Producing more pigs per sow per year--genetic contributions.

This review paper summarizes available knowledge on the genetic manipulation of litter size in pigs. Selection among breeds permits the exploitation of existing variation and this has already proceeded much further in Europe than in the United States. Crossbreeding strategies are available to enable the commercial herd to maximize sow productivity while ensuring carcasses acceptable to each particular market demand. These involve either the regular purchase of both replacement gilts and boars, or a high standard of management of the herd breeding program. Selection within purebred lines to increase further prolificacy seems possible, in spite of some contrary results from initial experiments. Success will only be achieved in well-designed and carefully executed programs with adequate population size that are continued for many generations. It is likely that breeders can continue to improve the potential of their stock at the commercial level, and this will be achieved by a degree of specialization between sire and dam lines.