Temporary storage of equine embryos necessitates cautious regulation of temperatures and holding in a special solution. Continuing storage, commonly anything in excess of a day requires that embryos be frozen or cryo-preserved under proscribed programs and stored in liquid nitrogen. When frozen, embryos remain stable for many years. This implies that embryos may be collected at times when a pregnancy is not normally desired and frozen for transfer at a later date. This can add considerable flexibility to numerous equine embryo transfers programs, and permits the international or inter-continental movement of equine genetics. Nonetheless, in addition to the concerns that apply to the transfer of fresh embryos, frozen embryos must be handled with care and liquefied under suitable conditions prior to their transfer into recipients, and at the appropriate stage of their cycle. It is atypical for pregnancy rates with frozen embryos to be as high as those obtained with fresh transferred embryos.

Embryo transfer has been recognized as a valuable process to optimize productivity of the mare. Methods to cool, which is 5OC and store embryos for up to 24 hours have been utilized to allow the transport of embryos to remote farms or facilities for transfer. While cooling embryos offers a limited duration of storage, embryo cryopreservation would allow storage for extended periods. The first colt was reported in 1982 after an embryo was cryopreserved by traditional methods. Conventional cryopreservation involves the revelation of an embryo to increasing concentrations of permeating cryoprotectants, dehydrating the embryo while cooling at approximately 0.5OC per minute. At approximately -6OC, the formation of ice crystals is induced and the temperature is further reduced. The embryo is eventually stored in liquid nitrogen at -196 OC. The melting process usually involves a series of steps, with embryos exposed to decreasing levels of cryoprotectants. Although successful, conventional cryopreservation is time-consuming, with the freezing process lasting approximately 1.5 hours, and embryos are usually frozen using specialized and often expensive equipment.

Procreation of a mare can be a luxurious plan. The cost of producing a foal depends on the initial value of the donor mare, and consequently her yearly reduction. Extra costs involve the maintenance of the mare, semen costs, and veterinary costs among others. Through the use of embryo transfer (ET), the costs of maintaining recipient mares must also be added. Further the fertility of the stallion, the success to obtain the embryo and achieve a pregnancy all influenced the final cost. The main cost of producing a foal comes from the decrease of the dam. Embryo transfer permits a mare to produce more foals per year and thus reduces the per foal depreciation cost. The second most important cost of producing pregnancies using ET originates from the maintenance of suitable recipient mares prior to the transfer. No less than three recipient mares have to be available per donor mare, so at the time of transfer one will be in good reproductive synchrony with the donor mare. New technology has enabled us to freeze horse embryos so that fewer recipient mares are needed leading to reduction of the overall cost. The cost of semen is low in comparison to all the other expenses. It shows that only the best stallions must be used, even if they are more expensive. When one has a valuable mare, producing a foal by ET is more economical, by having an inexpensive mare to carry the pregnancy. This added efficiency is the result of the donor mare producing several foals per year, as opposed to just one.