Whether a mare is maiden, recently foaled, or barren can influence her ability to conceive when breeding her with frozen semen. The first pregnancy obtained by using frozen-thawed semen in the equine species dates back to 1957 and was obtained using epididymal semen. However, for many years this reproductive technology has achieved limited progress in the horse compared with other species such as cattle. This was mainly because for a very long period of time only a few horse registries allowed the use of frozen-thawed semen. Hence, economic interests and resources allocated to research have always been minimal and as a result obstructing advances in this area.

It is a common belief of many veterinarians and breeders that the lifespan of frozen semen within the mare’s reproductive tract is reduced compared to fresh semen. While no experimental fertility trials have been conducted to directly address this question the type of damage that can occur to sperm membranes during the freezing and thawing process theoretically could reduce the longevity of semen. Sperm binding to the lining of the oviduct can extend fertilizing capacity in the mare and there is in vitro evidence that frozen-thawed sperm do not bind as well to the oviductal epithelium. Therefore, it is generally accepted that the optimum time for insemination of frozen semen is in the period from 12 hours prior and up to 6 hours after ovulation. The timing of insemination is a good topic for debate and many mare owners and veterinarians have their preferred method of choice. Generally opinion is split between a one dose post-ovulatory insemination or a two dose timed insemination protocol. We review the pros and cons of both options here.