An extremely fascinating aspect to equine embryo transfer is that donor mares may continue in competition or training and still produce a foal without becoming pregnant themselves. Embryo collections intended for mares that foal late in a year are being performed by transferring the embryo to a recipient or it is being frozen. The donor mare bred earlier the following year. The international movement of equine genetics through the import and export of frozen embryos have considerably lower the cost of transporting embryos as compared to that of the live mares. Then the international concerns regarding disease control are being tackled through embryo transfer talks. Embryo donor mares may be raised through artificial insemination or by natural cover. In any case, it is essential that the time of ovulation is known within 12 hours. Embryo recovery is usually attempted on day 7, where day 0 is the day of ovulation. A fertilized embryo can be recovered on day 7 in 60-70% of attempts through natural breeding and with assumption of the normal fertility of the stallion and mare. If artificial inseminations with older infertile mares or frozen semen are part of embryo program, then the rate of embryo recovery is low.

Either super-ovulation or administering hormones to increase the number of developing follicles oftentimes are involved in embryo transfer in cattle. Mares do not respond to super-ovulation. In the majority of embryo-collection attempts only one embryo, and rarely two embryos in the event of double ovulations is usually recovered. Embryo recovery attempts can be repeated on uninterrupted heats. So as to shorten the period between collection attempts, or to concentrate collections within a particular time frame, mares can be ’short cycled’ using prostaglandin. Careful supervision of follicle development and ovulation is important. After embryo recovery is attempted, and regardless if other collection attempts are planned, an injection of prostaglandin is given following a flush to remove residual fluid from the mare’s uterus. This will excite a return to normal estrus within a few days though the interval between estrus periods may be slightly longer than normal. Embryo collections should not as a rule multiply the risk of uterine infections or decrease mare fertility. In fact, embryo collection attempts by uterine flushing may be a useful diagnostic test and can form part of the treatment of subtle uterine infections. The fluid recovered from an infected uterus is normally cloudy and may contain flecks of pus, indicating the need for additional investigation or treatment.

Early embryonic development in the horse is exemplified through the formation of an atypical acellular glycoprotein “capsule” between the trophectoderm and the overlying zona pellucida. This structure is first detected between days 6 and 7 after ovulation and completely envelops the spherical conceptus until as late as day 23 of gestation. In the present study, a micromanipulator was used to remove the capsule from 15 embryos on day 6-7 after ovulation. None of these denuded embryos developed into ultrasonographically detectable pregnancies after surgical transfer into recipient mares whereas four of six control embryos handled and transferred similarly but without capsule removal developed normally, thereby demonstrating clearly a role for the capsule in embryonic survival. Moreover, observation of the embryonic investments after embryo collection and during micromanipulation led to the hypothesis that hatching of the horse embryo from its zona pellucida is assisted by the force of the expanding capsule, which causes the attenuating zona to literally burst open.