Estrus Suppression in Mares: What Works and What To Avoid

By Dr Duncan Houston

A mare in heat can be obvious. She may squat, urinate frequently, lift her tail, become distracted, squeal, kick, flirt with other horses, or feel completely different under saddle.

But here is the bit owners and riders often miss: not every difficult mare is hormonal. Tail swishing, girthiness, bucking, resistance, poor performance, aggression, or “mare-ish” behaviour can come from pain, training conflict, saddle fit, ulcers, lameness, ovarian disease, urinary disease, reproductive tract disease, or rider expectation.

Estrus suppression can be very useful in the right mare. The trick is choosing the right method, timing it properly, and not using hormones to hide a problem that was never hormonal in the first place.

Quick Answer

The most reliable medical options for suppressing heat behaviour in mares are progestogens, especially oral altrenogest, and veterinary-timed protocols such as oxytocin to prolong the luteal phase. Altrenogest is effective and widely used, but it must be handled carefully because it can absorb through human skin and is not suitable for every mare. Oxytocin can be a lower-cost option during the ovulatory season, but it must be timed correctly after ovulation. Intrauterine glass marbles are now generally a poor choice because efficacy is inconsistent and complications have been reported. (Merck Animal Health USA)

What Is Estrus in Mares?

Estrus is the “heat” stage of the mare’s reproductive cycle. This is when the mare is sexually receptive to the stallion and is under the influence of low progesterone and rising estrogen.

Mares are seasonally polyestrous, which means most cycle during the longer daylight months. Once regular cycles are established, the typical interovulatory cycle is around 21 days, although the length of estrus can vary, especially during seasonal transition. Merck Veterinary Manual notes that mares usually ovulate near the end of estrus, and that estrus may range from 2 to 8 days depending on season and individual variation. (Merck Veterinary Manual)

Common estrus signs can include:

• Frequent urination

• Tail lifting

• Squatting

• Winking of the vulva

• Interest in geldings or stallions

• Distraction under saddle

• Squealing or striking

• Sensitivity around grooming or leg pressure

• Reduced focus during work

The important word is can. Some mares show classic heat signs. Others show almost none. Some mares behave badly outside estrus. That is why diagnosis matters.

Is the Mare Really in Heat?

This is the first clinical checkpoint.

A review in Clinical Theriogenology notes that behaviour problems in mares are often blamed on estrus, but may or may not be directly connected to the estrous cycle. It recommends differentiating reproductive causes from non-reproductive causes before choosing suppression treatment.

In practice, I would be suspicious of a purely hormonal explanation if the mare is difficult all month, worse during saddling, worse on one rein, girthy, lame, reactive over the back, resentful when the rider’s leg is applied, losing condition, colicky, urinating abnormally, or changing behaviour suddenly.

Hormones may be part of the story. They are not always the whole book.

What Else Can Look Like Estrus Behaviour?

Before suppressing the cycle, your vet should consider other causes.

Musculoskeletal Pain

Back pain, sacroiliac pain, hock pain, stifle pain, foot pain, neck pain, and subtle lameness can all look like “attitude.” A mare that pins her ears when asked to canter may be sore, not hormonal.

Saddle Fit and Rider Factors

Poor saddle fit, girth pressure, dental pain, bit issues, rider imbalance, or inconsistent training can trigger resistance that gets blamed on the mare’s cycle.

Gastric Ulcers

Ulcers can cause girthiness, poor performance, irritability, appetite changes, and sensitivity. A hormonal treatment will not fix an ulcer.

Ovarian Disease

Granulosa-theca cell tumours and other ovarian abnormalities can cause behavioural changes, stallion-like behaviour, persistent anestrus, irregular cycles, or aggression. These mares need reproductive examination, ultrasound, and hormone testing, not just suppression.

Uterine or Vaginal Disease

Endometritis, uterine fluid, pneumovagina, urine pooling, vaginal discharge, or reproductive tract pain can make a mare uncomfortable.

Urinary Tract Problems

Frequent urination is not always estrus. Cystitis, bladder stones, kidney issues, or irritation can mimic heat-related urination.

Training and Environment

Herd changes, stall confinement, poor turnout, anxiety, overtraining, or inconsistent handling can all produce behavioural signs that appear “mare-ish.”

