Female pigs come into heat every 21 days, at which point certain physical traits and behaviors will be noticeable, to varying degrees, to a keeper trained in what to watch for. If you're attentive to your sow's reproductive schedule, you'll witness a series of indicators from her -- particularly in her vulva, ears and behavior -- that signal she is going into heat.
Why Heat Detection Is Important
Pigs have relatively short gestation periods, deliver larger litters than other livestock, and are considered easy to breed. The preferred method of breeding pigs is artificial insemination, as it doesn't require keeping a boar, an intact male pig, and the feed costs associated with him. Knowing when to inseminate, or serve, a female pig demands her keeper know when the sow's reproductive cycle will render her receptive to the procedure. Female pigs are successfully served when in heat, the ovulation period of her estrus.
Changes in the Vulva
For keepers intending to serve a virgin pig, known as a gilt, detecting estrus is sometimes a bit easier, as the pig's vulva -- the visible area of the pig's reproductive system -- will physically change in an easily observable way. Hormone release in pigs often causes the vulva to become swollen and change color to a deep shade of red; the clitoris, too, may become more prominent, flattening against the body. A watery discharge may be seen around the vulva. The hormonal changes are often less easily observed in darker pigs. The vulva becomes less reactive in bred sows.
Keep Eyes on the Ears
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Another sign of heat is a visible pricking of the sow's ears. Pigs' ears are naturally expressive body parts, able to indicate a variety of things to the trained observer, and this is especially true in the case of heat detection. In pigs with upright ears, such as the Large White or Berkshire breeds, look for the ears to become straight and rigid, pronounced such that they almost touch. Pigs with floppy ears, such as the Gloucestershire Old Spot or Large Black, are a bit less obvious, though you might notice ears twitching and frequently raising when in heat.
Standing Heat Is the Surest Sign
The best way to know when a pig is in heat is to observe her in standing heat, a condition indicating the hypothalamus, pituitary gland and ovaries are working together to render the pig reproductively receptive. The chief indicator is as the name implies: The pig will stand, rigid and motionless, with an arched back and pricked ears, often with a distant look in her eyes. If the sow is with other sows and stands while they mount her, she is in standing heat. Other signs include restlessness, often observed as fence-climbing, and an unwillingness to eat.