holstein DHIA cows. The results indicated that 14 and 10%, respectively, of the differences between herd averages in the two populations are heritable. Prediction equations which utilize both individual performance and herd average are developed. Selection according to these indexes should be 1.19 and 1.35 times as effective as selection based on individual performance alone, without reference to herd averages, in the Jersey and the Holstein population, respectively. Method B was used on records of 2,903 artificially sired Holstein DHIA heifers and of 735 of their dams. When the variance components were derived from an analysis within years, the heritability of herd differences was 6.5%, both for milk and butterfat production. When the analysis was within year-seasons, the resulting heritability estimates were much higher (28 and 24%). It is suspected that the large size of the latter estimates results from sampling variation and, therefore, that the latter values are too large.