Embryo Transfer in Mares: How It Works and When To Use It

By Dr Duncan Houston

Embryo transfer is one of the most useful tools in modern equine reproduction. It allows a mare to pass on her genetics without carrying the pregnancy herself. That can be valuable for performance mares, older mares, mares with medical problems, mares that should not carry the extra weight of pregnancy, or breeding programs trying to produce more than one foal from a high-value mare.

But embryo transfer is not a magic shortcut. The donor mare still has to ovulate, be bred at the right time, produce a viable embryo, and maintain a healthy uterine environment long enough for the embryo to be collected. The recipient mare then has to be correctly synchronised, healthy, hormonally suitable, and capable of carrying the pregnancy to term.

The real question is not just, “Can we do embryo transfer?” It is, “Is embryo transfer the best reproductive tool for this mare, this season, this budget, and this breeding goal?”

Quick Answer

Embryo transfer in mares involves breeding or inseminating a donor mare, flushing her uterus around day 7 or 8 after ovulation, finding and preparing the embryo, then transferring it into a synchronised recipient mare that carries the foal. It is commonly used for performance mares that need to stay in work, mares that should not carry a pregnancy, older or subfertile mares, and mares where producing more than one foal in a season is valuable. Success depends heavily on donor fertility, semen quality, embryo quality, recipient mare selection, and precise cycle timing. Merck Veterinary Manual notes that nonsurgical embryo collection is usually performed on day 7 or 8 after ovulation, and that both donor and recipient mares need breeding soundness evaluation and close ultrasound monitoring. (Merck Veterinary Manual)

What Is Embryo Transfer in Horses?

Embryo transfer, often shortened to ET, is a reproductive technique where the donor mare provides the genetics but does not carry the pregnancy.

The usual process is:

1. The donor mare is monitored with ultrasound.

2. She is bred or inseminated at the correct time.

3. Ovulation is recorded as day 0.

4. Around day 7 or 8 after ovulation, her uterus is flushed.

5. The recovered fluid is searched under a microscope.

6. If an embryo is found, it is washed, assessed, and prepared.

7. The embryo is transferred into a recipient mare.

8. The recipient mare carries the pregnancy, foals, and usually nurses the foal.

Merck describes embryo collection as a routine transcervical uterine lavage procedure, with the recovered fluid filtered and examined under magnification to locate the embryo. Once found, the embryo is washed and either transferred promptly to a recipient mare or prepared for storage and transport. (Merck Veterinary Manual)

In practice, embryo transfer is a team sport. The donor mare, stallion semen, recipient mare, reproduction vet, embryo lab, transport timing, and owner expectations all have to line up. When one piece is off, the whole plan can wobble.

Why Use Embryo Transfer?

Embryo transfer is most useful when you want the foal’s genetics from the donor mare, but carrying the pregnancy is not ideal for that mare.

Common reasons include:

• The mare is actively competing

• The mare needs to remain in training

• The mare has orthopedic disease, such as arthritis or laminitis risk

• The mare has poor body condition or medical risk

• The mare has a history of pregnancy loss

• The mare is older but still producing embryos

• The mare has uterine problems but can support an embryo for a few days

• The owner wants more than one foal from the same mare in a season

• The mare is valuable and pregnancy risk needs to be reduced

Merck notes that equine embryo transfer is used for mares with restricted reproductive potential, including older mares, mares with uterine disease or subfertility, and performance mares that need to remain nonpregnant so they can continue training and competing. (Merck Veterinary Manual)

This is why embryo transfer can be such a powerful option. It separates two jobs that are normally done by the same mare: producing the embryo and carrying the foal.

Which Mares Are Good Candidates?

A good donor mare does not have to be able to carry a foal to term, but she does need to be able to produce a viable embryo that can be recovered.

Good candidates may include:

• Performance mares in active work

• Mares with valuable genetics

• Mares that should not carry pregnancy due to injury or disease

• Mares that can conceive but lose pregnancies later

• Mares where multiple foals in one season are desired

• Mares that tolerate reproductive handling well

• Mares with good follicular activity and reasonable uterine health

The key clinical point is this: embryo transfer still needs the donor mare’s reproductive tract to work for the first week after ovulation.

