Embryo transfer allows the production of foals from valuable mares that are not capable of carrying a foal to term.
Mares in competition can also donate embryos and remain in training.
The first successful embryo transfer in a horse was reported in 1974. Most breed registries now allow the registration of foals resulting from an embryo transfer. The success rate of the embryo transfer depends upon many different factors but it is mostly related to the fertility and age of the donor and recipient mare, synchrony between donor and recipient mare and semen quality of the stallion.
Some subfertile mares have lesions in the reproductive tract that will prevent embryo recovery. These mares are candidates for more advanced reproductive technologies in which the oocyte of the donor mare is transferred to the oviduct of the recipient mare.
Factors affecting embryo transfer
* Donor Mare
The embryo recovery and pregnancy rates of subfertile donor mares are usually lower than the embryo recovery and pregnancy rates of young, fertile mares.
Subfertile mares have a high percentage of abnormal embryos that when transferred into young recipients do not survive. The embryos of subfertile mares are affected by the poor uterine environment between arrival in the uterus on day five and embryo flush on day seven or eight. Breeding soundness exams should be performed in all donor mares with questionable fertility.
Mares with uterine pathology need to be treated aggressively to improve uterine status and embryo quality.
The release of abnormal oocytes by subfertile donor mares is another important factor causing low embryo transfer pregnancy rates in these mares. Carnevale and Guinther (1995) demonstrated that the oocytes of old donor mares when transferred to young recipient mares resulted in a low pregnancy rate, suggesting that oocytes from old mares were defective. Reproductive failure in subfertile donor mares seems to occur not only because of uterine pathology, but also due to defective embryos prior to arrival in the uterus.
A major obstacle in the embryo transfer program is that the donor mare is limited to a single embryo per collection. Currently, no commercial protocol exists to induce the production of multiple follicles and, therefore, multiple embryos per collection. The substances and the protocol used to superovulate cows do not provide good results in mares. Equine pituitary extract (EPE) seems to provide the best superovulatory results in mares. (Alvarenga et.al., 2001; Scoggin et. al., 2002). Research mares treated with EPE had approximately three to four ovulations per cycle and two embryos recovered (Squires et al., 1999), which is still much lower than the average number of embryos recovered from superovulated cows. EPE is not approved for use in mares and research is under way to improve existing superovulatory protocols.
* Recipient Mare
The recipient mare should be young (less than 10 years old) and free of any uterine pathology (Grade I uterus). Mares that are cycling are preferred, however transitional and ovary-ectomized mares have been used with acceptable results (Lagneaux and Palmer, 1993; McKinnon et. al., 1988). Cycling recipient mares are synchronized with the donor mare with the use of prostaglandins, altrenogest, progesterone in oil, with and without estradiol. Synchronization results may vary and therefore at least two recipient mares per donor are recommended.
According to a three-year study involving more than 600 mares at Colorado State University (CSU), the age of the recipient mare, the day of ovulation, and the uterine tone on day five affected significantly pregnancy rate and embryonic loss (Carnevale et al., 2000). The recipient mare can ovulate a day prior, the same day or up to three days after the donor mare. Recipient mares are reevaluated based on palpation and ultrasound prior to the embryo transfer and checked for uterine tone and size of corpus luteum. Mares with good uterine tone, having no uterine fluid or edema and with the presence of a good size corpus luteum are considered acceptable for the embryo transfer.
An increasing number of reproductive facilities across the country are carrying a large recipient herd. Embryos can be collected from donor mares, cooled to 5°C and transported to these facilities by overnight or same day transport (counter to counter). Since cooled transport does not seem to affect pregnancy rate, it has encouraged many veterinarians to save time by shipping the embryos and eliminating the cost of keeping recipients.
* Semen Quality
The quality of the semen will affect significantly the results of the embryo transfer.
Embryo recovery from donor mares bred with frozen semen is usually lower than that for donor mares bred with fresh or cooled semen. Mares inseminated with fresh semen have higher chances of embryo recovery.
New alternatives for subfertile mares
Some subfertile mares fail to produce an embryo that can be transferred to a recipient mare. Problems associated with subfertile mares include ovulation failure, pathology in the oviduct, failure to transport an embryo from the oviduct to the uterus and uterine pathologies.
Oocyte transfer is the terminology used when the oocyte (egg) of a donor mare is transferred into the oviduct (Fallopian tube) of a recipient mare. The recipient mare is inseminated and the embryo is fertilized and developed within the recipient mare. CSU offers the largest commercial program for oocyte transfer. The reported pregnancy rate per transfer is around 35 percent for their commercial program and around 80 percent when research mares of good fertility and good semen quality are used. The lower pregnancy rate for the commercial program is due to older subfertile mares carrying defective oocytes and due to the use of stallions with poor semen quality.
GIFT (Gamete Intrafallopian Transfer)
This technique differs from oocyte transfer in which the sperm is transferred into the oviduct with the oocyte. It allows the use of a small number of sperm and it has the potential to produce pregnancies from frozen semen and from subfertile stallions. The embryo development rate was around 55 percent with the use of raw semen (Coutinho da Silva, 2002), but the results were low with the use of cooled semen. Further research is needed before this technique becomes commercially available.
The embryo transfer technique has not undergone any major changes during the past several years. However, continuous research has promoted more knowledge about the factors affecting embryo recovery and survival and therefore veterinarians have been able to improve the overall pregnancy results.
The major changes that have occurred in the industry over the past two years are the widespread use of transported cooled embryos to large reproductive facilities and the opportunity to successfully transfer oocytes of donor mares into recipient mares on a commercial basis.