Written by Kentucky Equine Research Staff.
Embryo transfer is a technique in which a mare (donor) conceives through natural or artificial insemination and the embryo is flushed from the uterus before it implants. The embryo is then introduced into the uterus of another mare (recipient) where it implants and matures. If all goes well, the recipient mare gives birth to the foal after a normal-length gestation. Embryo transfer can be used to produce a foal from a mare that has had difficulty carrying a pregnancy to term, and also for mares that are in an ongoing performance career.
Several studies have looked at the influence of the recipient mare on size, conformation, and growth of the embryo. The most successful donor mares should be healthy and preferably young (4 to 10 years of age). They can be maidens or may have had former pregnancies and deliveries that were uncomplicated.
Several studies have looked at the influence of the recipient mare on size, conformation, and growth of the embryo. In one study, pony embryos were transferred into Thoroughbred mares (termed “luxurious” gestation) and Thoroughbred embryos were transferred into pony mares (termed “restricted” gestation). The resulting foals were compared to foals produced from mares of the same breeds, and growth was tracked until all foals reached three years of age. Pony foals carried by Thoroughbred mares were larger at birth than pony foals carried by pony mares, and Thoroughbred foals carried by Thoroughbred mares were larger at birth than Thoroughbred foals carried by pony mares. All Thoroughbred foals were larger than all pony foals at both birth and age three. However, the researchers concluded that neither the “restricted” nor the “luxurious” gestation had any major or long-lasting effect on growth or development. In the three-year-old horses, “restricted” individuals were slightly smaller than controls and “luxurious” individuals were slightly larger than controls, but these measurements were not statistically different.
In another study, researchers used ponies, saddlebred horses, and draft horses to examine fetal growth, birth weight, fasting glucose levels, and glucose metabolism in foals produced by embryo transfer. Mares of each breed were used as embryo recipients, and only pony and saddlebred mares were used as embryo donors. Gestation was termed “enhanced” when embryos from smaller equines were placed in larger mares or “restricted” when embryos from larger equines were placed in smaller mares. Control pregnancies (pony mares and pony embryos; saddlebred mares and saddlebred embryos) were also included in the study.
Pony foals carried by draft mares were much heavier than pony foals carried by pony mares. They continued to be heavier when weighed at six months of age.
Saddlebred foals carried by pony mares were lighter at birth than saddlebred foals carried by saddlebred mares. By 30 days of age, this difference was no longer significant, but at six months of age, the foals from pony mares were still significantly lighter (29%) compared to those from saddlebred mares. Saddlebred foals from pony mares were significantly lighter than saddlebred foals from draft mares at birth and through 180 days of age. However, saddlebred foals from draft mares were not significantly different in body weight than saddlebred foals from saddlebred mares.
Fasting glucose levels showed significantly reduced plasma concentrations on days 30, 90, and 180 for pony foals carried by draft mares compared to pony foals carried by pony mares. Compared to saddlebreds, ponies appeared to have higher fasting glycemia at most times as well as reduced glucose metabolism at six months. Saddlebred foals from pony mares had higher fasting glucose concentrations but reduced insulin secretion compared to saddlebred foals from draft mares. Pony foals from draft mares showed decreased fasting glucose at most times, and when these young foals were given glucose tolerance tests, they showed some insulin resistance shortly after birth. This is a contrast to the control pony foals from pony mares in which insulin resistance developed at six months of age.