If the mare has severe uterine disease, chronic infection, cervical damage, oviductal problems, poor oocyte quality, or repeatedly fails to produce embryos, conventional embryo transfer may not be the right tool. In those cases, oocyte recovery and ICSI may be a better option. Penn Vet notes that oocyte aspiration and ICSI can be used for mares with chronic uterine infections, cervical problems, oviductal problems, and other issues that prevent them from conceiving or carrying a pregnancy. (vet.upenn.edu)

When Embryo Transfer May Not Be the Best Option

Embryo transfer is useful, but it is not the answer for every mare.

It may be a poor choice if:

• The mare does not ovulate normally

• Semen quality is poor

• Breeding timing cannot be managed properly

• The mare has severe uterine disease

• The mare repeatedly fails to produce embryos

• The mare is very old with poor oocyte quality

• The owner cannot access a suitable recipient mare

• The budget does not allow multiple attempts

• The breed registry does not allow the planned reproductive method

• The mare is unsafe to handle for repeated ultrasound and breeding work

This is where owners can burn money very quickly. If the mare is unlikely to produce an embryo, repeating standard flushes again and again may not be sensible. That is when the conversation should shift toward a full reproductive work-up, semen review, recipient strategy, or advanced options such as OPU-ICSI.

How the Embryo Transfer Process Works

1. Donor Mare Examination

Before breeding, the donor mare should have a reproductive assessment.

This may include:

• Breeding history

• Age and previous foaling history

• Uterine ultrasound

• Ovarian ultrasound

• Perineal conformation assessment

• Uterine culture or cytology if indicated

• Assessment for uterine fluid

• Review of previous pregnancy loss

• Semen plan

• Timing plan for ovulation and flush

Merck states that daily ultrasonographic evaluation of donor and potential recipient mares during estrus provides critical information about ovulation timing, embryo collection day, and recipient selection. (Merck Veterinary Manual)

The ovulation date matters enormously. In equine reproduction, “close enough” can become “no embryo recovered” very quickly.

2. Breeding or Insemination

The donor mare can be bred using natural cover, fresh semen, chilled semen, or frozen semen, depending on the stallion, breed registry, location, and veterinary plan.

The important detail is not just the semen type. It is timing.

Your vet needs to know:

• When the mare is likely to ovulate

• Whether ovulation should be induced

• Whether the semen is fresh, chilled, or frozen

• Whether the mare needs post-breeding treatment

• Whether the flush day should be adjusted

Frozen semen often requires more precise timing because the sperm lifespan is shorter. If breeding is mistimed, the embryo may never form, no matter how perfect the flush is.

3. Post-Breeding Uterine Management

Some mares need post-breeding treatment, especially if they are prone to fluid accumulation or post-breeding endometritis. This may include oxytocin, uterine lavage, anti-inflammatory treatment, or antibiotics when indicated.

This should not be interpreted as “flush every mare and add antibiotics or antifungals just because.” Treatment should match the mare. A young, clean, fertile mare is not the same as an older mare with poor uterine clearance.

The goal is to keep the donor mare’s uterus healthy enough to support the embryo until collection.

4. Embryo Collection

Equine embryos enter the uterus around 5.5 to 6 days after ovulation. In clinical practice, embryo recovery attempts are usually performed between 6.5 and 9 days after ovulation, with many mares flushed around day 7.5 or day 8 because recovery rates are good and embryos are easier to identify under the microscope. (Ymaws)

The flush itself involves passing a catheter through the cervix into the uterus, infusing embryo flush medium, recovering the fluid, and filtering it so the embryo can be found. Merck notes that multiple litres of flush medium may be used, and nearly all of the fluid should ideally be recovered. (Merck Veterinary Manual)

If no embryo is found, it does not always mean the procedure was done badly. It may mean the mare did not conceive, the embryo was lost early, semen timing was off, the embryo was missed, the embryo was not in the uterus yet, or the donor mare’s fertility is lower than expected.

5. Embryo Handling

Once the embryo is found, it is washed, graded, and either transferred fresh, shipped to another facility, or prepared for storage. Merck states that the embryo’s stage and quality should be recorded, and that after washing it should be transferred to a recipient mare within about 1 hour or prepared for short storage and transport. (Merck Veterinary Manual)

This is delicate work. Embryos are tiny, the handling is precise, and timing matters. It is not the part of the process where anyone should be learning casually over a coffee and a YouTube video.

