Oxytocin is one of the most familiar hormones in equine reproduction. Even if you have just started breeding or researching breeding your mare, you will have seen it mentioned a lot. That's because it is used for a number of different things in equine reproduction. Its uses in the mare extend far beyond inducing labor.
Oxytocin is most commonly associated with helping a mare clear fluid from her uterus after breeding. Anyone who has bred a mare has likely used it for just that. It is also used in some embryo recovery procedures, induction of parturition, management of retained fetal membranes, and occasionally to assist with milk letdown.
Understanding how and why oxytocin is used is particularly important when breeding older mares or mares that are susceptible to persistent post-breeding endometritis.
This article looks at the major uses of oxytocin in mares and explains what the hormone does, when veterinarians use it, and where the evidence is strongest.
What is oxytocin?
Oxytocin is a naturally occurring hormone produced primarily in the hypothalamus and released from the posterior pituitary gland. One of its most well-known functions is stimulating contraction of smooth muscle.
In the mare, this includes contraction of the:
- Uterus
- Myometrium
- Mammary gland
These effects make oxytocin particularly useful in equine reproductive medicine because many of the things that we need a broodmare to do in order to either get pregnant or nurse a foal require these organs to move — contract and expand.
The uterus is a muscular organ. When oxytocin stimulates uterine contractions, those contractions can help move material toward the cervix and out of the reproductive tract. This helps the uterus stay clean, and a clean uterus is required to house an embryo.
This is the fundamental reason oxytocin is so useful after breeding.
How is oxytocin used in mare reproduction?
The major applications can be summarized as follows:
We are now going to talk about each of these in greater detail.
Oxytocin after breeding: helping the mare clear her uterus
Breeding naturally causes an inflammatory response in the mare's uterus. This is a normal response to the introduction of semen, spermatozoa, bacteria and other material into the reproductive tract.
A healthy mare normally resolves this inflammation and clears the uterus within approximately 48 hours. However, some mares are less efficient at clearing the uterus.
These mares are particularly susceptible to persistent breeding-induced endometritis, sometimes called persistent post-breeding endometritis. This just means that fluid is accumulated inside the uterus, and this can interfere with the establishment and maintenance of pregnancy. The reason fluid in the uterus is undesirable is because it is unlikely that an embryo can survive if it comes into contact with fluid in the uterus. Some do, but most don't.
How does oxytocin help?
Oxytocin stimulates contraction of the uterine muscle.
Those contractions help the mare physically expel:
- Excess semen
- Inflammatory fluid
- Cellular debris
- Other material remaining in the uterus after breeding
This is particularly valuable in mares that have poor natural uterine clearance, because the contractions help push these things out.
A review of post-breeding endometritis reports that relatively small amounts of intrauterine fluid may sometimes be cleared with oxytocin alone, whereas larger accumulations may require uterine lavage followed by a uterotonic such as oxytocin. This is why it is important, especially in mares that tend to accumulate fluid, to ultrasound post-breeding and again post-ovulation to see if there is any fluid accumulation.
Why are older mares often treated with oxytocin?
Age is an important risk factor for poor uterine clearance.
Older mares can have reduced ability to physically clear fluid from the uterus because of changes affecting the reproductive tract, including reduced uterine contractility and altered cervical function. As any of us get older, muscle function can deteriorate.
However, being old does not automatically mean a mare needs oxytocin. Treatment is generally based on the mare's individual reproductive history and whether she is demonstrating poor uterine clearance.
Treatment may involve:
- Ultrasound examination to identify uterine fluid.
- Oxytocin to stimulate uterine contractions.
- Re-examination to determine whether the fluid has cleared.
- Uterine lavage when fluid accumulation is substantial.
- Further treatment based on the mare's individual findings.
Oxytocin is therefore not necessarily a "fertility drug." Rather, it helps address one of the physical problems that can prevent the uterus from returning to a healthy environment after breeding.
