Equiception - EN

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Donor mare management

Donor mare management

Equiception performs the embryo collection and transfer procedures, however we rely on the client’s regular veterinarian to appropriately manage and breed the donor mare prior to the day of flushing. Therefore, a crucial part of producing pregnancies via ET is out of our hands, with increasing success when both parties know what is expected and communicate well with each other.

This document describes Equiception’s recommendations regarding donor mare management and is designed to help understand what we expect from the veterinarian in charge of managing the donor mare.

Please feel free to contact Equiception with any questions you may still have after reading this document so we may work towards optimal results together.

Recipient mare wanted - Equiception - Equiception - EN


The best embryo donor mares are often younger than 15 years of age, reproductively healthy, in good body condition and free from stress and disease. Furthermore, it has been traditionally recommended that by the time a donor mare has reached the age of 8 to10 years, she has been allowed to carry a foal to term at least once. In order to optimise the donor mare’s reproductive health and minimise cervical functional issues due to fibrosis, it is recommended that she occasionally carry her own foal to term (every 3 to 4 years).

It is common to be presented with a less than ideal donor and it is not unusual to be presented with an older mare, which is still a maiden. In these mares, ET success rates tend to be lower and a tailored management is important.

Breeding mares - Equiception - Equiception - EN


To start breeding the non-pregnant donor mare as early as possible in the season, maintenance under lights (providing 16 hours of light per day and allowing 8 hours of darkness), starting 2 months before the desired date of the first ovulation of the year, is recommended. For example, light therapy instituted on December 1st will stimulate follicular development and ovulation by early February and should be continued until late March or early April.

It is important to realise that the light provided must be a minimum of 200 lux in all corners of the stable. Lux meters can be easily purchased online.

Pregnant donor mares that are expected to foal before April can also be maintained under the same artificial photoperiod to prevent them from returning to anoestrus after their foal heat cycle.

For donor mares that have recently foaled, embryo collection can be successful in the early post-partum period, however breeding on the foal heat should be decided on a case-by-case basis and only considered when the birth was uncomplicated and uterine involution is good.

icsi in horses - Equiception - Equiception - EN


At the beginning of the breeding season it is advisable to perform a thorough physical and reproductive exam on the donor mare. Procedures like uterine cytology, culture and biopsy may be indicated based on her history. The aim is to determine if the mare is in adequate physical condition or suffers from any medical issues and to evaluate her uterine and ovarian status (anoestrus, transitional or cycling).


Once the donor mare has started cycling she should be examined by ultrasonography per rectum on a routine basis to determine the optimal day to breed. Initially examinations may be performed every 2 to 3 days depending on the mare’s follicular development. Once a developing follicle reaches a diameter of approximately 40 mm, the ultrasound examinations should be performed daily. The donor mare can be bred when the dominant follicle is approximately 35-45 mm in diameter, depending on the breed (so called ‘cold blood’ mares, including Friesians, produce larger follicles and in these breeds the follicle should be given a chance to reach a bigger size). The goal is to inseminate the mare with fresh, chilled or frozen semen within 36, 24 or 12 hours prior to ovulation respectively. It may be preferable to inseminate immediately after ovulation if only a single dose of frozen semen is available, which may require ultrasound evaluations every 4 to 6 hours.

A timed ovulation may be induced within 36 or 40 hours after administration of a GnRH (Chorulon®) or LH analog (Ovuplant®) respectively. Ovulation induction will be most successful when the mare is in good oestrus (significant uterine oedema and soft cervix) with a follicle ≥ 35 mm in diameter.

Ultrasound examinations should continue to be performed daily to determine the exact day of ovulation, the number of ovulations and to evaluate uterine health post-insemination. Mares with uterine inflammation and fluid accumulation post-breeding should be treated (see below) to optimize embryo recovery.

The embryo flush procedure is scheduled based on the known ovulation date(s) and factors like the age of the donor mare, the type of semen used and information from previous embryo collection attempts. Usually embryo collection is performed on day 7 or 8 after ovulation. For older donor mares (≥ 20 years of age) and mares bred with frozen semen, it may be preferable to perform the embryo collection 9 days after ovulation as there may be a slight delay in embryonic development.



