If you understand what is happening with the egg and sperm it is easier breed correctly.
Meeting up in the oviduct:
Sperm need 8 hours swimming
Eggs life is 10 hours waiting
You need perfect timing!
As in a human, the female pig is born with a limited number of ovocyte primary follicles that will be available every twenty-one days. She will have about 400,000 in her ovaries.
During each twenty-one day cycle about fifty ovocytes will start the process of development. As they develop they will secret estrogen which will stimulate the hypothalamus and pituitary gland. As the days in the cycle progress the follicles will increase in size until they have a break in the follicular wall which frees the ovocyte follicles (ovules) in to the oviduct. This freeing process takes about three hours.
From the fifty ovocytes that started there are now about 25 ovules that have matured and been released. Once released they have a life span of only about ten hours. The fertilization of the ovules takes place in the oviduct close to the ovary.
The introduced sperm (spermatozoids) have a much longer viable life span than the ovules, they can survive from one to three days. To have a perfect breeding and fertilization the sperm must meet the ovocytes when the ovocytes have just been released or within the ten hour window.
To understand the perfect time to introduce the sperm we need to know about the time it takes them to reach the ovules. Once the semen is introduced using normal (not intrauterine) insemination the sperm take about three hours to move from the cervix to the oviduct. The sow releases Oxytocin from the pituitary gland that causes the uterus to contract which helps the sperm move along. Sperm are able to move about 5mm per minute so they need the contractions of the uterus to help them move along. About 30 percent of the sperm will reach the oviduct.
It takes the sperm about eight hours to reach the ovocytes in the oviduct. During this time the sperm are maturing by a process called capacitation during which cells in the uterus secrete substances that mature the sperm. Once mature they will be able to fertilize the ovocytes. On average it takes about eight hours for the sperm to travel to the ovocytes while maturing on the way.
If the exact time that sow would ovulate we could have a perfect one time insemination exactly 8 to 12 hours prior to the ovulation which would give the sperm the eight hours needed to reach the ovocytes exactly when they are released or just before.
An immediate conclusion would be that it is better to error on breeding a little too soon then too late as the sperm have a longer life span and are more capable of waiting for the ovocytes. If you inseminate at the exact time of ovulation the sperm will just be reaching the ovocytes when the ovocytes are at the end of their viable time frame resulting a poor fertilization rate.
If you know your sows, hard to know a gilt, you may be able to judge when it is about 12 hours before the sow is standing perfect and ovulating.
The Perfect Time