6. Recipient Mare Selection

The recipient mare is just as important as the donor mare. A beautiful embryo placed into the wrong recipient is a wasted opportunity.

A good recipient mare should be:

• Reproductively sound

• Healthy

• In good body condition

• Calm and manageable

• Free from uterine disease

• Able to carry and foal safely

• Properly synchronised with the donor

• Monitored with ultrasound

Merck notes that recipient mares should be reproductively sound, healthy, and in good body condition, and that synchronisation should be maximised using hormonal manipulation and daily transrectal ultrasound. Good pregnancy rates are reported when recipients ovulate from 1 day before to 3 days after the donor mare. (Merck Veterinary Manual)

Kansas State University’s embryo transfer program notes that if owners provide recipient mares, at least two recipients should be available per donor, and recipient mares should be either proven fertile with no reproductive problems or maiden mares, commonly 3 to 10 years old.

That “at least two recipients” point matters. If you only have one recipient and she is not synchronised properly, has uterine fluid, ovulates at the wrong time, or fails her exam, you may have an embryo and nowhere ideal to put it. That is the reproductive version of showing up to the airport with a ticket but no passport.

Fresh, Shipped, Frozen and ICSI Embryos

Embryos can be handled in different ways depending on the program.

Fresh Transfer

This is where the embryo is recovered and transferred promptly into a recipient mare at the same location or nearby. It is often the simplest and most direct approach when suitable recipients are available.

Shipped Embryo Transfer

If there is no recipient mare at the donor mare’s location, the recovered embryo may be shipped to a central transfer facility. Merck notes that pregnancy rates do not appear to differ from fresh embryos, especially when embryos are shipped the same day they are collected. (Merck Veterinary Manual)

This can be very useful for smaller breeding programs that do not have recipient herds on site.

Frozen or Vitrified Embryos

Freezing equine embryos is more challenging than freezing embryos in some other species. Merck notes that horse embryos are difficult to cryopreserve, likely because of their large diameter and embryonic capsule, and that morula or early blastocyst stage embryos under 6.5 days are preferable for cryopreservation. (Merck Veterinary Manual)

Frozen embryos can still be very useful, but expectations need to be realistic and centre-specific.

OPU-ICSI

OPU means ovum pick-up, where oocytes are collected directly from the mare’s ovaries. ICSI means intracytoplasmic sperm injection, where a single sperm is injected into an oocyte in the laboratory to create an embryo.

This can be especially useful when conventional embryo transfer is not working or when the mare cannot produce embryos through normal breeding. Colorado State University describes mature and immature oocyte collection using ultrasound-guided transvaginal aspiration, followed by ICSI either in-house or at a designated laboratory. (vetmedbiosci.colostate.edu)

Merck notes that OPU-ICSI is now used commercially in several regions and can use small amounts of frozen semen, including semen from deceased stallions. It also reports that, on average, about one transferable blastocyst may be produced per OPU-ICSI session, with 60% to 70% of recipient mares becoming pregnant after transfer of one embryo, although success varies. (Merck Veterinary Manual)

How Successful Is Embryo Transfer?

Success rates vary widely.

The biggest factors include:

• Donor mare age

• Donor mare fertility

• Semen quality

• Breeding timing

• Uterine health

• Embryo quality

• Recipient mare quality

• Recipient synchronisation

• Transfer technique

• Transport and handling

• Whether the embryo is fresh, shipped, vitrified, or ICSI-produced

Merck states that standard techniques generally result in an embryo recovery rate of about 75%, but this can be as high as 90% in young maiden or fertile mares and as low as 10% to 20% in subfertile mares. (Merck Veterinary Manual)

Kansas State University gives a more conservative program-level estimate, describing an average embryo recovery rate of approximately 60%, with young fertile mares bred to fertile stallions around 80% and older subfertile mares around 30%. It also reports pregnancy rates after transfer commonly varying from 60% to 80%, depending on donor mare fertility, uterine condition, embryo quality, and other factors.

The honest takeaway: embryo transfer can be very successful, but the mare in front of you matters more than the brochure.

A young fertile mare bred with good semen and a strong recipient herd is a very different project from a 21-year-old maiden mare with uterine fluid, frozen semen, and one questionable recipient mare.

