Animals used in intensive pig farming are characterized by their high productive efficiency. These genetic types have a very intense productive and reproductive cycle.

The high number of offspring per birth, the short generation interval and the high heritability of economic interest characters, have made it possible to obtain spectacular genetic progress in recent decades and the development of swine genetics.

Thanks to swine genetics, farming pigs are crossbred animals, commercial hybrids, whose parental breeds depend on what the market demands. Genetic selection has been carried out according to these premises and also considering the minimization of costs. For this, it has been fundamental to increase numerical productivity (number of weaned piglets per sow and year), mainly through the selection of prolificacy and also through adequate reproductive management.

Pig genetics allows, in intensive pig production, to use crossbred females and sometimes also crossbred males. The reason is twofold:

1. On the one hand, the high costs of genetic improvement are lowered.
2. On the other hand, certain advantages that crossbred animals present are taken advantage of; for example, a slightly larger litter size.

Although two, three or four breeds can be used in crosses, crossbreeding with three breeds is the most widespread in swine genetics. The most used breeds are usually Landrace and Large White to produce the hybrid female, but for the finishing male there is a remarkable variety of options depending on the destination of the product.

In pig genetic improvement programs there are:

• Maternal lines that, although they are also selected for productive characteristics, their selection is predominantly for reproductive characteristics and maternal aptitude.
• Paternal lines, selected for production characters and meat quality.

In general, in the first stage of a three-way cross (although there can be four if four lines are involved), two maternal lines give rise to a crossed female that is later mated or inseminated with a male from a paternal line.

Pig genetics has a pyramidal structure, in which different levels are distinguished. There are some animals that we call grandmothers and great-grandmothers (also grandfathers and great-grandfathers) that would be purebred, while hybrid animals appear in the production stratum.

Hybrid animals are the mothers that produce the piglets that will go to the slaughterhouse and, likewise, the piglets themselves, since they are the result of covering these hybrid mothers with semen from males of another or other breeds.

The reason for using crossbred animals is to take advantage of the complementarity of the breeds (some are better as mothers, while others provide, for example, greater muscular development), as well as the hybrid vigor, that provides a plus to the prolificacy of crossbred females and piglet survival.

Pig genetics is focused on obtaining greater economic performance from the animals. In pigs, only characteristics of economic importance are selected, those that, when improved, represent a greater profit for the producer.

The main characteristics or variables to be selected in pigs are classified as: reproductive, morphological, production and carcass.

The main objective of the reproductive traits is to reduce the costs of the weaned piglet, which is directly related to the numerical productivity of the sow. While the selection objective on production yields tries to improve the profit margin by reducing feed costs, considering the quality of the meat.