The female pig (sow) is ready to breed (reaches puberty) at 5 months of age and will show signs of being in heat. Some slow growing types and animals which are underfed will be older when they reach puberty. The sow will come into heat every 3 weeks throughout the year if she is not mated.
Most breeds of pig reach puberty at 5 months of age but some, e.g. the Chinese pig, come into heat for the first time at 3 months of age when they have enough good feed and water.
The pig should not be used for breeding when she comes into heat for the first time. It is wiser to allow her to grow for another month before using her for breeding. She will then be better able to carry and suckle a good litter of young. Only sows with 14 teats should be used for breeding so that all her litter can feed.
If the pig is not mated she will come into heat every 21 days, provided she has enough feed and water.
The female pig coming into heat is restless and may not eat. The vulva becomes pink and swollen. When the pig is pressed hard with the hands on either side of her back she will stand still, showing she is ready to accept the male. The sow will be in heat for 8 to 36 hours.
Healthy, well fed sows can be brought into heat so that breeding can be controlled.
Putting a sow which is in heat in with those which are not in heat will make some of the latter come into heat. A better method is to pen sows next to a boar so that they can see and smell him. The sows will come into heat especially if the boar is old and smelly.
Remember that failure to come into heat can be the result of poor or too little feed or a health problem in the sow.
Source : Pashu sakhi Handbook