What Does Embryo Transfer Cost?

Costs vary widely by country, clinic, recipient program, semen type, number of cycles, pregnancy fees, recipient lease arrangements, shipping, and whether ICSI or embryo freezing is involved.

The mistake is asking, “How much is one embryo transfer?” as if it is one item. It usually is not.

Costs may include:

• Initial reproductive exam

• Cycle tracking

• Ultrasound scans

• Semen collection, purchase, shipment, or insemination fees

• Donor mare breeding management

• Hormones and medications

• Embryo flush

• Embryo search and handling

• Recipient mare synchronisation

• Recipient mare lease or purchase

• Embryo shipment

• Embryo transfer

• Pregnancy check fees

• Recipient boarding

• Insurance

• Foaling and newborn care

Colorado State University’s 2026 assisted reproduction fee schedule lists separate line items such as annual embryo transfer enrollment, donor mare cycle management, embryo transfer fees, recipient lease and deposit, pregnancy fees, board, shipping, ICSI fees, embryo production, cryopreservation, and storage. That gives owners a good sense of why total costs can rise quickly across a season.

The practical advice: get a written estimate that separates one-cycle costs, recipient costs, pregnancy fees, failed-cycle costs, and repeat-attempt costs.

Cheap embryo transfer is only cheap if it works. A more expensive program with better recipient management can sometimes be the better investment.

Severity and Risk Framework

Low Risk: Strong Candidate

This is a young or middle-aged fertile donor mare, bred with good-quality semen, monitored closely, with access to a well-managed recipient herd.

What to do: embryo transfer is a sensible option if the goal justifies the cost.

Medium Risk: Performance Mare With a Tight Schedule

This mare is in work, travelling, or competing, but cycles normally and has no major uterine disease.

What to do: coordinate the training, breeding, ovulation, and flush schedule carefully. Avoid heat stress and heavy work around breeding and embryo recovery.

High Risk: Older or Subfertile Mare

This mare is older, has a history of poor embryo recovery, uterine fluid, endometritis, poor cervical function, poor oocyte quality, or previous reproductive failure.

What to do: do not assume repeated standard flushes are the best plan. Review semen, uterine health, ovulation timing, and consider advanced reproductive options such as OPU-ICSI.

Critical Risk: Medical or Welfare Concern

This includes a donor or recipient mare with fever, colic, severe uterine infection, severe laminitis, severe illness, unsafe handling, or major pregnancy risk.

What to do: stop chasing the breeding schedule and stabilise the mare. Reproduction can wait. A sick mare is not a reproductive project. She is a patient.

What Can Go Wrong?

Embryo transfer is generally safe when performed by experienced teams, but problems and disappointments can happen.

Possible issues include:

• No embryo recovered

• Poor embryo quality

• Embryo damaged during handling

• Recipient not synchronised properly

• Recipient mare not suitable on transfer day

• Pregnancy failure after transfer

• Early embryonic loss

• Twin or abnormal pregnancy issues

• Uterine fluid or inflammation in donor or recipient

• Transport delays

• Unexpected costs

• Registration rule problems

• Donor mare or recipient mare illness

Clinical Theriogenology notes that success depends on attention to detail, optimal donor mare management, careful recipient selection, correct embryo recovery, evaluation and handling, and gentle transcervical transfer technique. (Ymaws)

That is the key message. Embryo transfer is not one procedure. It is a chain of details. And chains are only as strong as the weakest link.

What Else Should Be Considered Before ET?

Donor Mare Age

Older mares often have lower embryo recovery and poorer oocyte quality. They may still produce embryos, but the owner should be prepared for lower efficiency.

Semen Quality

Before blaming the mare, assess the semen. Poor semen quality, delayed shipment, poor handling, and wrong insemination timing can all reduce embryo recovery.

Uterine Health

If the donor mare has fluid after breeding, chronic endometritis, cervical issues, or poor uterine clearance, she may need a targeted treatment plan before breeding.

Recipient Mare Availability

Recipient mare access is one of the biggest practical limiting factors. A good recipient program can make or break the result.

