In many cases, the value of an animal's genes to its progeny depends on the genes contributed by its mate. This is true for nonlinear merit traits, for which corrective mating systems can be used. However, the classic case is that the value of an animal may be higher if its mates are of a different breed, because the progeny will be more heterozygous and will exhibit heterosis.
Three types of crossbreeding system can be defined, according to the incidence of purebreds in the breed pedigree (Table 1). Fully structured mating systems can
Table 1 Mating systems that exploit heterosis
Mating system type Definition Heterosis expression Running cost Example
Fully structured Short path to purebreds on High High Multiple breed crosses both sides of the pedigree in poultry and pigs
Semi structured Short path to purebreds on Moderate Moderate Rotational crosses in male side of the pedigree temperate beef
Unstructured Short path to purebreds on Low Low Composite breeds in neither side of the pedigree tropical beef generate maximal heterosis, but with loss of structure this becomes impossible. However, more structure means more cost, because purebreds need to be maintained to feed into the system. This is more important for low-fecundity species, as a higher proportion of breeding females need to be in upstream populations.
Variations on classic crossbreeding systems can be found that fit better with the prevailing farming system. An example proposed by Andrew MacTaggart (personal communication) is shown in Fig. 3. At equilibrium, the cows in the commercial herd contain predominantly Brahman genes, even for a moderate level of within-herd female replacement. This is largely because the sires of imported heifers are pure Brahman, and their dams are part Brahman. High Brahman content makes for good mothering ability in the tough tropical environment that prevails. In contrast, the growing stock in the commercial herd typically contains mostly British genes, at a level that depends on migration rates, improving growth and product quality in slaughter stock. This system overcomes the fluctuations in breed content that affect classic rotational mating systems and better exploits the specific merits of different breed types.
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