This is why the best estrus suppression plans start with a question: does the behaviour actually follow the cycle?

How Do Vets Work Out Whether It Is Hormonal?

A practical work-up may include:

• Behaviour diary

• Timing signs against the mare’s cycle

• Reproductive ultrasound

• Palpation of ovaries and uterus

• Assessment for uterine fluid or inflammation

• Vaginal and perineal examination where needed

• Lameness examination

• Back and saddle assessment

• Dental check

• Gastric ulcer assessment if signs fit

• Blood testing for ovarian tumour markers where indicated

• Trial of suppression under veterinary supervision

A short, well-timed treatment trial can be useful. If the mare’s behaviour improves dramatically during true suppression and returns when treatment stops, hormones are more likely involved. If nothing changes, the problem is probably somewhere else. That is not failure. That is useful information.

Option 1: Oral Altrenogest

Altrenogest is the active ingredient in Regu-Mate. It is a synthetic progestogen, not natural progesterone. It is one of the most widely used and reliable options for suppressing estrus behaviour in mares.

In the United States, Regu-Mate is indicated for suppression of estrus in mares. The labelled dose is 0.044 mg/kg orally once daily for 15 consecutive days, which is also described as 1 mL per 110 pounds of body weight. (DailyMed)

Why It Works

Progestogens create a hormone environment similar to diestrus, when progesterone is high and the mare is not normally receptive to the stallion. This can reduce heat behaviour and make performance more predictable.

Merck Veterinary Manual notes that estrous behaviour can be suppressed using progestogens, including altrenogest, although progestogens may suppress behaviour without always suppressing follicle growth and ovulation in cyclic mares. (Merck Veterinary Manual)

That distinction matters. A mare may look behaviourally controlled while her ovaries are still active.

Pros

• Reliable for many mares

• Predictable onset and withdrawal response

• Useful for short-term show control

• Useful for longer management plans when appropriate

• Can be used strategically around competition or breeding schedules

Cons

• Daily dosing required

• Can be expensive long term

• Handling risk for people

• Can spill onto skin or surfaces

• Not suitable for all mares

• May mask underlying pain or disease if used without a diagnosis

Human Safety Warning

Altrenogest is a serious handling-risk medication. The product label states that it can be absorbed through unbroken skin, latex gloves are not protective, and vinyl, neoprene, or nitrile gloves should be used. Pregnant women, women who may be pregnant, and women of childbearing age need particular caution. (Merck Animal Health USA)

This is not a “splash a bit on the feed and she’ll be right” drug. It needs careful handling, safe storage, and proper dosing equipment.

When Not To Use It

Regu-Mate is contraindicated in mares with previous or current uterine inflammation because progestogen treatment may worsen low-grade uterine inflammation into a more serious infection. (Merck Animal Health USA)

That means a mare with discharge, uterine fluid, suspected endometritis, or reproductive tract disease should be assessed before using it.

Option 2: Injectable Progesterone and Compounded Progestogens

Injectable progesterone can be effective, but the formulation matters.

Merck Veterinary Manual describes progesterone in oil as one option and notes that progesterone in a biorelease vehicle is available by prescription from compounding pharmacies, with administration commonly every 7 to 10 days depending on the preparation. (Merck Veterinary Manual)

That is different from saying “a monthly hormone injection will fix it.” Some compounded products are not equivalent, and duration can vary.

The Big Warning: Not All Progestins Work in Mares

Some human contraceptive-type progestins have been used off-label in mares because they are convenient. That does not mean they work.

A theriogenology review notes that several injectable progestins used in humans have not reliably suppressed estrus behaviour in mares, and that medroxyprogesterone acetate failed to suppress estrus behaviour in the study cited by the authors. (Ymaws)

This is one of the common traps. A product may sound hormonal, long acting, and convenient, but still not bind equine progesterone receptors effectively enough to do the job.

Pros

• Less daily handling than oral altrenogest

• Can be useful where oral dosing is difficult

• Natural progesterone formulations can be effective

• May be practical for selected performance mares

Cons

• Injection site swelling or soreness can occur

• Compounded products vary

• Not all progestins are effective in mares

• Competition rules may apply

• Must be prescribed and monitored by a vet

For performance horses, injection site soreness alone can be a deal-breaker. A mare with a sore neck or rump before a major event is not exactly the management win we were aiming for.