Breed Registry Rules

Most breed associations now allow registration of foals born by embryo transfer, but rules vary, especially around multiple foals per season, frozen embryos, ICSI, and parentage testing. Merck notes that most breed associations allow registration of foals born by embryo transfer, and an increasing number allow multiple foals born in the same year, but owners still need to check their specific registry. (Merck Veterinary Manual)

Owner Goal

Are you trying to produce one foal, multiple foals, preserve genetics, keep the mare competing, rescue genetics from an older mare, or manage a mare that cannot carry safely?

The right reproductive plan depends on the goal.

When Is This an Emergency?

Embryo transfer itself is planned reproductive work, not an emergency. But donor mares, recipient mares, and pregnant recipients can still develop problems that need urgent veterinary attention.

Call your vet promptly if a donor or recipient mare has:

• Colic signs

• Fever

• Depression or not eating

• Foul vaginal discharge

• Heavy bleeding

• Severe pain after breeding, flushing, or transfer

• Signs of uterine infection

• Severe laminitis signs

• Sudden severe lameness

• Pregnancy loss signs

• Severe swelling or trauma

• Collapse

• A recipient mare showing illness after transfer

Also call your vet if a pregnant recipient later develops vaginal discharge, premature udder development, milk leakage, fever, colic, or signs of pregnancy compromise. The foal’s genetics may come from the donor, but the recipient mare is the pregnancy.

What Should You Do Next?

1. Define the Goal

Decide whether the priority is performance, genetics, pregnancy safety, multiple foals, or preserving a line.

A mare in full competition work needs a different plan from an older retired broodmare.

2. Get a Reproductive Work-Up

Do not start with the flush. Start with the mare.

Assess ovaries, uterus, cervix, breeding history, age, conformation, prior foaling history, and any reason she may have failed before.

3. Review the Stallion and Semen Plan

Fresh, chilled, and frozen semen all change timing and logistics. Make sure the semen plan matches the mare, the clinic, and the flush timing.

4. Secure Recipient Mare Access Before Breeding

Do not wait until the donor has ovulated to realise no recipient is available. KSU notes that communication with the recipient herd facility before breeding is important so the donor mare’s estrous cycle and ovulation date can be tracked for recipient planning.

5. Plan the Flush Date From Ovulation

Ovulation is day 0. Most clinical embryo collections are around day 7 or 8, but your vet may adjust timing based on semen type, embryo freezing goals, mare factors, and facility protocol. (Ymaws)

6. Ask for Realistic Success Expectations

Ask your vet for expected embryo recovery and pregnancy rates for this mare, not generic averages.

A realistic estimate should consider:

• Mare age

• Mare fertility

• Semen quality

• Previous embryo recovery

• Uterine condition

• Recipient availability

• Transfer type

7. Prepare for More Than One Attempt

Even in good programs, not every cycle produces an embryo and not every transfer becomes a foal. Budget and emotionally prepare for repeat attempts.

8. Consider OPU-ICSI Early if ET Fails

Do not wait until the season is nearly over to discuss advanced options. If the mare has severe uterine disease, repeated failed flushes, poor semen availability, or old age, OPU-ICSI may be worth discussing earlier. Penn Vet specifically describes oocyte recovery and ICSI as an option for mares not suited for embryo transfer. (vet.upenn.edu)

Common Mistakes Owners Make

Thinking ET Guarantees a Foal

It does not. ET improves options, but biology still gets a vote.

Choosing Recipient Mares Too Late

Recipient planning should happen before breeding the donor mare, not after the embryo is recovered.

Ignoring Donor Mare Uterine Health

A donor mare only needs to carry the embryo for about a week, but that week still matters.

Assuming Chronic Uterine Disease Is Always an ET Case

Some mares with uterine disease can still produce embryos for ET. Others cannot. If the uterus cannot support an embryo long enough for recovery, OPU-ICSI may be more appropriate.

Underestimating Cost

The flush is only part of the cost. Recipient lease, board, pregnancy fees, semen, scans, medications, shipping, and repeat cycles all matter.

Blaming the Recipient for Every Failure

Recipient quality matters, but failed pregnancy can also reflect embryo quality, donor age, semen issues, handling, timing, or early embryonic loss.

Forgetting Registry Rules

Check the breed registry before you begin, especially if you plan multiple foals, ICSI, frozen embryos, or international semen.

Can Embryo Transfer Help Mares With Uterine Problems?

Sometimes.

