Equine embryo transfer has become a widely accepted technique in which a donor mare is mated and the resulting conceptus (embryo)
is removed from her uterus and transferred to a suitable recipient mare, where it will undergo suitable maturation and development.
The first successful embryo transfer in horses was performed in 1972, and since that time, the procedure has gained widespread
- Embryo transfer is used so:
- Multiple offspring from one donor mare can be used.
- Donor mares can continue in athletic competition without having to carry a foal.
- Offspring can be produced from mares at an earlier age than usual (i.e. 2 years old).
- Mares that have experienced various reproductive problems can breed or carry.
- Mares with physical disabilities that make it difficult for them to carry a foal to term can still breed.
- Researchers can learn more about various aspects of equine reproduction.
- Rare or endangered equines have a better chance prospering.
Most breed registries allow this technique, and some even allow the registration of multiple foals in any given year, such
as the Quarter Horse. The breed registry guidelines should be consulted. Using reproductively normal donor mares and stallions
of known, good fertility, an embryo can be recovered approximately 70 percent of the time a recovery attempt is made. In contrast,
mares with poor reproductive histories or aged mares (greater than 18 years of age) have poor recovery rates (less than 30
percent). Aged mares also have fewer normal oocytes, which contribute to poorer embryo recovery. Remember, embryo transfer
is not a cure for infertility!
In recent years, a purified eFSH product has come available to potentially increase the number of ovulations and recovery
of more than one embryo per attempt. The selection of the stallion for use in an embryo transfer program is also extremely
important. The use of fresh or fresh-cooled semen from a stallion of known good fertility can increase your embryo recovery
rate. In contrast, the use of poor-quality semen or frozen semen in general will lower your embryo collection rates. The worst-case
scenario is to breed older subfertile mares with poor-quality semen or frozen semen.
Initial mare exams
Each donor mare should undergo a complete reproductive examination prior to enrolling in an embryo transfer program to
help ascertain a realistic probability of harvesting an embryo successfully.
Donor mares should be cycling normally and in good physical condition. Any uterine pathology or physical abnormalities should
be corrected prior to the breeding season. For best results, donor, recipients and stallions should be on the same premises
and managed by the veterinarian in charge of the embryo transfer program to maximize embryo recovery.
Recipient mares ideally should be young, reproductively normal mares that have undergone a thorough physical and reproductive
examination. The proper selection and management of recipient mares may be the most important factors affecting a successful
embryo transfer program.
Both donors and recipients must be reproductively managed to be in synchrony with each others' reproductive cycles. Ideally,
two to three recipients should be available for each donor mare. Cycles usually are synchronized using a combination of progesterone-estrogen
(P and E) and prostaglandin. Ovulation usually is induced by the use of HCG or GnRH analog. The recipient should ovulate from
two days before the donor or up to three days after the donor. Ideally, the recipient should ovulate one day after the donor
mare. Alternatively, ovariectomized mares that are suitably prepared with hormones also can be used as recipients.
Embryo transfer procedure
The most common method of embryo recovery is the non-surgical transcervical uterine lavage technique. This procedure usually
is carried out on day seven post ovulation (where detected ovulation is day zero). Donor mares that are older (18 years of
age) generally are flushed on day eight because the embryo seems to be smaller and less developed in these older mares. Because
the equine embryo doesn't enter the uterus until about day six post ovulation, earlier attempts to recover the embryo may
result in not recovering an embryo. Recent work has indicated that the non-surgical transfer of day-nine embryos can be as
successful as day seven or eight post-ovulation embryos.