Uterine lavage and oxytocin are frequently used together in mares with significant uterine fluid. The lavage physically washes material out of the uterus — it's often referred to as a "flush" because it does just that. Oxytocin then encourages the uterus to contract and expel remaining fluid. This combination makes physiological sense: the lavage removes material mechanically, while uterine contractions help with subsequent drainage.
Timing oxytocin after breeding matters
One of the more interesting aspects of oxytocin therapy is that when it is given matters. The mare's uterus changes in its responsiveness to oxytocin during the estrous cycle. Following your vet's instructions is important here.
The mare needs to be in heat, but you wouldn't be breeding her if she wasn't, so that is probably obvious. Before ovulation, when estrogen is high and progesterone is low, the uterus is particularly responsive to oxytocin. After ovulation, the uterine response changes and higher doses may be required to achieve a similar effect.
There is also an important reason that you should not administer oxytocin immediately after insemination simply as a matter of routine. The sperm need time to move through the reproductive tract and reach the oviduct. You just spent money on a stud fee and shipping in order to get the sperm in there, so don't try and get rid of it too soon!
It takes about 4 hours after breeding for the sperm to make the journey up to the oviducts, where it will sit and wait for ovulation to occur. You should be safe to give oxytocin 4 hours after breeding, but some like to wait until about 6 hours.
Oxytocin in embryo recovery
If you have done embryo transfer with one of your mares, you likely will have seen oxytocin used in the process. An embryo is normally recovered from a donor mare approximately seven to nine days after ovulation by flushing the uterus with specialized fluid.
The goal is to recover the embryo while recovering as much of the flush medium as possible. You really don't want to miss any fluid or leave it in the mare, as your embryo might be in it! Oxytocin can be used when you don't think that you got all the fluid out. It's the same mechanism of stimulating uterine contractions that helps push the remaining fluid out.
The Merck Veterinary Manual describes oxytocin being administered during embryo flushing in some cases to minimize retention of a large amount of flushing medium within the uterus. An AAEP presentation similarly describes oxytocin as an occasional aid to recovery of uterine lavage fluid during embryo collection.
Interestingly, recent research suggests that the technique used to massage the uterus during embryo flushing may have a greater effect on embryo recovery than oxytocin itself. A 2024 study found that uterine massage during flushing improved embryo recovery compared with gravity drainage, while oxytocin was administered during the flushing protocol.
In most cases, experienced practitioners will have their own tried and tested methods and not rely on one single step, but rather use many tools as part of their process.
Oxytocin to induce foaling
Oxytocin stimulates uterine contractions and can be used to induce labor in mares that are ready to foal. It is very important to understand that this should only be given by a vet, and that mares often go for long gestations with no issues. This is not a solution for a long gestation, because unless the mare is on the verge of foaling, it won't do you any favors to stimulate uterine contractions — more on that later.
Modern research has focused on low-dose oxytocin protocols that attempt to induce foaling only when the mare is physiologically ready. A recent review concluded that low-dose oxytocin is currently considered one of the most reliable and safest methods for inducing parturition when appropriate clinical criteria indicate that the mare is ready to foal.
Studies have shown that appropriately selected mares can often foal relatively soon after low-dose oxytocin administration. One study reported normal foals being delivered within two hours in the majority of treated mares.
Why can't oxytocin simply be used to make a mare foal?
Because the fetus and placenta need to be developmentally ready. We get it — having a mare "go over" can be a painful waiting game for us humans, but for the horse, long gestations can be normal, and needed. The last thing you want to do is push a foal out before he or she is ready!
The scary part is that, if used incorrectly, oxytocin can stimulate uterine contractions even when a pregnancy is not yet ready for normal parturition. High doses can therefore induce premature delivery. You do not want this!
For this reason, induction of foaling should only be undertaken when there is a valid veterinary reason and evidence that the mare is sufficiently prepared. Research specifically warns that induction before adequate fetal maturity can result in premature or dysmature foals.