Option 3: Oxytocin To Prolong Diestrus

Oxytocin is an interesting and often underused option for estrus suppression in mares.

The aim is not to “sedate” the mare or shut down the ovaries. The aim is to prolong the life of the corpus luteum, which keeps progesterone levels higher and delays the mare’s return to estrus.

A review in Clinical Theriogenology describes a commonly used protocol of 60 units of oxytocin given intramuscularly once daily from day 7 to day 14 of the mare’s cycle. The same review states this approach prolongs corpus luteum function in about 60 to 70% of treated mares when properly timed. (Ymaws)

Why Timing Matters

Oxytocin suppression depends on knowing when the mare ovulated. Day 0 is ovulation. Treatment is then timed for days 7 to 14.

If you guess the timing, you can miss the window. This is why ultrasound monitoring is so useful.

Pros

• Often cheaper than long-term altrenogest

• Less daily handling risk for humans than oral altrenogest

• Can provide a longer period out of heat

• Useful during the ovulatory season

• Minimal behavioural dulling when it works

Cons

• Requires ovulation tracking

• Does not work in every mare

• Less useful during anestrus or irregular transition

• Requires injections

• Not ideal if you are trying to breed that cycle

• Should be vet-directed

Oxytocin is a lovely option when it is timed properly. When it is guessed wildly, it becomes expensive optimism in a syringe.

Option 4: Intrauterine Glass Marbles or Balls

Intrauterine glass marbles were historically used to try to trick the uterus into behaving as if a conceptus were present, thereby delaying luteolysis and suppressing estrus.

They are now much harder to justify.

A review notes that intrauterine marbles have unreliable efficacy, a high probability of failure or expulsion, and documented complications, including endometrial damage and pyometra. (Ymaws)

A PubMed-indexed case series reported reproductive complications after long-term intrauterine glass marble use, including chronic endometritis, pyometra, and marbles or glass shards adhered to the endometrium. (PubMed)

Pros

• Low medication cost

• No daily oral dosing

• May work temporarily in some mares

Cons

• Variable and unreliable response

• May be expelled

• Can be forgotten

• Requires ultrasound monitoring

• Can damage the uterus

• Can cause chronic endometritis or pyometra

• Can compromise future fertility

My practical take: there are usually better options. I would not make a glass marble my first-line plan for a valuable performance or breeding mare.

Option 5: Ovariectomy

Ovariectomy is surgical removal of the ovaries. It is permanent and should only be considered in very specific cases.

It may be appropriate when:

• There is ovarian pathology

• A granulosa-theca cell tumour is present

• The mare is not intended for future breeding

• Severe behaviour is strongly linked to ovarian activity

• Medical management has failed

• Welfare or safety is a major concern

It is not a casual performance hack. Once the ovaries are removed, the mare cannot breed naturally. Also, not all behaviour improves after ovariectomy if the original cause was pain, training, gastric ulcers, or temperament.

Option 6: Managing the Season With Light, Routine and Workload

Light management is often used to bring mares into cyclicity earlier for breeding. It is not usually an estrus suppression tool.

Merck notes that exposing mares to 16 hours of light per day can hasten the onset of cyclicity, but mares generally need 8 to 10 weeks to respond. (Merck Veterinary Manual)

This can be helpful for breeding programs, but it will not solve heat behaviour in the way altrenogest or properly timed oxytocin might.

That said, routine still matters. Some mares are much more reactive when their workload, turnout, herd situation, or feeding schedule is chaotic. You cannot hormone your way out of poor management forever. The horse will lodge a complaint, usually at the trot poles.

Choosing the Right Estrus Suppression Method

Option	Best For	Main Advantage	Main Concern
Oral altrenogest	Short or long-term behaviour suppression	Reliable and predictable	Daily handling and human safety risk
Injectable natural progesterone	Mares difficult to dose orally	Less daily handling	Injection soreness, compounded variability
Oxytocin	Ovulatory-season suppression after confirmed ovulation	Lower cost and often practical	Must be timed correctly
Intrauterine marble	Rarely preferred now	No daily medication	Unreliable and potential uterine complications
Ovariectomy	Ovarian disease or severe refractory cases	Permanent ovarian hormone removal	Surgery and loss of breeding ability
Management changes	Mild signs, stress-linked behaviour	Low risk and useful foundation	May not control true estrus behaviour

The right choice depends on whether the mare is being managed for performance, breeding, embryo transfer, showing, safety, or comfort.

