Can You Breed an Obese Mare Safely?

By Dr Duncan Houston

Breeding an overweight mare is not automatically impossible, but it should never be treated as routine.

A mare can be fat, cycle normally and still get pregnant. That is the awkward part. The bigger concern is what is happening metabolically underneath the body condition score: insulin dysregulation, laminitis risk, abnormal fat deposits, PPID, inflammation, poor uterine clearance, pregnancy weight gain and the future health of the foal.

The question is not simply, “Can this mare conceive?”

It is:

Is this mare metabolically safe to breed, healthy enough to carry a pregnancy, and likely to produce a healthy foal without putting herself at avoidable risk?

Quick Answer

An obese mare should not be bred until her metabolic health has been assessed. Body condition alone does not prove infertility, but obesity is commonly associated with insulin dysregulation, equine metabolic syndrome and laminitis risk, which can make pregnancy and lactation more dangerous.

Before breeding, the mare should have a body condition assessment, insulin testing, PPID testing where indicated, hoof evaluation, diet review and a reproductive examination. Aim for a moderate body condition, usually around 5 to 6 out of 9, with controlled insulin and no active laminitis before attempting pregnancy. The MSD/Merck body condition chart describes score 5 as moderate and score 6 as the acceptable upper limit for broodmares, while scores 7 and above are considered obese or unacceptable because of metabolic and laminitis risk. (MSD Veterinary Manual)

The Key Correction: Obesity Is Not the Same as EMS

This is where many articles become too blunt.

Obesity means the mare is carrying excess body fat.

Equine metabolic syndrome means the mare has a metabolic risk state, with insulin dysregulation as the key feature. Many EMS horses are obese or have regional fat deposits, but not every obese horse has proven EMS, and not every insulin-dysregulated horse looks dramatically obese.

Merck Veterinary Manual states that obesity and a cresty neck are not sufficient to diagnose EMS, and absence of obesity is not sufficient to exclude insulin dysregulation. Testing should focus on documenting insulin dysregulation. (Merck Veterinary Manual)

That distinction matters for breeding.

A moderately overweight mare with normal insulin regulation, no laminitis history and a normal reproductive tract may be managed very differently from a cresty, insulin-dysregulated mare with previous laminitis.

Why Does Obesity Matter in a Broodmare?

Fat tissue is not just storage. It is metabolically active tissue.

In obese horses, excess fat may be associated with:

• Insulin dysregulation

• Higher laminitis risk

• Increased inflammatory signalling

• Reduced exercise tolerance

• Heat intolerance

• Mechanical stress on feet and joints

• More difficulty losing weight safely once pregnant

• More complex nutritional management during late pregnancy and lactation

The main danger is not that fat magically stops the ovaries working. Current reviews show the relationship between obesity, insulin dysregulation and fertility in mares is complex and sometimes contradictory. Obesity and insulin dysregulation can alter the follicular environment and endometrium, but obesity alone has not consistently been linked to markedly reduced conception rates in mares. (PMC)

The practical veterinary concern is this:

An obese mare may still get pregnant, but pregnancy may expose an already fragile metabolic system.

Does Obesity Stop Mares Cycling Normally?

Not usually in the simple way people expect.

In mares, higher body condition is often associated with more persistent cyclicity. Some heavier mares continue cycling through winter or return to cyclicity earlier than lean mares. (PMC)

That means an obese mare may appear reproductively “active” and still have an underlying metabolic problem.

The evidence does not strongly support the idea that obesity or insulin dysregulation is a major direct cause of ovulatory failure in mares. A major review states that obesity and insulin dysregulation are not considered significant predisposing factors for ovulatory failure, although EMS may alter the follicular environment and the clinical relevance remains unclear. (PMC)

So the article should not say, “Obese mares often skip ovulation” as if that is established fact.

A better version is:

Obese mares may cycle normally, but insulin dysregulation and altered follicular or uterine environments may still affect fertility, embryo quality or pregnancy risk.

Does Obesity Cause High Progesterone or Hormonal Imbalance?

The evidence is not strong enough to make that a simple claim.

Some studies suggest obese mares may have higher progesterone concentrations or slightly altered luteal activity, but the overall published data are inconclusive. A major review concludes that obesity does not appear to markedly alter corpus luteum function. (PMC)

That matters because owners often hear that every problem in a fat mare is “hormones.”

Sometimes the mare’s cycle is not the main issue at all.

