Embryo Transfer in Horses 

What is Embryo Transfer 

Equine embryo transfer has been used since 1970s and has increased in popularity over the past years. Embryo transfer is a process that involves breeding a donor mare to a stallion and then transferring the resulting very young embryo (7-8 days old) into the uterus of a reproductively competent recipient mare, which carries the foal to term and raises it until weaning. The resultant foal will be the genetic offspring of the donor mare and stallion.   

The Advantages 

There are many advantages when it comes to embryo transfer, including: 

  • It allows the donor to continue a performance career whilst the recipient mare carries the foal. 
  • More than one pregnancy per year can be achieved from the single mare. 
  • Foals can be obtained from mares that are subfertile or cannot viably carry embryos themselves. 
  • Younger mares of 2 & 3 years of age can produce offspring. 

The Disadvantages 

  • Some breed registry societies have restrictions on the number of foals that can be registered out of a mare per season. 
  • High costs are involved due to the drugs, veterinarian fees, mare care, stud fees and processing fees (usually between $4,500-$8,000 per successful cycle). 
  • Intensive management is required of donor and recipient mare. 
  • Usually only one embryo can be achieved per cycle, as you cannot super ovulate the mare. Sometimes twin embryos may be achieved though. 
  • Genetic health of the recipient mare is not always known. 

The Procedure 

The first step of the process is to synchronize the reproductive cycles of the donor and recipient mares.  It is important that the recipient mare is at a similar stage in her cycle as the donor mare to ensure that her uterus is ready to accept the embryo. 

The donor mare is bred to the stallion.  This can be fresh, chilled, or frozen semen. It is important that the mare is monitored, and the time of ovulation is known. 

The flushing of the embryo is usually done at 7 or 8 days following the detection of ovulation. A sterile catheter is placed in the uterus of the donor mare and the embryo removed by flushing a special fluid into the uterus.  The recovered fluid passes through a filter allowing the microscopic embryo to be retrieved.  The embryo is then assessed and graded according to its appearance and is then either prepared to be transferred or frozen. 

The embryo is then inserted into the recipient mare and a pregnancy scan is performed on the recipient mare 6-7 days following the transfer of the embryo. 

Success Rates 

A survival rate of 75-80 % of good quality embryos recovered typically occurs following transfer. But it is also important to consider that embryo recovery rates may be as high as 75% or as low as 25%. There are many factors that influence the recovery rate, including: 

  • Age and fertility of donor mare 
  • Quality of semen used 
  • Day of recovery 
  • Number of ovulations 



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