The Holstein breed, which is composed almost
completely of American Holstein genes, has largely
replaced other breeds of dairy cattle throughout much
of the world. Production and conformation traits have
been emphasized in the breeding of Holsteins because
of moderately high heritability and ease of data
collection. However, female fertility, calving ease, calf
mortality, health and survival have been ignored until
very recently. Problems related to functional traits,
coupled with increased inbreeding on an international
scale, have resulted in tremendous interest in crossbreeding
among commercial dairy producers. Purebred
sires will continue to be sought to breed almost
all dairy heifers and cows for cross-breeding. Most
cross-breeding systems with dairy cattle will make
use of three breeds to optimize the average level of
heterosis across generations.
For further information see: Hansen (2006).