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Embryo transfer research boosts odds of successful pregnancy

Article

Embryo transfer allows the production of foals from valuable mares that are not capable of carrying a foal to term.

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.

Success factors

The first successful embryo transfer in a horse was reported in 1974.Most breed registries now allow the registration of foals resulting froman embryo transfer. The success rate of the embryo transfer depends uponmany different factors but it is mostly related to the fertility and ageof the donor and recipient mare, synchrony between donor and recipient mareand semen quality of the stallion.

Some subfertile mares have lesions in the reproductive tract that willprevent embryo recovery. These mares are candidates for more advanced reproductivetechnologies in which the oocyte of the donor mare is transferred to theoviduct of the recipient mare.

Factors affecting embryo transfer

* Donor Mare

The embryo recovery and pregnancy rates of subfertile donor mares areusually lower than the embryo recovery and pregnancy rates of young, fertilemares.

Subfertile mares have a high percentage of abnormal embryos that whentransferred into young recipients do not survive. The embryos of subfertilemares are affected by the poor uterine environment between arrival in theuterus on day five and embryo flush on day seven or eight. Breeding soundnessexams should be performed in all donor mares with questionable fertility.

Mares with uterine pathology need to be treated aggressively to improveuterine status and embryo quality.

The release of abnormal oocytes by subfertile donor mares is anotherimportant factor causing low embryo transfer pregnancy rates in these mares.Carnevale and Guinther (1995) demonstrated that the oocytes of old donormares when transferred to young recipient mares resulted in a low pregnancyrate, suggesting that oocytes from old mares were defective. Reproductivefailure in subfertile donor mares seems to occur not only because of uterinepathology, but also due to defective embryos prior to arrival in the uterus.

A major obstacle in the embryo transfer program is that the donor mareis limited to a single embryo per collection. Currently, no commercial protocolexists to induce the production of multiple follicles and, therefore, multipleembryos per collection. The substances and the protocol used to superovulatecows 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 approximatelythree to four ovulations per cycle and two embryos recovered (Squires etal., 1999), which is still much lower than the average number of embryosrecovered from superovulated cows. EPE is not approved for use in maresand research is under way to improve existing superovulatory protocols.

* Recipient Mare

The recipient mare should be young (less than 10 years old) and freeof any uterine pathology (Grade I uterus). Mares that are cycling are preferred,however transitional and ovary-ectomized mares have been used with acceptableresults (Lagneaux and Palmer, 1993; McKinnon et. al., 1988). Cycling recipientmares are synchronized with the donor mare with the use of prostaglandins,altrenogest, progesterone in oil, with and without estradiol. Synchronizationresults may vary and therefore at least two recipient mares per donor arerecommended.

According to a three-year study involving more than 600 mares at ColoradoState University (CSU), the age of the recipient mare, the day of ovulation,and the uterine tone on day five affected significantly pregnancy rate andembryonic loss (Carnevale et al., 2000). The recipient mare can ovulatea day prior, the same day or up to three days after the donor mare. Recipientmares are reevaluated based on palpation and ultrasound prior to the embryotransfer and checked for uterine tone and size of corpus luteum. Mares withgood uterine tone, having no uterine fluid or edema and with the presenceof a good size corpus luteum are considered acceptable for the embryo transfer.

An increasing number of reproductive facilities across the country arecarrying a large recipient herd. Embryos can be collected from donor mares,cooled to 5°C and transported to these facilities by overnight or sameday transport (counter to counter). Since cooled transport does not seemto affect pregnancy rate, it has encouraged many veterinarians to save timeby shipping the embryos and eliminating the cost of keeping recipients.

* Semen Quality

The quality of the semen will affect significantly the results of theembryo transfer.

Embryo recovery from donor mares bred with frozen semen is usually lowerthan that for donor mares bred with fresh or cooled semen. Mares inseminatedwith fresh semen have higher chances of embryo recovery.

New alternatives for subfertile mares

Some subfertile mares fail to produce an embryo that can be transferredto a recipient mare. Problems associated with subfertile mares include ovulationfailure, pathology in the oviduct, failure to transport an embryo from theoviduct to the uterus and uterine pathologies.

Oocyte transfer

Oocyte transfer is the terminology used when the oocyte (egg) of a donormare is transferred into the oviduct (Fallopian tube) of a recipient mare.The recipient mare is inseminated and the embryo is fertilized and developedwithin the recipient mare. CSU offers the largest commercial program foroocyte transfer. The reported pregnancy rate per transfer is around 35 percentfor their commercial program and around 80 percent when research mares ofgood fertility and good semen quality are used. The lower pregnancy ratefor the commercial program is due to older subfertile mares carrying defectiveoocytes 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 transferredinto the oviduct with the oocyte. It allows the use of a small number ofsperm and it has the potential to produce pregnancies from frozen semenand from subfertile stallions. The embryo development rate was around 55percent with the use of raw semen (Coutinho da Silva, 2002), but the resultswere low with the use of cooled semen. Further research is needed beforethis technique becomes commercially available.

Conclusion

The embryo transfer technique has not undergone any major changes duringthe past several years. However, continuous research has promoted more knowledgeabout the factors affecting embryo recovery and survival and therefore veterinarianshave been able to improve the overall pregnancy results.

The major changes that have occurred in the industry over the past twoyears are the widespread use of transported cooled embryos to large reproductivefacilities and the opportunity to successfully transfer oocytes of donormares into recipient mares on a commercial basis.

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