Oxytocin use in the mare during breeding
Oxytocin acts on smooth muscle in the body, causing it to contract. One such area of tissue, which is of tremendous importance relative to breeding, is the mare’s uterus and so oxytocin use when breeding a mare can be a truly beneficial aid to improving uterine clearance and, in appropriately selected mares, pregnancy rates.
It has been recognized for some time that fluid build up in the uterus can be detrimental to establishing or maintaining pregnancy, and with a view to removal of such fluid there has been research into the use of oxytocin.
Up to 2 cm depth of fluid can usually be treated with oxytocin alone; more than 2 cm may require lavage as well
Uterine fluid is most commonly seen by ultrasound post-breeding, and is usually present as a result of a natural response to a foreign object in the uterus – semen. In most mares, this fluid poses no problem to the establishment or maintenance of pregnancy, and is cleared within 24 – 48 hours or less by the mare herself. In some mares however, especially older multiparous mares, or mares with poor reproductive conformation, a condition known as “delayed uterine clearance” (“DUC”) may occur. This results in the fluid not being adequately cleared and may result in persistent, at least low-grade, endometritis (and sometimes a significant inflammatory response). This causes the conceptus to encounter an inhospitable environment when it enters the uterus about 6 days post-ovulation, with a high risk for subsequent pregnancy loss.
Uterine fluid encountered in estrus mares during an ultrasonic evaluation prior to breeding (often during an examination of the follicular status of the ovaries to anticipate correct time of cover or insemination) has been identified as being detrimental to establishing or maintaining pregnancy1. Appropriate use of oxytocin alone is often beneficial and adequate to promote fluid clearance, however with greater fluid depth, uterine lavage may be required in addition. Approximately 2 cm of fluid depth is a useful clinical threshold for deciding when lavage is more likely to be indicated, although treatment should be based on the mare’s overall clinical findings rather than fluid depth alone. If fluid is lavaged pre-breeding, the resulting exudate should also be evaluated for inflammatory cells and bacteria as a uterine infection may be the cause. Per ultrasound, anechoic fluid is not necessarily sterile, and echogenic fluid is not necessarily infectious. Ultrasonographic appearance may provide useful clinical information, but cytology and appropriate bacteriological/fungal investigation are required when infection is suspected. Post-breeding, some increased echogenicity is not unusual owing to the normal post-breeding inflammatory response.
Routine Antibiotic Lavages No Longer Recommended!
One popular post-breeding treatment combination used in the past which while widely used has more recently been modified, was intramuscular oxytocin use when breeding a mare combined with an intra-uterine infusion of a broad-spectrum antibiotic.2. With rising concerns about the over-use of non-targeted antibiotics producing increasing numbers of resistant pathogens, we and others have stopped using post-breeding antibiotics in the combination unless indicated by identification of a sensitive pathogenic presence in the uterus3 and have not noted a reduction in pregnancy rates. It is important however to understand that oxytocin promotes uterine clearance; it does not treat bacterial or fungal infection. When infectious endometritis is suspected, diagnosis should be based on appropriate cytological and microbiological evaluation, and antimicrobial treatment should be directed at an identified pathogen. This applies to both pre- and post-breeding infectious endometritis and is to be clearly differentiated from the non-infectious post-breeding induced endometritis (PBIE) which is rarely present as a result of infectious agents, but owing to the normal uterine inflammatory response to sperm.
Pre-breeding Usage of Oxytocin in the Mare
If uterine fluid is identified pre-breeding, the same parameters should be observed. With high-risk mares manifesting uterine fluid upon entry to estrus, it is not uncommon that we commence the oxytocin protocol at the onset and keep treatment up through to 3½ days post-ovulation – the treatment being on a six-hourly rotation. For convenience, we use a 6 am, midday, 6 pm, midnight rotation, but that can be modified to be convenient for the user. The timing should, however, be manipulated so that oxytocin is given no fewer than 2 hours prior to breeding (to avoid continued uterine contractions when sperm are introduced) nor preferably earlier than 4 hours post-breeding to avoid interfering with sperm transport.