What matters more clinically is whether she has:

• Insulin dysregulation

• Laminitis risk

• PPID

• Poor uterine clearance

• Endometritis

• Older mare reproductive changes

• Poor semen timing or quality

• Early embryonic loss

How Does EMS Affect Fertility?

EMS may affect fertility through several possible pathways, but the research is still developing.

Possible mechanisms include:

• Altered follicular fluid environment

• Altered oocyte or embryo development

• Increased inflammatory signalling

• Changes in endometrial function

• Higher risk during pregnancy because pregnancy itself alters insulin sensitivity

• Increased laminitis risk during dietary or hormonal changes

A review of obesity and insulin dysregulation in mares notes that maternal metabolic status can affect reproductive processes and may have implications for foal health before and after birth. It also notes that the complete effect on reproduction and offspring is not fully understood. (PMC)

This is the balanced clinical message:

EMS is not a guaranteed infertility diagnosis, but it is a major risk marker.

A mare with EMS may still conceive, but breeding her before insulin is controlled is poor planning, especially if she has a laminitis history.

Why Pregnancy Raises the Stakes

Pregnancy naturally changes insulin sensitivity.

Mares develop physiological insulin resistance during mid to late pregnancy to help direct glucose to the placenta and fetus. This is normal. The concern is what happens when a mare is already insulin-dysregulated before pregnancy begins. (PMC)

An obese or insulin-dysregulated mare may have less metabolic room to adapt.

Pregnancy also adds:

• More body weight

• More load through the feet

• Changing nutritional needs

• Reduced margin for pasture mistakes

• Greater difficulty managing weight loss safely

• Higher consequences if laminitis develops

The real fear is not infertility alone.

It is a pregnant mare developing laminitis, becoming painful, needing restricted movement and requiring strict diet management while also needing enough nutrition to support the pregnancy.

That is a horrible corner to paint yourself into.

Does Obesity Increase Uterine Infection Risk?

This needs careful wording.

Endometritis is a major cause of infertility in mares, but obesity alone has not been clearly proven to increase the incidence of endometritis. A major review notes that obesity and insulin resistance are suggested to affect immune responses, but no clear correlation has been reported between body condition and endometritis incidence in mares. (PMC)

That does not mean uterine health can be ignored.

Before breeding an obese or metabolically abnormal mare, it is still sensible to assess:

• Uterine fluid

• Culture

• Cytology

• Cervical function

• Vulvar conformation

• Previous breeding history

• Endometrial quality in older or repeat-breeder mares

A positive culture alone does not automatically prove a true uterine infection. Merck states that endometrial culture should be used as a diagnostic adjunct and that a positive culture should be accompanied by evidence of inflammation before diagnosing endometritis. (Merck Veterinary Manual)

What About PPID or “Cushing’s Disease”?

Pituitary pars intermedia dysfunction, or PPID, is common in older horses and can complicate breeding decisions.

PPID may be associated with:

• Long or delayed-shedding coat

• Muscle loss

• Abnormal fat distribution

• Increased drinking and urination

• Increased infection susceptibility

• Laminitis when insulin dysregulation is also present

Merck notes that horses with PPID are at high risk for laminitis if insulin dysregulation is also present. (Merck Veterinary Manual)

That is the key point.

A PPID mare should not automatically be placed in the same category as an obese EMS mare. Some PPID horses are lean and need more calories, not less. The 2025 Equine Endocrinology Group PPID recommendations state that insulin dysregulation should be assessed in all PPID patients, and that PPID horses with normal insulin regulation confirmed by dynamic testing do not need a carbohydrate-restricted diet. (Squarespace)

For breeding, the question is not simply, “Does she have PPID?”

It is:

Is her PPID controlled, is insulin dysregulation present, and can pregnancy, lactation and medication be managed safely?

Can Pergolide Be Used in Breeding or Pregnant Mares?

This is a specialist decision.

Pergolide is the standard treatment for PPID, but the 2025 Equine Endocrinology Group recommendations state that the safety and efficacy of pergolide in breeding, pregnant and lactating animals has not been assessed. They also note that effects on fertility, lactation and fetal development are not known. (Squarespace)

This does not mean every PPID mare must stop treatment or cannot be bred.

It means the mare needs an individual plan involving the treating veterinarian, reproduction veterinarian and owner.

Do not stop pergolide before breeding or foaling without veterinary direction. Missing or stopping treatment can allow ACTH to rise, and uncontrolled PPID may create its own risks.