Oxytocin and retained placenta
Oxytocin, and those uterine contractions it stimulates, can be a very useful tool if a mare has a retained placenta.
After the mare has delivered her foal — that's stage 2 of labor — she will deliver the fetal membranes, which is stage 3. The complete delivery of the placenta is important, but it doesn't always happen in a timely manner. A placenta that has not been expelled within approximately three hours after foaling is generally considered retained and requires veterinary attention. Retained fetal membranes can lead to serious complications including metritis, laminitis and systemic illness.
The U.S. federal labeling for oxytocin includes use in horses as a uterine contractor to precipitate or accelerate normal parturition and postpartum evacuation of uterine debris. That said, treatment of a retained placenta is generally an issue that you need to get your vet involved with, rather than something that should be managed by simply giving a mare an arbitrary dose of oxytocin.
Oxytocin and milk letdown
Oxytocin can assist with milk letdown due to the way it stimulates contractions.
Oxytocin does not produce the milk itself. But it can cause the contraction of the myoepithelial cells surrounding the mammary alveoli, helping eject milk into the ducts. This is known as the milk-letdown reflex.
This can be useful if a mare has produced adequate milk but fails to release it normally because the milk-ejection reflex is disrupted. An important clarification is that this will not stimulate the production of milk — rather, it lets the milk down once it has already been produced. A mare lacking in milk because she has not produced enough is a different problem, and not one that oxytocin will help with.
The U.S. regulatory labeling for oxytocin describes its action on mammary smooth muscle as producing milk letdown when the udder is in the appropriate physiological state.
Oxytocin and the mare's natural estrous cycle
This is not a treatment — this is just something we think is an interesting FYI about oxytocin.
Oxytocin is naturally involved in the mare's reproductive physiology. During the normal estrous cycle, oxytocin interacts with the endometrium and prostaglandin F2α (PGF2α), which is involved in luteolysis — the regression of the corpus luteum.
Research has demonstrated pulses of oxytocin occurring alongside pulses of PGF2α during the period when luteolysis occurs. This means oxytocin is part of the mare's natural mechanism for controlling the lifespan of the corpus luteum, and therefore the length of the estrous cycle.
An interesting research finding: oxytocin can prolong the corpus luteum
This seems counterintuitive, given what we just told you about oxytocin being part of the process of lysing the CL. If oxytocin is involved in luteolysis, you might expect giving a mare oxytocin to cause her corpus luteum to regress. Nope.
Research has actually demonstrated the opposite under certain experimental conditions. Continuous administration of oxytocin during specific stages of the mare's cycle prevented luteolysis in most treated mares and prolonged corpus luteum function. Another study found that repeated oxytocin administration during days 7–14 after ovulation prevented normal luteolysis in treated mares.
This is not a routine breeding treatment. It is important because it demonstrates how complicated the relationship between oxytocin, its receptors and PGF2α is in the mare.
It should not be interpreted as a recommendation to administer oxytocin to manipulate a mare's cycle. What it should be interpreted as is: talk to your vet. Often we hear of things like oxytocin so frequently that we can forget there are complicated mechanisms behind the way all these elements work, and how they impact one another.
Does every mare need oxytocin after breeding?
No.
Oxytocin is particularly useful when there is evidence or a history of poor uterine clearance, but it isn't necessarily required for every mare after every breeding. A healthy mare normally clears the inflammatory response caused by breeding herself.
Routine treatment decisions should therefore take into account:
- Mare age
- Previous reproductive history
- Number of previous foals
- Cervical function
- Uterine clearance
- Presence of intrauterine fluid
- Previous history of post-breeding endometritis
- Timing relative to ovulation
- Type of breeding
- Veterinarian assessment
The goal is not simply to make the uterus contract as much as possible. The goal is to achieve effective uterine clearance without causing excessive or uncoordinated contractions.
Can too much oxytocin be a problem?
Yes.