Treatment Interval and Dosage
As the half-life of oxytocin in the mare is only 6.8 minutes4, multiple treatments with oxytocin are required for optimal and maintained clearance. The first intramuscular or intravenous injection of oxytocin (see below for dosage discussion) should be at around 4 hours after breeding, which allows adequate time for all sufficiently progressively motile sperm to enter the oviducts (which are not affected by oxytocin use when breeding a mare) before stimulating uterine contractions. Some practitioners prefer to start treatment sooner; however, the traditional recommendation is to wait approximately 4 hours after breeding to allow adequate time for sperm transport to the oviducts. If uterine lavage is to be used it is typically performed using saline or Lactated Ringer’s Solution and is then followed by oxytocin. In lesser-amount fluid situations or if being used prophylactically, three more intramuscular or intravenous injections of oxytocin at six hourly intervals (the first of which takes place 6 hours after the uterine lavage and initial oxytocin injection) will often prove adequate. Note that oxytocin treatment commences post-breeding and not post-ovulation. In highly susceptible mares, we have found it beneficial to monitor the mare using ultrasound and continue oxytocin treatment on a six-hourly treatment schedule through to 3½ days post-ovulation. If indicated, we will repeat the uterine lavage up to and including the day after ovulation, or in more severe cases (and rarely) 2 days post-ovulation. The effectiveness of oxytocin for physical evacuation of uterine contents decreases as the cervix closes after ovulation. Consequently, treatment is most effective when instituted while the cervix remains sufficiently relaxed, and persistent fluid later in diestrus requires individualized veterinary assessment rather than simply continuing oxytocin indefinitely.
In the past a suitable dosage level of oxytocin use when breeding a mare was considered to be 40 International Units (i.u.) or more. Research has demonstrated that 20 IU intravenously markedly enhances uterine clearance5. Subsequent research has supported the use of lower doses, including 10 IU intravenously or 20 IU intramuscularly, particularly prior to ovulation7. The research indicated that the higher dosages result in a universal constriction – a tetanic clamp-down – of the smooth muscle, whereas the lower dosage results in methodical contractions starting at the distal (cranial) portion of the horns working caudally towards the cervix, which results in a more complete and systematic evacuation of the fluid6. Injections can be either intramuscular or intravenous, and after treatment with 20 IU, it has been shown that >90% of intrauterine content is eliminated within 30 minutes5. Once ovulation has occurred, the uterine response to oxytocin is reduced7, so increasing the dosage to 20 i.u. intravenously or 25 i.u. intramuscularly may be appropriate. This dosage however should not be exceeded in order to avoid the possibility of the tetanic contractions8.
Alternatives to Standard Oxytocin
Some users of this protocol may favour the use of the synthetic prostaglandin analogue Cloprostenol (Estrumate™, Intervet) rather than oxytocin owing to it having a longer half-life (active period) than oxytocin. Additionally, intramuscular rather than intravenous use has been suggested as resulting in a slower, longer duration of action with this drug. Research on the use of prostaglandin analogues at the post-ovulatory stage however has indicated that there may be an interference with the development of a fully functional Corpus Luteum9, 10, 11, and its use is therefore not recommended beyond 12 hours after ovulation12. Other researchers have suggested that progesterone secretion of the CL is only temporarily affected and rebounds by day-7 post ovulation if the synthetic prostaglandin treatment is carried out over only the first 48 hours post-ovulation and at a lower dosage (250 micrograms IM)11. As a matter of convenience one may also use either “Biorelease” oxytocin (BET Pharm, Lexington, KY, USA), Carbetocin (where those are available) or as with the noted timing limitations, Cloprostenol (“Estrumate©” Merk Animal Health) for the evening treatment to provide up to a 12 hour coverage, thereby preventing the need for middle-of-the-night treatments. With the latter hormone, this is a happy compromise between practicality and not risking compromise of CL function with prolonged post-ovulation use of a prostaglandin13. “Biorelease” oxytocin can be used without concern about interfering with CL function, and can consequently be used after the 12 hour limit; Carbetocin is a longer-acting oxytocin analogue that has been investigated as an alternative to repeated oxytocin administration. However, published research has produced conflicting findings concerning its effects on luteal function and progesterone secretion. Earlier work reported an adverse effect on luteal function14, whereas more recent research found no detrimental effect on progesterone secretion15, pregnancy rate or conceptus size when Carbetocin was administered during the periovulatory and early post-ovulatory period in mares resistant to endometrial infection. Differences in dosage, timing and treatment protocol may contribute to these differing findings. Neither Biorelease oxytocin nor Carbetocin are universally available. As oxytocin, prostaglandin and Cloprostenol are all produced in thin liquids, dependent upon required concentration and dose a low volume may be used which can facilitate the use of 1-ml insulin syringes. These cause minimal distress to the mare or the person performing the injection owing to the smaller needle size.
What Mares may Benefit from Oxytocin Use During the Breeding Period?