Can Obesity Affect the Foal?

Possibly, but the evidence is still developing.

A major review notes that maternal metabolic status, body condition and nutrition during pregnancy can influence foal metabolic pathways and later health. In horses, maternal insulin resistance, inflammation and altered fetal growth may predispose foals to metabolic changes and possible orthopedic health problems. (PMC)

Another important finding is that diet may matter as much as body condition. The same review notes that feeding regime and post-feeding hyperinsulinaemia may play a role in foal glucose and insulin dynamics. (PMC)

So the practical advice is not simply “make the mare thin.”

It is:

Breed mares in healthy condition, avoid overfeeding, control insulin, and feed pregnancy diets based on measured need rather than tradition.

Should You Breed an Obese Mare?

Lower Risk

The mare:

• Has a body condition score around 5 to 6 out of 9

• Has no active laminitis

• Has no previous laminitis or only a well-managed historical episode

• Has normal insulin testing

• Has no uncontrolled PPID

• Has a normal reproductive tract

• Has no persistent uterine fluid or infection

• Has a suitable exercise and diet plan

What to do: breeding may be reasonable with routine reproductive management and continued metabolic monitoring.

Moderate Risk

The mare:

• Has a body condition score around 6.5 to 7 out of 9

• Has a cresty neck or regional fat deposits

• Has unknown insulin status

• Has mild previous foot soreness

• Is older

• Has possible PPID signs

• Has not had a pre-breeding reproductive examination

What to do: pause breeding plans until metabolic and reproductive testing are completed.

High Risk

The mare:

• Has confirmed insulin dysregulation

• Has EMS

• Has previous laminitis

• Has a body condition score of 7 or higher

• Has a large cresty neck

• Has uncontrolled PPID

• Requires severe grazing restriction

• Has persistent uterine fluid or repeat-breeder history

• Is not sound enough for safe exercise

What to do: do not breed yet. First control insulin, reduce body condition safely, stabilise hoof health and complete a reproductive work-up.

Poor Candidate to Carry a Pregnancy

The mare has:

• Active laminitis

• Severe insulin dysregulation

• Chronic painful feet

• Recurrent laminitis despite strict management

• Severe obesity with poor mobility

• Uncontrolled PPID

• Significant systemic illness

• Serious cardiac, respiratory or orthopedic disease

• A uterus unlikely to maintain pregnancy

What to do: carrying a pregnancy may not be safe. If the mare has valuable genetics, embryo transfer or OPU-ICSI may be discussed, but these do not correct poor oocyte quality or all metabolic effects.

When Is This an Emergency?

This topic is usually about planning, but some signs are urgent.

Call a veterinarian immediately if the mare has:

• Hot, painful feet

• Strong or bounding digital pulses

• Short, pottery steps

• Reluctance to turn

• Repeated weight shifting

• A rocked-back stance

• Sudden severe lameness

• Colic signs

• Fever

• Depression

• Refusal to eat

• Foul vulvar discharge

• Vaginal bleeding during pregnancy

• Premature udder development

• Milk leakage before term

• Suspected abortion

• Severe swelling, weakness or collapse

Active laminitis should be treated as an emergency. Do not breed a mare with active or unstable laminitis.

What Else Can Cause Poor Fertility in an Obese Mare?

Do not blame everything on weight.

Important differentials include:

• Poor breeding timing

• Poor semen quality

• Semen shipping or handling problems

• Failure to ovulate

• Persistent anovulatory follicle

• Age-related oocyte decline

• Older maiden mare cervical dysfunction

• Persistent breeding-induced endometritis

• Bacterial or fungal endometritis

• Poor uterine clearance

• Pneumovagina

• Urovagina

• Poor vulvar conformation

• Endometrial fibrosis or endometrosis

• Uterine cysts

• Oviductal dysfunction

• Early embryonic loss

• Stallion subfertility

• PPID

• EMS

• Chronic pain or systemic disease

In practice, the mistake is often treating “fat mare” as the diagnosis and skipping the actual fertility work-up.

A heavy mare can also have bad timing, poor semen, endometritis or an ageing uterus. Biology likes teamwork, unfortunately.

What Should You Do Before Breeding?

1. Body Condition Score the Mare

Use a 1 to 9 scoring system and physically palpate the ribs, neck, withers, shoulders, back and tailhead.

