One of the most important components of a successful embryo transfer is the recipient mare. Recipient mares should be between 3 and 15 years of age and within a 200kg weight range of the donor mare, in good physical condition and easy to handle. A potential recipient mare should be critically evaluated before making the decision to use her as a surrogate. A general physical examination should be performed, followed by a thorough reproductive evaluation to determine if the mare has any condition that may affect her ability to become pregnant after transfer or prevent her from carrying a foal to term. Recipients may receive 2 or maximally 3 embryos in a given year. If they fail to come pregnant or keep a pregnancy after these 2 or 3 opportunities, they should no longer be used as embryo recipients.
At Equiception we maintain our recipient mares under a stimulatory artificial photoperiod beginning on approximately December 1st in order to have them cycling and ready to receive embryos early in the breeding season. It is possible to use hormone-treated noncycling or ovariectomized mares as recipients, however pregnancy rates are slightly lower in these mares as compared to synchronized cycling mares.
As Equine embryos are collected on day 7-9 after ovulation they are usually transferred into a recipient mare that ovulated during a window that ranges from 4 days after the donor mare (-4 synchrony) to 1 day prior to the donor mare (+1 synchrony). Successful pregnancies can be obtained after transferring into recipients that ovulated up to 6 days after the donor mare, however, it is most commonly accepted that pregnancy rates are highest if the recipient mare ovulated 1 or 2 days after the donor.
Depending on the situation it may be necessary to actively synchronize the reproductive cycle of the donor mare with a single recipient mare, or, when dealing with a large herd of recipients, oestrous synchronization is not typically required as there are usually enough mares that ovulate spontaneously on any given day to accommodate the collected embryos. Synchronization of oestrus of the donor mare with a single recipient mare is usually accomplished by the administration of prostaglandins. A single dose per mare may be all that is needed, if both the donor and the recipient are in di-oestrus with a responsive corpus luteum (CL), otherwise two doses 14 days apart may be necessary. It is recommended to check the ovarian status of the mares before deciding on the timing of the prostaglandin treatment. To increase the probability that the recipient mare(s) will ovulate slightly after the donor mare, it may be beneficial to stagger the onset of therapy in donors and recipients. Timing of subsequent examinations following prostaglandin treatment is dependent on the size of the largest follicle present. Once both the donor and recipient(s) are in oestrus, appropriate synchronization of ovulation can be obtained by administration of an ovulation-inducing agent (hCG or deslorelin). Mares should be examined daily after administration of hCG or deslorelin to determine the exact day of ovulation. Once the donor mare has ovulated, the recipient mare can be induced so that she will ovulate 1 to 2 days after the donor mare.
It is important to track the recipient mare’s reproductive cycles closely to evaluate follicular development, endometrial oedema pattern, follicle size at ovulation, the exact day of ovulation, number of ovulations, development of the corpus luteum and to detect any reproductive abnormalities. Besides cycle synchrony with the donor, the overall quality of the recipient’s oestrous cycle leading up to ovulation is also an important factor in recipient selection.