Mares that are likely to benefit from the use of an oxytocin protocol such as one outlined above include those with known delayed uterine clearance problems, mares with uterine fluid presence pre- and/or post-breeding, older mares that may have uterine lymphatic issues, mares post-breeding with frozen and in some cases cooled semen, and – in our experience – some other “problem mares” that have defied pregnancy establishment for no apparent reason. In our practice, we will use oxytocin prophylactically in mares that we consider may present breeding problems even in the absence of identified problems (including fluid), and have seen good – if inexplicable – success. While the treatment is cheap, easy and carries minimal risks, as well as being a common clinical practice in some breeding programs, evidence supporting routine prophylactic treatment of all apparently normal mares is limited. Treatment should therefore be individualized according to reproductive history, uterine findings and breeding circumstances.
Oxytocin Use in the Mare being Inseminated with Frozen Semen
Use of an oxytocin protocol following insemination with frozen semen is of significance, as sperm are a major stimulus for the post-breeding uterine inflammatory response, while components of seminal plasma can modulate that response. When semen is processed for freezing, it is typically centrifuged to allow an increase of sperm concentration and removal of seminal plasma which is then replaced by the freezing extender. This means that even in mares with a healthy uterine environment there is a greater risk of increased post-breeding inflammatory response, and in those mares categorized as “susceptible”, the already-present risk is magnified by the absence of the mitigating seminal plasma. To some extent, the same is seen with extended fresh or cooled semen which although it may still contain seminal plasma (although in some cases it too has been predominantly removed prior to re-extension) has had the seminal plasma diluted. It is one of life’s ironies that seminal plasma has the potential to be toxic to sperm, so this dilution is important particularly for semen being shipped, but the removal also has the potential for negative impact on the post-insemination uterine responses. This dilution effect is often misunderstood when seen in some mares and categorized as “a reaction to the extender” which is unlikely and is more likely to be an indicator of a mare which is susceptible to delayed uterine clearance and/or post-breeding endometritis.
An additional point related to oxytocin use in the mare being bred with frozen semen arises when a timed insemination protocol is being used with two semen doses. Typically, these are inseminated at an interval of 24 and 40 hours after treatment with an ovulation promoter – so at an interval of 16 hours between. While some mares may routinely have cleared the post-breeding inflammatory response from the first insemination by 16 hours, some may not and the remaining inflammatory cells may pose a hazard to sperm from the second insemination. In our experience, use of oxytocin approximately 4 and 10 hours after the first insemination, while maintaining the interval required before the second insemination, can assist uterine clearance and will be likely to assist in producing a more hospitable environment for the second-insemination sperm.
A Possible Benefit in Reducing the Incidence of “Low Progesterone Mares”?
A related point worthy of serious consideration has been raised. Some of the above-mentioned research has established that the presence of systemic low levels of prostaglandin post-ovulation interferes with CL formation. This can then have a negative impact on subsequent progesterone release. There is an increasing belief among some researchers that low progesterone levels in the mare may be present because of delayed uterine clearance (DUC) and/or low-grade breeding-induced endometritis (BIE) causing low levels of prostaglandin to be released. This endometritis may be as a result of either undetected uterine pathogenic presence (that should have been identified in a pre-breeding uterine culture and cytology evaluation), or delayed uterine clearance issues causing failure of clearance of normal post-breeding inflammatory response fluids and a sub-clinical endometritis. Appropriate diagnosis of uterine disease and delayed uterine clearance should precede empirical hormonal supplementation; progesterone/progestin therapy should be reserved for mares in which a specific indication has been established. As research continues to establish that progestin supplementation is not without risks, including suppression of endogenous progesterone levels and mare immune function (affecting both uterine condition and response to vaccines16, 17), the importance of this point becomes increasingly relevant.
Other Related Issues
One important point is that if during the pre- or post-breeding evaluations the cervix is found to be tight, treating the mare with systemic Estradiol Cypionate (ECP) and/or topically applying Buscopan or a cream made with the prostaglandin E product Misoprostol to the cervix may also be considered. Failure of the cervix to relax can be a significant contributor to retention of uterine fluid. Note that higher dosage rates of estradiol prior to ovulation should be carried out with caution as this may delay ovulation. Additionally, only a single use of Misoprostol is recommended per estrus.
Summary for Oxytocin Use when Breeding a Mare
The use of suitable low-dose repeated oxytocin treatments in the mare during the breeding cycle is well-documented as being potentially beneficial, particularly in those mares termed “susceptible” to PBIE, having delayed uterine clearance issues. An important key to successful treatment is to recognize the short half-life of oxytocin – and hence the need for repeated treatment in those mares susceptible – and also the importance of keeping dosages under 25 International Units to avoid a tetanic clamp-down effect of uterine contractions, rather than systematic contractions moving caudally which will assist fluid clearance. Below is a useful “primer” for possible treatment rationale:
Practical approach to uterine fluid management
No fluid, normal mare → No routine treatment necessarily required.
Small amount of fluid (<~2 cm) → Oxytocin may be appropriate, particularly in a mare with a history of delayed clearance.