Do not judge only from the side view. A mare can hide a lot under a winter coat or look “broodmare round” when she is actually obese.

Target a moderate body condition before breeding.

2. Assess the Neck Crest and Fat Deposits

Regional fat can matter even when the overall body condition score is not extreme.

Look for fat at:

• Neck crest

• Tailhead

• Behind the shoulder

• Over the ribs

• Around the mammary glands

• Supraorbital fossae above the eyes

Cresty neck and regional fat deposits increase concern for insulin dysregulation.

3. Test Insulin Regulation

Ask your veterinarian whether the mare needs:

• Basal insulin

• Oral sugar test

• Oral glucose test

• Post-feeding insulin

• Laminitis radiographs if historical hoof damage is suspected

A normal-looking mare can still be insulin-dysregulated. A fat mare can also occasionally have acceptable testing. Test, do not guess.

4. Test for PPID Where Indicated

Consider PPID testing if the mare is older or has:

• Delayed shedding

• Long or abnormal coat

• Muscle loss

• Fat redistribution

• Increased drinking or urination

• Recurrent infections

• Laminitis

• Abnormal sweating

• Poor topline

Assessment for insulin dysregulation should also be pursued in PPID horses. (Squarespace)

5. Review Hoof Health

Before breeding, check:

• Digital pulses

• Hoof rings

• White line stretching

• Sole depth

• Previous rotation

• Current comfort

• Farrier schedule

• Radiographs if there is any concern

Pregnancy weight gain is not friendly to already compromised feet.

6. Build a Weight and Diet Plan

A metabolic mare usually needs:

• Tested low-NSC hay if insulin dysregulation is present

• Controlled pasture or no pasture

• No grain or sugary treats

• A suitable ration balancer

• Measured forage

• Gradual weight loss

• Exercise only when sound

• Regular body condition reassessment

Merck states that EMS management focuses on maintaining a normal body condition and using low-carbohydrate hay to help prevent laminitis. (Merck Veterinary Manual)

7. Complete a Reproductive Examination

Before semen is ordered, assess:

• Ovarian activity

• Uterine fluid

• Cervical function

• Vulvar conformation

• Culture and cytology where indicated

• Endometrial biopsy in older or repeat-breeder mares

• Previous breeding history

A mare should not be bred just because she eventually lost weight. The reproductive tract still needs to be ready.

8. Decide Whether She Should Carry the Pregnancy

For some mares, producing a foal through embryo transfer may be safer than carrying one.

This may be considered if the mare has:

• Valuable genetics

• Severe orthopedic disease

• Chronic hoof pain

• Poor uterine environment

• High pregnancy risk

• Need to remain in work

However, embryo transfer does not solve every issue. If obesity or insulin dysregulation affects oocyte or embryo quality, using a recipient mare may not fully remove the reproductive risk.

How Should an Obese Mare Be Prepared for Breeding?

Start Months Before the Season

Do not start weight loss when the mare is already about to be inseminated.

A sensible programme may take several months.

The aim is:

• Gradual fat loss

• Preserved muscle

• Controlled insulin

• Sound feet

• Stable routine

• A uterus ready for breeding

Do Not Crash Diet

Sudden severe restriction can create welfare problems, gastric risk and poor compliance.

Forage should be measured, not removed.

Use Exercise Carefully

Exercise improves insulin sensitivity and supports weight loss, but only if the mare is sound.

Do not exercise a footsore or laminitic mare for weight loss. That is not fitness. That is punishment wearing a lunge line.

Monitor During Pregnancy

Once pregnant, continue monitoring:

• Body condition

• Neck crest

• Digital pulses

• Hoof comfort

• Insulin where indicated

• ACTH in PPID mares

• Diet and pasture exposure

• Fetal and placental health

• Vaccination and parasite plan

• Foaling risk

Pregnancy is not the time to stop thinking about metabolism.

Common Mistakes Owners Make

Breeding Because the Mare Is Already Cycling

Cycling does not prove she is metabolically safe to breed.

Assuming Obesity Always Means Infertility

Obese mares may cycle and conceive. The main concern is metabolic and laminitis risk, not guaranteed failure to ovulate.

Ignoring Insulin Testing

Body condition does not diagnose EMS. Insulin testing is the decision-maker.

Blaming Every Failed Cycle on EMS

Poor semen timing, endometritis, older maiden mare issues and oocyte ageing may be the real problem.

