Advantages of embryo transfer
• One mare may produce more than one foal a year.
• Makes it possible to produce offspring out of mares which are still competing.
• Aged mares and those which for clinical or physical reasons are no longer able to carry normal pregnancies to term may still produce foals.
• Obtain foals out of valuable mares without risking their health.
• Fillies may start their breeding career earlier, without the inconvenience of becoming pregnant at an early age.
• One may buy or sell embryos which are offspring of both mare and stallion of one’s choice.
• In one single year, it may be possible to define which stallion is the best option for one given mare, comparing the foals born in that season to one mare, by several different sires.

The following text aims to be a simple explanation of the technical procedure of embryo transfer in the equine. Understanding the workflow of this process is very important to enable the breeder to choose well both stallion and mare, and also to analyze the real cost of this method of reproduction.
 
What is embryo transfer (ET?)
ET is the method by which one mare’s (the donor) embryo is transferred into another, the receptor mare. This technique has resulted from many years of research and study. 

Technically, ET consists in harvesting the donor mare’s embryo. This mare is bred or inseminated as close as possible to ovulation. In order to be able to detect ovulation, we must perform daily ultrasound examinations, starting from the first day of estrus (heat) to ovulation.  This period of daily examinations may vary between 4 and 10 days, depending on the length of estrus. It may extend a bit further, in case of the mare needing some kind of reproductive therapy. The embryo is harvested on the eighth or ninth day after ovulation (this is why ovulation must be determined precisely. The recovered embryo is then transferred into the receptor mare immediately after harvesting; or cooled down and shipped, in case of the receptor mare being at a different place than the donor mare; or it will be frozen.

Technical nomenclature
It is very important to become familiar with the fundamental ET technical nomenclature, to enable one to understand what is happening to the mares during the period their embryos are harvested.

Ovulation detection
This means to observe the follicle until ovulation. It takes place during the mare’s estrus (season) by palpation of the ovary and by trans-rectal ultrasonography. It may last from 4 to 10 days depending on the mare. This procedure is the key element of ET.

Donor mare
A mare generally of very high genetic, commercial or sentimental value, who will donate embryos, enabling her to produce foals without becoming pregnant. They may be mares during their athletic campaign, or those incapacitated of getting into foal due to several health problems.

Recipient mare (surrogate mother)
A mare which will receive embryos out of donor mares. She is going to be in foal, without having been bred or inseminated, of a foal which is not hers. Pregnancy is identical to that of normal mares, 345 (11 months) on average.

Embryo harvest
Act of collecting an embryo from a mare’s uterus by infusing the womb with an appropriate solution, which is later recovered.

Surgical transfer
Act on inovulating, i.e., putting an embryo into a receptor mare’s uterus by means of a surgical procedure.

Non-surgical transfer
Same as above, but without the need for performing surgery.

Embryo transfer rates
Below are listed rates and mean results of equine embryo transfers done all over the world, enabling us to evaluate cost-benefit of the technique for each case.

Embrionary recovery
60%. This is the percentage obtained by the number of harvests performed divided by the number of embryos obtained. This index varies greatly according to the donor’s fertility and to the quality of the employed semen. For instance, in young mares (up to 8 years of age) the rate of embrionary recovery may reach 80%, while in aged mares or in those with reproductive problems, we will have a recovery average of 30%, meaning that in 10 harvests we will obtain 3 embryos. Normally, this average is obtained by using semen of proved fertility.
Pregnancy rate
70%. This is the number of embryos obtained and transferred divided by the number of receptor mares in foal. This index is valid for non-surgical transfer, meaning that for each 10 embryos transferred we will have an average of seven receptors pregnant.

Pregnancy loss
10%. Indicates the number of mares in foal which “loose” their embryo up to 60 days of pregnancy. This rate is identical both for mares with a natural pregnancy and for embryo recipient mares.