Embryo transfer can help mares that can conceive and support the embryo for the first few days but cannot reliably carry to term. That is common in some mares with later pregnancy loss, performance demands, physical disease, or pregnancy risk.

But if the mare cannot provide a healthy enough environment for early embryo development and recovery, conventional ET may fail. In those cases, oocyte recovery and ICSI may bypass the uterus more effectively because oocytes are collected from the ovary and embryos are produced in the laboratory. Penn Vet describes oocyte aspiration and ICSI as useful for mares with chronic uterine infections, cervical problems, oviductal problems, and other issues that prevent conception or pregnancy maintenance. (vet.upenn.edu)

The decision is not “uterus problem equals ET.” The decision is: can this mare produce a recoverable embryo through standard breeding, or do we need to collect oocytes instead?

Prevention: How To Improve the Chance of Success

You cannot control every part of embryo transfer, but you can reduce avoidable failure.

A better ET plan includes:

• Pre-season reproductive exam

• Body condition assessment

• Good dental, hoof, and general health care

• Accurate ultrasound tracking

• Correct semen timing

• Prompt treatment of post-breeding uterine fluid

• High-quality semen handling

• Recipient mares arranged before breeding

• At least one backup recipient where possible

• Clear communication between donor vet and recipient facility

• Written cost estimate

• Breed registry confirmation

• Pregnancy check schedule

• Backup plan if no embryo is recovered

• Early discussion of OPU-ICSI if the mare is older or subfertile

The best embryo transfer programs are boring in the right way. Everyone knows the ovulation date, the flush date, the recipient options, the semen plan, the costs, and the backup plan. No last-minute reproductive circus, thank you very much.

FAQ

When is the embryo flushed from the donor mare?

Most equine embryos are flushed around day 7 or 8 after ovulation, with day 0 being the day of ovulation. Clinical embryo recovery attempts are commonly performed between 6.5 and 9 days after ovulation, depending on the goal and semen type. (Ymaws)

Does embryo transfer hurt the mare?

Embryo collection is usually performed nonsurgically through the cervix using uterine lavage. Most mares tolerate it well, although restraint, stocks, and sometimes sedation may be needed. Merck notes that sedation is not used in most cases, but may be needed for uncooperative mares. (Merck Veterinary Manual)

How many recipient mares do I need?

Ideally, more than one suitable recipient is available. KSU recommends at least two recipient mares per donor when owners provide recipients, so the mare closest in timing and with the best uterine condition can be selected.

What is the success rate for embryo transfer in horses?

Success varies widely. Merck reports embryo recovery around 75% with good standard techniques, up to 90% in young fertile mares, and as low as 10% to 20% in subfertile mares. KSU reports average embryo recovery around 60% and pregnancy rates after transfer around 60% to 80%, depending on mare, stallion, embryo, and program factors. (Merck Veterinary Manual)

What is the difference between embryo transfer and ICSI?

In embryo transfer, the donor mare is bred or inseminated, and the embryo develops inside her uterus before being flushed and transferred. With OPU-ICSI, oocytes are collected from the mare’s ovaries, fertilised in the laboratory by injecting a sperm into the oocyte, and any resulting embryo is transferred to a recipient mare. Colorado State University and Penn Vet both describe oocyte collection and ICSI as advanced options when conventional embryo transfer may not be suitable. (vetmedbiosci.colostate.edu)

Final Thoughts

Embryo transfer is one of the best reproductive tools available for mares that should not, cannot, or do not need to carry their own foal.

It can allow a performance mare to stay in work. It can reduce pregnancy risk for a medically compromised mare. It can help preserve valuable genetics. It can give older or subfertile mares another chance, as long as expectations are realistic.

But success depends on details: ovulation timing, semen quality, donor uterine health, embryo handling, recipient mare selection, synchronisation, and honest planning. The embryo is tiny. The logistics are not.

The smartest embryo transfer plans start early, involve a reproduction vet, secure recipients before breeding, and have a backup plan if the mare does not produce an embryo. Done well, embryo transfer can be brilliant. Done casually, it can become a very expensive way to learn that timing matters.

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If you are considering embryo transfer for a performance mare, older mare, medically compromised mare, or valuable breeding mare, ASK A VET™ can help you work through the key questions before you commit: whether ET is suitable, what risks to discuss with your reproduction vet, and when advanced options like OPU-ICSI may make more sense.