Emily Townsend

ANS 242 Section 1

Honors Option

11/23/02

Equine Embryo Transfer

Commonly used in cattle, embryo transfer has now emerged as an innovative tool within the equine reproductive industry. During embryo transfer, a six to eight day old embryo is flushed from the uterus of the donor mare and transferred to a recipient mare, who carries the foal to term. Embryo transfer (ET) has significantly impacted the cattle industry through economic and genetic gain. This procedure achieved popularity once technology enabled the freezing of embryos, thereby making the procedure cheaper, easier, and more reliable. Despite the popularity in cattle, the first equine ET did not occur until 1972, and the procedure wasn’t widely accepted by the horse industry until the early 1980’s. In the late 1980’s when equine embryo cooling technology was developed, ET became more practical. In spite of the late recognition, ET’s impact on equine reproduction has been significant.

Equine embryo transfer provides many opportunities for horse enthusiasts and breeders alike. The prospect for an expanded gene pool and more adaptable breeding programs has greatly increased since the introduction of ET. The importation and exportation of frozen equine embryos promotes a greater overseas exchange of genetic material, which creates an expansive and varied gene pool for breeding programs throughout the world. Not only can one mare produce multiple foals per season, but she is able to be bred at any point throughout the year. Breeders can inseminate, harvest, and freeze the embryo regardless of the season or foaling date. In addition, ET offers a method to produce foals from unsound mares although it is not a cure for infertility. Donor mares are able to reproduce without refraining from athletic competition, and two-year old mares may reproduce without the physical stress and risk of foaling. A high quality mare becomes more profitable through additional foals and the sale of embryos. Through embryo sale, horse enthusiasts can produce better quality foals without the purchase of a high quality broodmare. The per-foal depreciation of a mare decreases significantly with the use of ET. From a genetic perspective, ET encourages the perpetuation of rare or endangered equines as well as exceptional bloodlines. Additionally, many researchers employ ET as a tool to study aspects of equine reproduction. The advantages of using ET range from financial, genetic, or scientific gain, but each application has a broad affect on the equine industry.

Just as many benefits of ET are financial, the disadvantages of the procedure are of a financial nature as well. To create a successful embryo transfer, both the donor and two or three recipient mares must maintain synchronized estrus cycles with the aid of prostaglandins alone or in combination with exogenous progesterone. This, of course, increases the cost of ET and increases the amount of labor involved. In addition, daily teasing and rectal palpations contribute to the added labor. Veterinarian costs, including daily ultrasounds and the actual procedure, significantly increase the price of ET. A sample veterinary charge per diagnosed pregnancy is $4,060. Due to less donor mare depreciation, the cost of producing a foal from a $50,000 mare decreases from approximately $15,000 to $12,000 using ET. Conversely, using ET on a $5,000 donor mare bumps the cost from $5,000 to $10,000. Breeders must consider these costs when evaluating the practicality of ET and it’s relative economic position within their program.

For successful embryo transfers, several conditions must be met. The donor mare must be in good physical condition with a normal cycle. Additionally, the recipients mares need to be reproductively sound and ideally between ages 3 and 10. Proper selection of the recipient mares is considered the key to ET success. As previously mentioned, both the donor and recipient mares must remain in synchronized estrus cycles, where the recipient mare ovulates two days before to three days after the donor mare. Ideally, the recipient mare should ovulate one day after ovulation of the donor mare, and the donor mare should be artificially inseminated as close to her ovulation as possible with at least 500 * 10^6 motile spermatozoa.

After meeting each condition, the donor mare is inseminated and the embryo can be recovered. Prior to day six, the embryo has not entered the uterus and recovery attempts are typically unsuccessful. The highest recovery rates of 76% and 78% occur on day seven and day eight respectively. After day eight, the embryo becomes too large and fragile for successful recovery. During embryo recovery, a catheter with a cuff to seal the cervix is passed into the uterus through the vagina. Once the catheter is in place, the uterus is flushed with two liters of Dulbecco’s phosphate buffered saline containing 1% newborn calf serum. Then, the solution is expelled via catheter through an embryo recovery filter. This process is repeated three or four times, with 95% of the fluid being recovered. After removing the catheter, the embryo recovery filter is examined with a stereo dissecting microscope or poured into a search dish to locate the embryo. Once located, the embryo is rinsed in a culture media to prepare for the transfer. The embryo can be stored in the media for 12-24 hours, but immediate transfer is recommended.

The transfer to the recipient mare occurs through surgical or non-surgical methods. The former consists of a standing flank laparotomy. This exteriorizes the uterine horn, which is then punctured by a cutting-edge suture needle. The embryo is deposited into the uterine lumen, the uterine horn is returned to the abdomen, and the incision is closed. Using this method, there is a 10% increase in successful transfers over the non-surgical method. Non-surgically, an artificial insemination pipette or an “insemination gun” is manually inserted into the vagina and passed through the cervix to the uterine body. The embryo can be deposited into the uterine body or into the uterine horn with trans-rectal manipulation. The latter method is much less invasive yet not as successful, which raises several ethical questions.

As the procedures of embryo transfer advance, ethical issues arise. Naturally, a mare produces one foal per year, yet this technology significantly increases that figure. Combating Mother Nature with man-made technology always creates problems. Unfortunately, the consequences are still unknown. Without regulation, ET could create a large impact on the gene pool and horse population. In certain cases, ET is used to produce foals from reproductively unsound mares. That scenario potentially harms the next generation by passing on the genes for the unsoundness. Additionally, the surgical method is a highly invasive procedure. Is it justifiable to force an unnecessary surgery on a recipient mare simply to produce a foal without donor mare depreciation? The current actions of the equine industry say “yes”, but ultimately the answer depends on personal interpretation and motivation.

Over the past 20 years, embryo transfer made a significant impact on the equine industry. The procedure has provided many additional opportunities for horse breeders and enthusiasts. Financial advantages and disadvantages of the procedure exist, as well as genetic and scientific gains. Although it is a valuable technique of assisted reproduction, its efficiency is limited by unreliable or ineffective methods to super-ovulate mares. As technology advances, ET is likely to impact the equine industry with greater force.