Artificial insemination, or AI, has been performed in horses since the end of the 18th century but embryo transfer in horses has lagged behind other species, especially cattle. Initially, difficulties in transporting equine semen restricted use of AI to the premises where the semen was collected. However, improvements in technology have allowed AI using cold and frozen semen to become common in today's horse industry. When natural breeding was compared to AI, there was no difference in pregnancy rates when breeding fertile mares, so breeding mares by AI is very effective. However, in AI breeding the timing near ovulation is critical and extremely critical when using frozen semen. In fact, frozen semen is also placed deep in the uterine horn of the mare adjacent to the follicle that is going to ovulate because this increases pregnancy rate.

Embryo transfer allows a breeder to obtain more than one foal from a valuable mare per year. Although cattle are superovulated to produce multiple ova for breeding, Dr. Jacob Scherzer indicates in the Compendium that this process has not been as successful in horses as in cattle. Superovulation has been shown to produce on average almost two embryos from these mares whereas mares without superovulation produce about one half of an embryo per mare. Consequently, mares used in embryo transfer are four times more likely to produce an embryo than if they are not superovulated. For embryo transfer, the mare's uterus is flushed with a sterile fluid and embryos are recovered and then either transported directly into a recipient mare or can be frozen for later implantation. For more information on embryo transfer in horses or AI, contact your equine veterinarian.