Competition and Medication Rules

If the mare competes, check the rules before starting any hormone treatment.

Altrenogest rules vary between organisations, countries, sexes, and disciplines. Equestrian Australia reported that FEI guidance allows altrenogest for suppressing estrus in mares when used according to manufacturer recommendations, but its use in male horses is not allowed. It also notes that medication records may still matter for FEI horses. (Equestrian Australia)

Do not assume a drug is permitted because another rider uses it. Racing, FEI, national federations, pony clubs, and breed organisations may have different rules.

Severity and Risk Framework

Low Risk: Mild, Predictable Heat Behaviour

The mare shows mild, cycle-linked signs such as tail lifting, urination, distraction, or flirtiness, but she is safe to handle, comfortable, sound, and normal between cycles.

What to do: track her cycle for 2 to 3 months, improve management consistency, and consider veterinary-guided suppression if performance is affected.

Medium Risk: Performance Is Clearly Affected

The mare becomes difficult to ride, distracted, reactive, or uncomfortable during specific cycle stages, but there are no red flags for pain or illness.

What to do: arrange a reproductive exam, confirm cycling pattern, rule out pain, and discuss altrenogest or oxytocin options.

High Risk: Behaviour Is Severe, Sudden, or Not Cycle-Linked

The mare is aggressive, dangerous, suddenly changed, painful, resistant all month, or showing signs not limited to estrus.

What to do: do not jump straight to hormones. She needs a proper veterinary assessment for pain, ovarian disease, lameness, ulcers, urinary disease, reproductive tract disease, and training factors.

Critical Risk: Medical Red Flags

The mare has colic signs, fever, vaginal discharge, blood in urine, severe abdominal pain, collapse, severe lameness, rapid deterioration, or suspected medication reaction.

What to do: stop riding and call your vet urgently.

When Is This an Emergency?

Estrus behaviour itself is not usually an emergency. But signs blamed on estrus can sometimes be serious.

Call your vet promptly if your mare has:

• Colic signs

• Fever

• Depression or loss of appetite

• Vaginal discharge

• Foul smell from the vulva

• Blood in urine

• Straining to urinate

• Severe flank or abdominal pain

• Sudden dangerous aggression

• Persistent stallion-like behaviour

• One-sided ovarian enlargement

• Severe lameness

• Rapid behaviour change

• Suspected pregnancy

• Reaction to medication

• Human exposure to altrenogest

Also seek advice immediately if altrenogest is spilled on human skin, especially if the person is pregnant, may be pregnant, or is of childbearing age. The product label instructs immediate washing after skin exposure and warns that the drug may affect menstrual cycles or pregnancy. (Merck Animal Health USA)

What Should You Do Next?

1. Track the Behaviour

Write down when the signs occur, how long they last, what they look like, and whether they match a cycle.

Track:

• Urination

• Tail lifting

• Winking

• Aggression

• Girthiness

• Performance changes

• Rider comments

• Turnout changes

• Feed changes

• Lameness or soreness

• Weather and workload

If signs appear every 18 to 23 days and last a few days, hormones become more likely. If the mare is angry every Tuesday because that is jumping day, I would look elsewhere.

2. Get a Veterinary Exam

Do not medicate first and diagnose later.

A vet may check ovaries, uterus, vaginal health, urinary signs, pain, lameness, and general health.

3. Confirm the Cycle With Ultrasound

Ultrasound allows your vet to identify follicles, ovulation, corpus luteum, uterine fluid, and abnormalities. This is especially important if oxytocin is being considered because timing depends on ovulation.

4. Decide the Goal

Are you trying to manage one competition weekend, an entire season, breeding timing, safety, transport, or a chronic behavioural issue?

Short-term and long-term plans are different.

5. Choose the Safest Practical Option

For many mares, altrenogest is the most predictable. For some mares, oxytocin is an excellent lower-cost alternative. For others, the right answer is treating pain rather than suppressing estrus.

6. Check Competition Rules

Before treating a competing mare, check the rulebook for the discipline and country. Keep medication records.