Excessive uterine contractions can actually interfere with effective drainage in some circumstances. A review of post-breeding endometritis notes that excessive doses can produce tetanic uterine contractions, potentially contributing to retention of uterine fluid.
This is one reason dosage and timing need to be individualized. Oxytocin is a powerful hormone, not simply a harmless "uterus cleaner."
The bottom line
Oxytocin is one of the most versatile hormones used in equine reproduction. Its primary action is simple: it stimulates smooth-muscle contraction. But that single action has many applications.
In the breeding mare, oxytocin can help the uterus clear itself after breeding, assist mares susceptible to persistent breeding-induced endometritis, and help evacuate fluid after uterine lavage. It can also be used during specialized embryo-recovery procedures.
In the pregnant and postpartum mare, oxytocin can induce parturition when a mare is appropriately prepared, assist with retained fetal membranes, promote postpartum uterine contraction, and facilitate milk letdown. And at the physiological level, oxytocin is part of the mare's natural reproductive cycle and interacts with the hormonal mechanisms controlling luteolysis.
For breeders, the important point is that oxytocin is not one treatment for one problem. It is a hormone with multiple roles, and its usefulness depends heavily on why it is being used, when it is administered, and what is happening in the individual mare.
Accurate reproductive records can be extremely useful when managing mares that repeatedly require treatments such as oxytocin. Recording breeding dates, ovulation, ultrasound findings, uterine fluid, lavage, medications and pregnancy outcomes creates a history that can help a veterinarian identify patterns from one breeding season to the next.
Sources and further reading
Equine post-breeding endometritis
- Maischberger E, Irwin JA, Carrington SD, Duggan VE. Equine post-breeding endometritis: A review. Irish Veterinary Journal. 2008;61:163. The principal source for the sections on post-breeding inflammation, uterine clearance, oxytocin treatment, uterine lavage, timing and excessive uterine contractions.
Oxytocin and luteal function
- Stout TA, Lamming GE, Allen WR. Oxytocin administration prolongs luteal function in cyclic mares. Journal of Reproduction and Fertility. 1999;116(2):315–320.
- The uterus as a source of oxytocin in cyclic mares — research demonstrating that oxytocin is released into the uterine lumen and participates in PGF2α release and luteolysis.
- Diestrus administration of oxytocin prolongs luteal maintenance and reduces plasma PGFM concentrations and endometrial COX-2 expression in mares. Theriogenology. 2013;79(4):616–624.
- Effect of chronic administration of oxytocin on corpus luteum function in cycling mares. Journal of Equine Veterinary Science. 2020.
Embryo recovery
- Effect of embryo flushing technique, number of flushing attempts and previous experience of operator on embryo recovery. Journal of Equine Veterinary Science. Found that operator experience and flushing technique influenced embryo recovery, and that oxytocin did not affect recovery rate.
- The effect of uterine massage and number of embryo flushing attempts on embryo recovery in mares. Theriogenology. 2024;224:94–101.
Parturition and retained fetal membranes
- Hillman RB, Ganjam VK. Hormonal changes in the mare and foal associated with oxytocin induction of parturition. Journal of Reproduction and Fertility Supplement. 1979.
- Clinical observations on the course of oxytocin- or prostaglandin E2/oxytocin-induced parturition in mares — research examining cervical preparation and induction of parturition.
- Gilbert RO. Retained Placenta in Horses. Merck Veterinary Manual. Updated 2024.
- Effective oxytocin treatment on placental expulsion after foaling in heavy draft mares — clinical study of oxytocin for placental expulsion.
Postpartum uterine function
- Gündüz MC, Kaşikçi G, Kaya HH. The effect of oxytocin and PGF2α on the uterine involution and pregnancy rates in postpartum Arabian mares. Animal Reproduction Science. 2008;104(2–4):257–263.
Lactation
- Characteristics of milk ejection, associated intramammary pressure changes and oxytocin release in the mare — research examining oxytocin and the milk-ejection reflex in mares.