Deeper or persistent fluid (>~2 cm) → Consider uterine lavage followed by an ecbolic.
Echogenic/purulent-appearing fluid or recurrent fluid → Cytology ± culture/other appropriate microbiological testing.
Known susceptible mare → Individualized post-breeding monitoring and ecbolic treatment.
Recognized Higher Risk Semen Use (Cooled/Frozen) → Individualized post-breeding monitoring and ecbolic treatment.
Persistent fluid despite treatment → Investigate cervical function, uterine conformation, lymphatic clearance, endometrial pathology and infectious disease rather than simply escalating oxytocin.
References:
1: Pycock, J.F. and Newcombe, J.R. 1996. The relationship between intraluminal uterine fluid, endometritis and pregnancy rate in the mare. Equine Pract. 18:19-22.
2: Pycock, J.F. 1994. Assessment of oxytocin and intrauterine antibiotics on intrauterine fluid and pregnancy rates in the mare. Proc. 40th AAEP Convention
3: Pycock, J.F. Personal communication
4: Paccamonti DL, Pycock JF, Taverne MA, Bevers M, Van Der Weijden GC, Gutjahr S, Schams D, Blouin D. PGFM response to exogenous oxytocin and determination of the half-life of oxytocin in nonpregnant mares. Equine Vet J. 1999 Jul;31(4):285-8.
5: LeBlanc MM, Neuwirth L, Mauragis D, Klapstein E, Tran T. 1994. Oxytocin enhances clearance of radiocolloid from the uterine lumen of reproductively normal mares and mares susceptible to endometritis. Equine Vet J 26:279-282.
6: Cadario, M.E., Merritt, A.M., Archbald, L.F., Thatcher, W.W., LeBlanc, M.M. 1999 Changes in intrauterine pressure after oxytocin administration in reproductively normal mares and in those with a delay in uterine clearance, Theriogenology 51(5):1017-25.
7: Gutjahr S, Paccamonti DL, Pycock JF, Taverne MA, Dieleman SJ, van der Weijden GC. 2000. Effect of dose and day of treatment on uterine response to oxytocin in mares. Theriogenology 54(3):447-56.
8: E Maischberger E, Irwin JA, Carrington SD, Duggan VE. 2008. Equine post-breeding endometritis: A review. Ir Vet J. 2008; 61(3):163–168.
9: Brendemuehl JP 2001. Effect of Oxytocin and PGF2a on Luteal Formation, Function, and Pregnancy Rates in Mares, Proc. 47th AAEP Convention. 239.
10: Troedsson MH, Ababneh MM, Ohlgren AF, et al. 2001. Effect of periovulatory prostaglandin F2α on pregnancy rates and luteal function in the mare. Theriogenology 2001. 55;9:1981-1899.
11: Nie G.J., Johnson K.E., Wenzel J.G.W., Braden T.D 2002. Effect of periovulatory ecbolics on luteal function and fertility. Theriogenology 58; 2-4:461-463.
12: LeBlanc MM. 2001. The Chronically Infertile Mare. Proc. 54th AAEP Convention. 391-407.
13: Canisso IF, Segabinazzi LGTM, Fedorka CE. 2020. Persistent Breeding-Induced Endometritis in Mares — A Multifaceted Challenge: From Clinical Aspects to Immunopathogenesis and Pathobiology. International Journal of Molecular Sciences. 21(4):1432 – https://doi.org/10.3390/ijms21041432
14: Diel de Amorim M, Bramer SA, Rajamanickam GD, Klein C, Card C. 2023. Serum progesterone and oxytocinase, and endometrial and luteal gene expression in pregnant, nonpregnant, oxytocin, carbetocin and meclofenamic acid treated mares. Theriogenology 198:47-60 – https://doi.org/10.1016/j.theriogenology.2022.10.019
15: Khan Y, El-Shalofy A, Kaps M, Gautier C, Aurich C 2024. In mares resistant to endometrial infection, periovulatory treatment with ecbolic drugs does not influence uterine clearance or luteal development. Animal Reproduction Science 267:107548 – doi.org/10.1016/j.anireprosci.2024.107548
16: Crabtree JR, Mouncey R, Wilsher S. 2023. Effect of altrenogest on endogenous progesterone during early pregnancy in recipient mares. JEVS 125:104761 – see our paper summary.
17: Kapadia CL, Crook RA, Coleman S, Unger G, Douglas RH, Fedorka CE. 2026. Synthetic progestin treatment alters cytokine expression and impairs vaccine response in horses. Veterinary Immunology and Immunopathology; 296:111111 – doi.org/10.1016/j.vetimm.2026.111111
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