Embryo Transfer (ET)

Embryo transfer is the removal of an embryo from a donor mare and placing it in the uterus of a recipient mare. The recipient mare carries the pregnancy and delivers the foal as if it was her own. The foal retains the genes of the donor mare and the stallion.


  • ET was first described in horses in 1972
  • It has been available commercially since the 1980’s
  • Prior to 2002 the use of ET was limited by breed regulations that restricted the registration of foals from individual mares to one per year. Subsequent to a pending lawsuit, the AQHA removed restrictions and allowed the registration of multiple foals from a single mare per year.

Traditionally ET has been used in older mares that fail to get pregnant or fail to carry a pregnancy to term. This limits the technology because older mares and infertile mares are less likely to donate embryos than reproductively healthier younger mares.
More recently ET has been used in younger mares to expand their progeny and allow them to continue their performance career.
ET is being used in conjunction with other assisted reproduction techniques such as cooled semen, frozen semen, Intracytoplasmic sperm injection and Oocyte transfer.

Currently, most clients have their mares bred locally with shipped semen and 7 days later the embryo is recovered from the donor mare, packaged and stored in an Equitainer and then shipped to a large embryo transfer facility where it is placed in a recipient mare.

  • Embryos are collected from donor mares on day 6,7 or 8.
  • Day 6 embryos are smaller and harder to harvest.
  • Day 8 embryos are the largest size that can comfortably fit into a pipette for transfer.
  • Method:
    • A catheter is introduced through the cervix, once in position the cuff of the catheter is inflated with air and the catheter is then drawn back against the internal opening of the cervix to assure a tight seal.
    • A buffered solution (Dulbeccos phosphate buffered saline) into the uterus, 1-2 liters at a time, and recovered by gravity through a 75 micron filter.
    • The procedure is repeated at least 3 times, but often times up to 6 times.
    • Embryo recovery rates per flush:
      • 31% on first 3 liters
      • 15% on subsequent 2 liter flushes
    • The filtered media is placed in a search dish and examined under magnification (7x-15x magnification with stereo dissection microscope)
    • Once located the embryo is washed, graded and packaged in a straw for immediate transfer into the recipient mare or placement in cooled storage for shipping.
    • Preferred Recipient Mares:
      • Maiden mares or young mares that have had 1 or 2 foals are preferred.
      • Quiet and well handled recipient mares
        • 1000-1200lb
        • 3-10 years old
        • Halter broke
  • Typically the embryo is shipped on an airplane to an embryo transfer center where many mares are kept. Usually the embryo can be placed into a mare that spontaneously ovulated on the same day as the donor mare.
  • Where large numbers of recipient mares are not available synchronization of ovulation can be provided by hormonal therapy. A minimum of 2 mares is required per donor mare. Recipient mares that have ovulated 1-day prior or up to 3 days after the donor mare are candidates.

Currently most transfers are done nonsurgically.
Success rates of 75% per embryo transferred are expected with nonsurgical transfer
There is no higher rate of embryonic loss with ET than with leaving the embryo in the donor.

  • Frozen embryo technology lagged behind other species and other equine assisted technologies
  • Probably due to the inability to repeatedly harvest multiple embryos in the horse, and breed association restrictions on the transferring frozen embryos.
  • Advantages include:
    • Embryos can be stored indefinitely
    • Important genetic lines can be preserved indefinitely
    • Frozen embryos can be held until a recipient mare is available, negating the need for multiple recipient mares that are synchronized.
    • Embryos can be shipped inside and outside our borders at our convenience without regard to matching of recipient cycles.
  • Success rates are higher (70%) when smaller embryos are used, day 6.


  • Producing more than one ovulation per heat cycle
  • Advantages:
    • Collection of multiple embryos per collection
    • Improving pregnancy rates per breeding /embryo recovery attempt
  • Problems:
    • Mares respond inconsistently to superovulation treatments
    • Lack of availability of commercial superovulatory drugs
    • Considerable cost of superovulation
    • Possible lowered viability of multiple embryos
    • Mares are very refractory to some drugs that work well in other species
  • Treatments tried:
    • GnRH: shown to ineffective in mares during the normal breeding season
    • Equine Pituitary Extract
      • Obtained from pituitaries from slaughtered horses
      • Crude extract
      • Contains 10% LH and 5% FSH
      • Has been shown to effectively induce multiple ovulations in treated mares
      • 2-4 ovulations per cycle, still lower than obtained in other species
      • It appears there are a reduced number of embryos per ovulation using this drug.


  • Porcine FSH (follicle stimulating hormone)
    • Porcine origin FSH
    • 7 ovulations per cycle
  • Equine FSH
    • E FSH: a recently released commercial product (Bioniche Animal Health)
    • Useful in increasing ovulations per cycle and embryos recovered.
    • The average number of embryos recovered in treated mares was 2


What Tucson Horse Owners Need to Know About Equine...
Influencing The Sex Of Offspring

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