One mare conceives, while another one carries the foal. That is how embryo transfer maximizes the production of a valuable mare.
This assisted breeding procedure produces foals despite your mare’s training, age, or condition, and also allows breeders to double or quadruple the foal output. We’ll explore reasons you might consider embryo transfer, and the facts about this option.
Why to Use Embryo Transfer
Breeders have different motivations for choosing this method of equine reproduction.
For the investment. You can increase the number of foals from your mare by transferring embryos to surrogate mares early in the season, and then letting the mare carry a foal herself.
For a show mare, embryo transfer allows breeders to keep her in training and earn money by selling her foals. In other words, your dual-career mare can compete on the show circuit and still breed. Two winning sport horse mares, the Hanoverian Breanna and Westfalen Dorina, have done so for their owners.
Vickie Meisenburg, DVM, and co-owner of EquiGen LLC in Archer, Fla., explains, “We do a lot of dressage mares. People are getting smart. They have up and coming, FEI potential young mares. They see they need progeny on the ground, but can’t take the mare out of training.”
A young mare that produces foals early earns a show record while her foals increase in value. “With a five-year-old, her value is increasing exponentially,” says Meisenburg. “But take her out of training a year, and you’re behind.”
For sentiment. You can replace a special mare, without putting her through the stress of gestation and foaling. Even if she’s older and never bred, she can still give you a foal.
For the mare with a history of breeding problems. A broodmare may have a history of repeated abortions or be unable to deliver a foal. Anja Stoll, DVM, University of California, Davis, says, “Older mares that have had trouble carrying a foal to term in the past are good candidates for embryo transfer, as well as mares that have other conditions, such as laminitis or other severe lameness, that would not allow them to carry a foal to term.” If the mare can conceive, she’s a candidate for embryo transfer. And if she still produces eggs (oocytes), oocyte transfer is another method. She donates an oocyte, which is transferred to the recipient mare for fertilization.
For the breed. To continue certain bloodlines, your mare can contribute her genes toward breed improvement. Through embryo transfer, she can furnish foals to meet the demand for the pedigree she represents.
Producing a foal through a surrogate dam involves several parties: the mare’s owner, the owner of the surrogate, veterinarians who specialize in equine reproduction, and the officials of breed registries.
For you, a performance mare won’t have to take time off. “It disrupts their training minimally,” says Meisenburg. “They can go back to training the night after being flushed. We don’t have to sedate them.”
Clarice Cooper of Camelot Stallion Station, Maysville, Ga., agrees. “We get a lot of show mares. They have only a short amount of time for breeding, and they are in and out really quick.”
Most donor mares are sport horses, Arabians, or Quarter Horses. Registries such as the American Hanoverian Society, Arabian Horse Society, and American Quarter Horse Association (AQHA) have established rules to accept foals produced through embryo transfer. Rules of the Paint and Appaloosa registries are similar to those of AQHA.
AQHA donor mares must be enrolled before the embryo’s collection: 3,821 mares were so listed in 2007. From the embryos collected in 2006, AQHA registered 4,069 foals in 2007. Like other registries, AQHA requires genetic testing to verify parentage—donor mare, sire, and foal.
How Transfer Succeeds
Your usual veterinarian may have participated in training for the procedure, but the specialists have more experience. The mare will undergo rectal palpation often, so you need a reproduction specialist who’s skillful in examinations. Such professionals are also used to handling mares with reproductive system problems. “The number of pregnancies vary depending on the experience of the person performing the procedure and the quality of mares available,” says Stoll.The transfer involves collecting the embryo from the donor and depositing it into the recipient. Transfer is non-surgical for both mares.
“The donor mare should be free of uterine infection and cycling normally,” says Stoll. And once that is confirmed, “The donor mare is monitored and inseminated as for a standard pregnancy.”
Typically the recipient is a less-valuable mare. Still, health, breeding soundness, and manners are important. Heather Graves, co-owner of EquiGen, says, “We look closely about their handleability. They need good ground manners, and they need to let us handle their feet.”
