Breed pigs by introducing a sexually mature boar to a female pig that is in estrus, or via artificial insemination of a female that is in estrus. Attempting to breed a pig outside of estrus does not result in a pregnancy, even if the animal is healthy and well-fed.
Female pigs have estrus cycles of 21 days. During the course of this cycle, hormonal changes cause the reproductive cells inside the female's ovaries to mature: a process called ovulation. If the pig has not ovulated, there are no cells for the male's sperm to fertilize if it is bred, resulting in no pregnancy. Estrus can be triggered by presenting the female with a mature male or by keeping it with other females that are already in estrus.
Once the female is in estrus, it can be introduced to the male. A sexually receptive female stands rigidly and allows itself to be mounted. A farmer may mate the male with the female at set times, or he may simply pen the male with the female and allow them to mate at will. The first method is called hand mating and the second is pen mating.
Artificial insemination requires healthy boar sperm and a catheter. The female's vulva is cleaned; then, the catheter is lubricated and inserted into the vagina. It must be inserted deeply enough to reach the cervix. Care must be taken so as to not to cause urine to flow into the catheter, because urine kills sperm. If any urine flows into the catheter, it must be replaced.
Once the catheter is correctly inserted, a bottle of sperm is attached to the other end. The sperm flows into the female and travels up through the cervix via a series of uterine contractions.