In many breeding programs trait recording is expensive and therefore the number of animals from which traits are recorded is rather small in relation to the whole population of animals. Then, the genetic improvement is realized with a limited number of animals in which the selection is practiced. Subsequently, selected animals are used to disseminate the genetic improvement realized in the recorded population into the whole population. The selection of a limited number of animals, the multiplication in the next generation with a much larger number of animals and the production of the “production” animals in very large numbers in the final generation, leads to a pyramidal structure of such a breeding and production program. Besides, the pyramidal structure will allow to apply crossbreeding (hybridization), which can fully utilize the advantages of heterosis.
The dissemination of the selection response depends of the structure of the breeding programs. In commercial pigs and poultry programs, selection takes place in the top of the breeding program. In the pure lines in the top genomic selection is applied. Via a few “multiplying generations” the selection response obtained in the top is disseminated into the animals producing meat or eggs. see the figure below where the structure of the Hendrix-Genetics (ISA) breeding program is outlined:
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Within the commercial breeding schemes, e.g. for poultry and pigs, the selection response is realized in specialized lines. In commercial poultry (broiler) breeding programs usually a four-way cross is applied. The two female lines are selected for fertility and egg quality and the male lines for growth traits. Crossbreeding results in a high number of healthy chicks due to the full exploitation of heterosis. In the broiler scheme above, the selection takes place in the pure lines in a limited number of Great Grandparents. When the selected Great Grandparents are multiplied in sufficient numbers, they are crossed. The generation interval in the poultry selection lines is very low, less than a year to accelerate the selection response. The A*B line cross gives the A*B F1 progeny that act as Grandparent and the C*D line cross the C*D F1 Grandparent. The pure line and the F1 animals are owned by the breeding company to protect the characteristics of their lines and the realized genetic improvement in these lines. They have the possibility, by maintaining different selection lines to create animals that can produce for different markets and offers the opportunity for a quick reaction at market changes.
In commercial pig breeding programs usually a three-way cross is applied. See e.g. the Hypor program in 2014:
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