How Far We've Come

I was flattered when I was asked to discuss how far we've come in equine reproduction. I found it interesting to go back and review the many changes that have occurred.

Stallions

The use of artificial insemination has increased over the past several decades in all breeds except the Thoroughbred. Commercial skim milk extenders containing antibiotics provide a convenient means of diluting semen and protecting sperm. Semen extenders have been developed for maintaining sperm at 5°C during transport so mares can remain at home with semen shipped to them. Since major breed registries now allow breeding with frozen-thawed semen, the number of foals born from this is likely to increase dramatically.

Techniques have been developed that allow insemination of lower numbers of sperm into mares using techniques such as rectally guided insemination with a pipette or endoscopic insemination of sperm directly onto the uterotubal junction. This is useful when the amount of semen is limited, such as frozen semen from a dead stallion. The ultimate in low-dose insemination is the injection of one sperm into an egg, called "intracytoplasmic sperm injection (ICSI)." This technique is now commercially used to obtain pregnancies from subfertile stallions, stallions with low sperm numbers or poor semen quality, epididymal sperm, or frozen semen. With ICSI, a straw of semen can be thawed, diluted 100-1,000-fold, refrozen, then thawed again and used for sperm injection.

New techniques using the flow cytometer have been developed to evaluate several attributes of sperm simultaneously in a matter of seconds. Using molecular techniques, scientists are now looking at specific seminal plasma proteins and sperm proteins that might be related to fertilization, sperm binding to the oocyte and, ultimately, fertility. In the future, it might be possible to select stallions based on these genetic markers as an indication of fertility.

Thanks to research by Dr. Sue McDonnell (PhD, Certified AAB) at the University of Pennsylvania, we now know more about stallion sexual behavior and how housing affects sex drive and hormone levels.

Mares

Without a doubt, the most significant change in equine reproduction in the past several decades has been the advent of ultrasonography. This technique has allowed early pregnancy detection, twin reduction, evaluation of early embryonic death, fetal sexing, estrus detection, and determination of whether a follicle is close to ovulation. The use of hormones for induction of ovulation has allowed us to more precisely time breeding or insemination. This would include the use of human chorionic gonadotropin (hCG), gonadotropin-releasing hormone, (GnRH), deslorelin, and more recently recombinant equine luteinizing hormone (LH). The progestin Regumate has been a valuable tool for estrous suppression, estrous synchronization, and pregnancy maintenance. Oxytocin use after breeding has become commonplace for combating post-breeding endometritis.

Equine embryo transfer was first introduced in the late 1970s. New developments for cooling and storage of equine embryos allows collection of the embryo at one farm and embryo shipment to another facility for transfer. Multiple foals can be registered from one mare in some breeds. Equine follicle-stimulating hormone (eFSH) can be used to superovulate mares, which might increase the efficiency of equine embryo transfer.

Mares no longer able to produce embryos can have oocytes (eggs) transfered directly from the ovary using a transvaginal ultrasound probe. Upon oocyte transfer into recipient mares, approximately 30-40% become pregnant.

Ovaries can be harvested from a dead mare, shipped to a facility where the oocytes are collected, matured, and transferred into recipients, and get live foals. The same is true for shipping testicles from a stallion that died prematurely. Sperm can be collected from the epididymis and frozen for later use.

It would be difficult to write a review of the advances in equine reproduction and not mention cloning. If someone had told me 10 years ago that we would be cloning horses, I would have thought they were crazy. However, here we are in 2006, several foals have been produced, and tissues from many mares, stallions, and geldings have been collected for future cloning. In the next several decades, similar advances will be made in reproduction.

About the Author

E.L. Squires, MS, PhD

Edward Squires, BS, MS, PhD, is a professor at University of Kentucky and the Director of Industry Relations for The Gluck Equine Research Center. Squires authored Understanding The Stallion, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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