Accumulation of uterine fluid after insemination (persistent mating induced endometritis) is the most common reproductive problem encountered in middle aged to older (≥ 15 years old) donor mares. Prior to insemination the uterus will often appear normal, however the day after breeding it will contain a moderate to large volume of echogenic fluid. Usually the inflammation is secondary to antigenic stimulation from spermatozoa and not due to a bacterial infection. If left untreated, the mare may fail to rid herself of this inflammatory fluid by the time the embryo enters the uterus, decreasing the chance of embryo survival.

Treatment of PMIE consists of uterine lavage using large volumes (1 to 3 litres) of sterile saline or lactated Ringer’s solution together with the administration of ecbolic agents like oxytocin (10 to 20 units, IM or IV). Uterine lavage should continue until the effluent fluid is clear and in extreme cases this may require volumes up to 5 litres or even more. It is not unusual for mares with PMIE to require treatment on multiple consecutive days, hereby aiming to have treatment completed by day 5 after ovulation (the embryo enters the uterus on day 6). As a rule of thumb, if the intrauterine fluid is >1cm deep on ultrasound examination, a uterine lavage will be required for the best possible outcome.

Smaller volumes of intra-uterine fluid usually respond well to treatment with ecbolics alone. Depending on the severity of the clinical signs a single administration of an ecbolic agent may be sufficient to help clear the fluid from the uterus. In more severe cases the donor mare will require a higher administration frequency, for example every 4 to 6 hours until the next ultrasound examination is planned (with the first administration right after completion of the uterine lavage).

Oxytocin causes uterine contractions for 30 to 45 minutes.

Once a mare has exhibited clinical signs of PMIE, it is likely that the issue will reoccur following future inseminations. Limiting the number of inseminations (the goal is a single well-timed insemination), prophylactic uterine lavage as early as 4 to 6 hours after insemination and strategic administration of ecbolic agents can help prevent (the severity of) PMIE in these mares.


A clinical suspicion or diagnosis of infectious endometritis may be made by a combination of ultrasonography (presence of intra-uterine echogenic fluid), presence of inflammatory cells (neutrophils) on uterine cytology, growth of organism(s) on culture, and detection of bacterial or fungal DNA on PCR analysis of a uterine sample. Infectious endometritis is most commonly due to bacterial organisms (Streptococcus equi subspecies zooepidemicus, Escherichia coli, Klebsiella pneumonia and Pseudomonas aeruginosa) and less so due to fungal organisms (Candida albicans and Aspergillus fumigatus).

Treatment usually consists of uterine lavage to help remove debris, fluid and organisms and infusion of an appropriate antimicrobial agent into the uterus for 3 to 5 days. The mare can be short-cycled post-ovulation and checked if she is ‘clean’ at the onset of oestrus. Depending on the outcome, she can be bred that cycle or re-treated.

Prevention of infectious endometritis is based on correction of predisposing factors, such as a Caslick procedure in mares with poor perineal conformation. In order to minimize uterine contamination and infection it is important to strictly adhere to principles of hygiene, use sterile equipment, and evaluate the mare closely after insemination and embryo collection procedures.


It is common for the older maiden mare (≥ 15 years of age) to have a cervix that fails to relax during oestrus. As a consequence these mares may accumulate a significant volume of inflammatory fluid following insemination. The key to successful reproductive management of these older maiden mares is anticipation of cervical issues. A vaginal speculum examination and a digital (manual) evaluation of the cervix when the mare is in heat will determine the degree of cervical relaxation and the probability of issues post-breeding.

Mares at risk should be inseminated only once, the uterus should be lavaged 4 to 6 hours after breeding to remove residual sperm, inflammatory cells and fluid from the uterus, post-lavage the cervix can be gently dilated manually and administration of ecbolic agents may also aid in the evacuation of fluid from the uterus.



To give Equiception the chance to synchronize a mare with your recipient mare it’s important to inform us about the cycle of your mare.


To give Equiception the best chance of properly synchronizing a recipient mare with your donor mare, please contact us to update us on the following:

  • When the donor mare is given prostaglandin – size of follicles present on ovaries.
  • When the donor mare is coming into heat – oedema and follicle of 30mm or greater.
  • When the donor mare’s ovulation is induced. It is important to update us if there are multiple dominant follicles present (so we can synchronize additional recipient mares).
  • Inform us on the exact day of ovulation of the donor mare. If it is likely that a second (or third) follicle will ovulate in the near future, an examination 2 days later should be done to determine if other ovulations occurred.

More information on the use of a donormare can be found here.