First: where we use the word breed, you can also read selection line. In commercial pig and poultry breeding selection in specialized selection lines is much more common than selection within breeds. Second: in all crossbreeding systems before animals are crossed, they are first selected for the relevant traits. Crossbreeding does not make selection redundant. ThirdThird: crossbreeding schemes require a strict implementation by all participants. Hence. Fourth: crossbreeding requires maintenance of the pure breeds. This may seem obvious, but often it is not easy to maintain the pure breeds. For example, in Brazil, the cross between Holstein and Zebu breeds is common. Zebu cows are inseminated with Holstein bulls imported from US or Europe or tested under local circumstances. However, the Zebu breed needs to be maintained as well. With the low reproductive capacity in cattle, this is not easy to achieve. For instance, when the aim is to have a population with 100,000 F1 cows and the replacement rate per year is 25% and each cow produces each year one calf, then 25,000 F1 cows per year are required. This means that 50,000 Zebu cows need to be inseminated with Holstein semen. But another 50,000 Zebu cows are needed to replace the Zebu population. These figures are even larger in reality, because it was assumed that all female calves survive to reproductive age, and that all cows produce a calf. To achieve this, the Zebu population needs to be as large as the F1 population. In pigs and poultry, the issue of maintenance of pure breeds is much easier because the required pure breed population is much smaller due to higher reproductive capacity for females.
As listed in the previous paragraph, crossbreeding is applied for several reasons and they can only be realized when the chosen crossbreeding system is strictly implemented.
In these Several crossbreeding systems heterosis percentages vary as can be seen in the table below (the crossbreeding systems will be outlined in the subchapters):
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In the pure-breed cross the heterosis in the F1 is 100 %. In the F1 population the difference in allele frequencies between the F1 and one of the parent breeds is half the difference between the two parent breeds. This fact makes that e.g. when the F1 is mated with one of the parent breeds in a backcross the heterosis in the F2 is 50 % compared to the original two way cross. The % of heterosis depends of the difference in allele frequencies between the dam and the sire population. As stated before, the heterosis is most pronounced and most valuable for the improvement of traits with a low heritability like health and fertility traitscan be distinguished. For reading the mating schemes, it is important to know that the sire is always the first parent (the one on the left) and the dam always the second parent (on the right). Furthermore, if a mating is between brackets, it means that that is performed first and the resulting crossed animal is used as parent. For example, A * (AB) means a sire of the A line is mated with a dam that is a crossbred of a sire of the A line and a dam of the B line.
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