Stallion semen is frozen by a special process, using semen extenders which may vary from horse to horse. Once frozen (down to -140°C) the semen - usually in half ml plastic straws - is stored in liquid nitrogen inside insulated storage tanks. It is removed immeadiately before insemination and thawed quickly to body temperature, then placed in the mare's uterus using sterile pre-warmed equipment.
AI with thawed frozen semen requires special management of the mare's cycle because the sperm cells are viable for a shorter period of time inside the mare than those in either fresh or chilled semen. On the other hand, frozen semen is always available and does not have to be ordered and collected to fit in with projected ovulation time.
The shorter "life" of the sperm cells after insemination means that AI must occur within 6 hours of the mare's ovary releasing the egg at ovulation of a follicle - either before or after.
To get the timing right and the maximum chance of a pregnancy, the mare must be scanned often enough to pin-point the time of ovulation - with some mares, this means every 6 hours. Frozen semen AI is greatly assisted by the use of ovulating drugs