The assumed purpose of breed and crossbreeding evaluation efforts is to
allow and encourage optimum use of animal germplasm to improve efficiency of
livestock production under the many diverse world production-marketing environments.
This requires clear definition of production efficiency and of the effects of
alternative genetic changes in performance
traits on both output value and input cost under defined production-marketing systems.
Prediction of optimum breed use in production systems (ranging from pure breeds
to specific or rotation crossbreeding to formation of new composite breeds)
requires knowledge of breed differences and
crossbreeding heterosis and of any important deviations from
expectations for only average and dominant gene effects. This information
should include individual, maternal and possible paternal effects on such
traits as reproduction, viability, growth, body composition and female
production of milk, wool or eggs, and how these are affected by environmental
differences. Experimental approaches for obtaining the needed estimates of
breed and crossbreeding parameters are described, with examples from the major classes of livestock. In poultry and
swine, central tests can be used to characterize available commercial
stocks, but designed crossing experiments are needed to fully estimate the
parameters required to predict the optimum choices of breeds and system of
breeding. In some ruminant species, and
especially in dairy cattle, much of the needed information on breed, heterosis
and even deviations of interse mated crossbred populations from additive dominance
expectations, can be obtained from well planned field records on animals of known pedigree. However, more complete and
critical evaluation of exotic breeds and their crosses with indigenous breeds can be accomplished in designed
experiments, including the combination of either 1) indigenous breeds, F1
reciprocal backcrosses, F2, F3, and for maternal heterosis, F1 crosses with a common unrelated breed of sire,
or 2) a diallel design, including reciprocal three-breed crosses to
measure breed of female performance and maternal heterosis effects, with
extension to matings of each set of F1 and F2 and two backcross females with a common breed of sire to
measure effect of exotic breed fraction, independent of heterosis, and
any deviations from additive/dominance expectations. Experiments directly comparing multi-breed composites (F3 +
generations) with constituent breed crosses and parental purebreds provide the
most complete evaluation for the role of composites relative to specific
or rotation crossbreeding, when the required proportion of industry purebred
matings is also considered.
