Sire Evaluation and Herd Level of Milk Production

Age-corrected milk records of 40,374 Holstein cows tested in 1,073 herds in Ontario were studied to determine if the level of herd production had an effect on the genetic expression of milk production. Heritability of milk production was estimated from paternal half-sib correlations at each of seven levels of herd production. The estimates obtained varied from 0.24 ± 0.05 to 0.36 ± 0.04, lowest values being associated with the lowest and highest levels of production. A significant quadratic component (P = 0.07) was obtained when these estimates were regressed on herd level of production. The sire component of variance increased from low to higher levels of herd production. The environmental component of variance increased in magnitude from the lowest to the highest production level, with a large increase at the highest level. Correlations among the contemporary comparison proofs of 19 sires, each evaluated on the production of 20 or more daughters at four levels of herd production, ranged from 0.73 to 1.01, indicating little change in ranking of sires across herd level of production.