The gilt or sow usually ovulates at 30-36 hours after her first standing reflex. The ova lives no more than about 3-4 hours if not fertilized. Sperm live approximately 24 hours in the female tract, and it may take 1-3 hours to reach the oviduct after insemination. The sperm must reside in the female 8-12 hours before it is capable of fertilizing ova.
Although there is a possibility of conception when inseminating any time during standing heat, timeliness of mating(s) within this period will increase conception rates and litter sizes. If you want control of optimum breeding time, you will check for heat every twelve hours (twice daily). Gilts and sows should be bred 12-24 hours after they are found in standing heat. A second breeding should follow twelve hours later.
Throughout the estrous period, sows and gilts have fluctuating degrees of receptivity to a breeding service. This is true for both natural and artificial services. Sows and gilts have a ‘standing reflex’ periodically throughout the estrous period, and it is this ‘reflex’ which enables the boar or inseminator to service the female. This ‘reflex’ is triggered primarily by odors from the boar, along with sight and touch, and is an intense reaction which causes the female to become rigid and receptive. This ‘reflex’ also results in more dramatic uterine contractions that help to transport the seminal fluid to the oviducts. It is in this highly-receptive state that a female should be serviced, if best results are to be obtained.
However, a sow or gilt cannot maintain this state of receptivity throughout the estrous period. She will have peaks and valleys throughout estrus in which she will show a ‘standing reflex’ followed by periods of ‘refractoriness.’ Typically, a sow will be receptive for 5-15 minutes and ‘refractory’ for approximately 45 minutes or more.
To optimize breeding success with AI, boars should be maintained in a completely separate room or area prior to heat checks and breeding. If this practice is followed, sows and gilts tend to have more dramatic ‘standing reflexes’ when the boar is introduced. This occurs because the females are not continually ‘reflexing’ throughout the day in response to continual boar contact. Additionally, if boars are in contact with sows and gilts only when heat checking, there is less chance that a female may enter a ‘standing reflex’ shortly prior to the check and be in the ‘refractory’ state at the time of the check. This will greatly reduce the possibility of an estrous female falsely giving indications of not being in estrus.
Furthermore, be ready to service the female immediately during her period of ‘standing reflex’ caused by her contact with a boar. This is the optimal time for insemination, and it should not be delayed because the semen or supplies are not ready. It is important to time inseminations properly from the onset of estrus. Never perform inseminations on a female not exhibiting ‘standing reflex.’
Sows will peak once per hour or longer if continually exposed to a boar. This ‘standing reflex’ may last 5-15 minutes and is characterized by a dramatic rise followed by a gradual fall.