7. Monitor Response

Record whether the behaviour improves, stays the same, or worsens. If suppression does not help, reassess the diagnosis.

8. Recheck Long-Term Cases

Mares on ongoing suppression should not disappear from veterinary oversight. Reassess comfort, reproductive tract health, performance, side effects, competition compliance, and breeding plans.

Common Mistakes Owners Make

Assuming Every Behaviour Problem Is Hormonal

This is the big one. Many mares blamed for being hormonal are actually sore, stressed, confused, ulcer-prone, poorly fitted, or poorly understood.

Using Altrenogest Carelessly

Altrenogest needs careful handling. Latex gloves are not protective, and human exposure matters. (Merck Animal Health USA)

Using Random Compounded Hormones

Not all progestins work reliably in mares. Convenience is not the same as efficacy. (Ymaws)

Timing Oxytocin Without Ovulation Tracking

Oxytocin protocols depend on the day of the cycle. Guessing the timing is a classic way to waste money and confidence.

Using Glass Marbles as a First-Line Option

Given the potential for expulsion, poor response, chronic endometritis, pyometra, and glass-related complications, intrauterine marbles are usually not the best first choice. (PubMed)

Forgetting Future Breeding Plans

Some treatments may interfere with breeding timing, fertility evaluation, or uterine health. Always tell your vet if the mare may be bred later.

Ignoring Competition Rules

Medication rules can change and vary by organisation. Check before you treat.

Prevention: Building a Better Mare Management Plan

You cannot prevent every estrus-related behaviour, but you can make the pattern easier to manage.

A good plan includes:

• Tracking cycles

• Recording behaviour changes

• Regular reproductive exams if signs are severe

• Proper saddle fitting

• Dental checks

• Lameness and back assessment

• Consistent turnout

• Stable herd management

• Ulcer risk assessment

• Appropriate workload

• Heat and stress management

• Clear competition medication records

• Early vet involvement if behaviour changes suddenly

The goal is not to make every mare hormonally silent. The goal is to keep her comfortable, safe, rideable, and fairly managed.

FAQ

What is the best way to stop a mare coming into heat?

The most reliable medical option is usually a progestogen such as oral altrenogest. Oxytocin can also be useful in properly timed cases during the ovulatory season. The best option depends on the mare, season, competition rules, breeding plans, and whether the behaviour is truly cycle-related.

Does Regu-Mate stop ovulation?

Not always. Regu-Mate can suppress estrus behaviour, but cyclic mares may still develop follicles or ovulate while behaviour is controlled. Merck Veterinary Manual notes that progestogens may suppress behaviour without effectively suppressing follicle growth and ovulation. (Merck Veterinary Manual)

Is oxytocin a good alternative to Regu-Mate?

It can be. Oxytocin can prolong corpus luteum function and keep some mares out of heat for longer, but it must be started at the right time after ovulation. The commonly described protocol is 60 units intramuscularly once daily from day 7 to day 14, under veterinary guidance. (Ymaws)

Are glass marbles safe for mares?

They are not my preferred option. Research and case reports have described inconsistent results and complications including expulsion, chronic endometritis, pyometra, and glass shards adhered to the endometrium. (PubMed)

Should I suppress estrus if my mare is aggressive?

Only after a proper assessment. Aggression may be hormonal, but it can also come from pain, ovarian disease, ulcers, lameness, fear, training conflict, or poor handling. If the behaviour is sudden, severe, dangerous, or not cycle-linked, investigate before medicating.

Final Thoughts

Estrus suppression in mares can be extremely useful, but only when the target is correct.

Altrenogest is often the most predictable option. Oxytocin can be a smart lower-cost alternative when ovulation timing is known. Compounded injectable progesterone can work, but product choice matters. Glass marbles are increasingly difficult to justify given their variable success and uterine risks.

The most important step is not choosing the drug. It is confirming the problem.

A mare that is genuinely struggling during estrus deserves help. A mare that is sore, ulcerated, lame, or misunderstood deserves a diagnosis. Good mare management means knowing the difference.

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If your mare’s behaviour changes during her cycle, or you are unsure whether suppression, ultrasound tracking, pain assessment, or reproductive investigation is the right next step, ASK A VET™ can help you work through the signs and decide when veterinary support is needed.