For this event to take place, the egg must be released when a male sperm is available for fertilization. This process of releasing an egg from the ovary is referred to as ovulation. The egg travels from the ovary to the tube called the oviduct. It is in the first one-third of the tube that a sperm or group of sperm need to rendezvous with the egg in order for fertilization to occur.
Timing is everything.
It has long been thought that it is best if the sperm are already in the oviduct waiting for the egg to make her grand entrance. We now know that this is not always necessary. If sperm is placed in the mare's reproductive tract up to 10 hours after ovulation, there is still a good chance that pregnancy will occur. After ten hours, the odds seem to drop at a rate of about 10 percent per hour. Twenty to 24 hours after ovulation, the chances of fertilization are near zero.