The recipient does not have to be a proven broodmare. “We haven’t had any trouble with any of the mares rejecting foals,” says Graves. “Even if you have a mare that’s foaled before, you don’t necessarily know the history.”
The breed of the recipient doesn’t matter. The surrogate mare won’t change a foal’s inherited qualities.
Size, though, matters some. Stoll explains, “The size of the recipient mare plays a semi-important role. Embryos from a large-size donor mare should be transferred to a large sized recipient mare, in order for the fetus to grow to its full potential in utero. Embryos from large donor mares can be placed in a smaller recipient mare, but the result is usually a smaller sized foal.”
Cooper uses draft mares “for the size and the temperament. Our main concern is temperament, because the foal learns from the mare. And, physically, our babies are born with more body mass and look more fully developed.”
For the transfer, you are dealing with two mares’ reproductive cycles. “The recipient mare has to be synchronized to the donor mare’s cycle, meaning ovulation of the two mares needs to be timed close together and not more than one to three days apart,” says Stoll. With the timing crucial, reproduction clinics often have two or even three recipient mares available.
The veterinarian must know the exact date the donor mare ovulates. Hormone therapy helps control the estrus cycle, with the veterinarian timing a prostaglandin injection to bring the mare into heat within one to five days.
Previous medication treatments can affect the mare’s cycling. A show mare that has been on Regumate (altrenogest) must resume a normal estrus cycle, which could take two to 12 months. Graves says, “Some may have been on Regumate for years. It does affect their ovaries and ovulation pattern.”
Once ovulation does occur, the donor mare is then bred through artificial insemination or live cover. If she conceives, the embryo enters the uterus on Day 6 post-ovulation.
On Day 6, 7, or 8, the veterinarian flushes the embryo from the mare. To recover it from the donor mare’s uterus involves introducing a sterile solution. Meisenburg explains, “We use solutions that are state of the art, a warmed fluid with sugars in it. In some mares, the uterus is in better condition after the flushing.”
The collected embryo is then placed in a storage solution. If the recipient is onsite and ready for the transfer, the veterinarian places the embryo into the recipient, using an embryo transfer gun. “It is gently deposited into the lumen of the uterus,” says Meisenburg.
Graves adds, “We have a better success rate when the mares are here and we transfer directly.” If the recipient mare is offsite, the cooled embryo is prepared for shipment.
The embryo can also be frozen (cryopreservation) for later transfer. It must be slightly smaller for this, and so is collected at Day 6.
A successful embryo transfer can take two or three cycles. Retrieving the embryo from the donor mare is about a 50 to 60 percent chance, so the veterinarian usually repeats the process.
With an embryo transferred, the next step is confirming the pregnancy in the surrogate mare. Ultrasound can locate the embryo as early as Day 12. If the recipient is not pregnant at 30 days, repeat the procedure, so you avoid losing more time.
Often, the breeding will result in collecting two embryos. “We tend to aim for twins,” says Meisenburg. “If I wait for a 40 mm follicle, I can get multiple embryos.”
From a single breeding and paying one stud fee, you can have the two embryos implanted into two recipients. “We got two foals from our mare this year,” says sport horse breeder Gina Fisk, Santa Fe, N.M. “We got twins from the German stallion Fidertanz, and both took in the recipient mares.” Over the last three years, Fisk has bred several foals through embryo transfer.
Researchers have also increased the number of ovulations using follicle stimulating hormone (FSH). “Superovulation” with this hormone isn’t as successful for breeders, however; in practice, more follicle doesn’t result in a greater number of embryos collected.
Overall, the pregnancy rate through embryo transfer compares to the mare carrying her own foal. “With healthy young donor mares, and healthy young stallions, we get about an 85 to 90 percent success rate,” says Graves. For the cryopreserved embryos, statistics show a 40 to 60 percent pregnancy rate.
Older mares reduce the percentages. Meisenburg explains, “It’s like all of us—as we get older, tissues loosen, hormones slide, and therein lie problems.”
For your success, consult with your equine practitioner to plan your strategy. But be assured, your exceptional mare can produce more foals through embryo transfer.