University of Illinois Extension

Swine Reproduction Papers

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Selection for Reproduction
SwineReproNet Staff


Boars serve the dual purposes of contributing genetic material for reproduction and for determining when females are fertile. To accomplish both of these needs effectively, the boar should be mature and have high libido.


Boars reach sexual maturity around 6 to 7 months of age. However, they begin mounting as early as 4-5 months of age, but are not fertile. Once puberty is reached, measures of maturity such as levels of male hormones and even fertility estimates increase as the boar ages up to 2 years of age. Therafter, little increases are observed.

Fertility Measures

Measures of fertility for a boar can include testes size, libido (which includes aggressive behavior, vocalization (chanting), and physical size and appearance, and semen characteristics. Selection for boars can initially include testes size, which has a moderate heritability. This trait is related to sperm production and even to advanced age of puberty in siters and daughters.

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