Waiting Until Pregnancy to Manage Weight

Weight and insulin should be controlled before conception, not halfway through gestation.

Removing Too Much Feed Too Quickly

A fat mare still needs forage, protein, minerals and a safe routine.

Assuming Pergolide Is Simple in Pregnancy

PPID medication decisions in breeding and pregnant mares need veterinary planning because safety and efficacy data in breeding, pregnant and lactating animals are limited. (Squarespace)

Forgetting the Foal

The mare’s metabolic state and diet may influence the foal’s metabolic development, growth and possibly orthopedic health. (PMC)

Can These Problems Be Prevented?

Many can be reduced with earlier management.

Prevention starts long before the breeding season:

• Keep broodmares in moderate body condition year-round

• Body condition score monthly

• Monitor neck crest and regional fat

• Test insulin in easy keepers and laminitis-prone mares

• Test older mares for PPID when signs fit

• Avoid unrestricted lush pasture in high-risk horses

• Feed analysed forage when metabolic risk exists

• Use a ration balancer rather than grain where possible

• Maintain regular farriery

• Exercise mares appropriately while sound

• Investigate poor fertility instead of repeating cycles blindly

• Use culture and cytology before treating suspected uterine infection

• Plan pregnancy nutrition rather than simply “feeding for two”

The best broodmare plan is boring, measured and early.

Very annoying. Very effective.

Will the Mare and Foal Be Okay?

Many overweight mares can still become safe breeding candidates once weight, insulin and reproductive health are addressed.

The outlook is better when:

• Body condition improves before breeding

• Insulin is controlled

• No active laminitis is present

• Hooves are stable

• PPID is diagnosed and managed

• The uterus is healthy

• The mare is monitored during pregnancy

• Diet and pasture are controlled

• Foaling is supervised

The outlook becomes more guarded when:

• Laminitis is active or recurrent

• Insulin remains high

• PPID is uncontrolled

• The mare is severely obese

• Movement is already painful

• Uterine disease is present

• The mare is older with poor oocyte quality

• Pregnancy would worsen an existing welfare problem

The strongest decision is sometimes not breeding this season.

That is not failure. That is good veterinary judgement arriving before the invoice and the emergency.

FAQs About Breeding Obese Mares

Can an obese mare still get pregnant?

Yes. Obesity alone does not always prevent cycling or conception. The bigger concern is whether the mare has insulin dysregulation, laminitis risk, PPID or uterine disease.

What body condition score is best before breeding?

A moderate score is safest for most mares. Score 5 out of 9 is moderate, while score 6 is generally an acceptable upper limit for broodmares. Scores 7 and above increase concern for metabolic and laminitis risk. (MSD Veterinary Manual)

Should a mare with EMS be bred?

Not until EMS is controlled. A mare with EMS should have insulin managed, laminitis risk reduced, diet controlled and hoof health stabilised before breeding is considered.

Can a mare with PPID be bred?

Possibly, but she needs careful assessment. PPID should be controlled, insulin dysregulation should be tested for, and medication decisions need veterinary guidance because pergolide safety in breeding, pregnant and lactating mares has not been fully assessed. (Squarespace)

Does weight loss improve fertility?

It may improve metabolic safety and reduce laminitis risk, but fertility depends on more than body condition. The mare may still need reproductive ultrasound, culture, cytology, biopsy, semen review and accurate ovulation timing.

Final Thoughts

Breeding an obese mare is not automatically irresponsible, but breeding an obese mare without testing and preparation is asking biology to be generous.

Sometimes it will be.

Sometimes it will be spectacularly expensive.

The main decision points are:

• Is she truly obese or just a heavier broodmare type?

• Does she have insulin dysregulation?

• Has she ever had laminitis?

• Is PPID present and controlled?

• Is her uterus healthy?

• Is she sound enough to carry extra pregnancy weight?

• Can her diet be managed safely through pregnancy and lactation?

• Is breeding this year better than waiting?

The aim is not to make every broodmare skinny. Thin mares also have reproductive and welfare problems.

The aim is a metabolically stable mare in moderate condition, with healthy feet, a clean reproductive tract and a realistic pregnancy plan.

That is how you protect the mare, the foal and the breeding season.

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If you are considering breeding an overweight, EMS-positive or PPID mare, ASK A VET™ can help you organise her body condition, insulin results, hoof history and reproductive questions before you build the final plan with your local equine veterinarian or reproduction specialist.