Equine Reproduction and Infertility: Understanding Causes in Mares and Stallions

Introduction

Successful reproduction in horses depends on far more than normal breeding behavior. Both mares and stallions can appear outwardly healthy yet struggle with infertility due to subtle, underlying medical conditions. Because breeding is time-sensitive and emotionally and financially significant, understanding reproductive health is essential for making informed decisions and setting realistic expectations.

Infertility is not a single disease—it is a multifactorial problem involving anatomy, hormones, infection, age, management, and overall health.

How Normal Equine Reproduction Works (Brief Overview)

In the Mare

• Ovulation occurs toward the end of estrus (heat)

• The uterus must clear inflammation and fluid quickly after breeding

• A healthy uterine environment is essential for embryo survival

In the Stallion

• Sperm production occurs continuously in the testes

• Proper temperature regulation is critical for sperm quality

• Normal libido does not always equal normal fertility

Disruption at any step can reduce the chance of conception.

Common Causes of Infertility in Mares

Endometritis

What it is: Inflammation or infection of the uterine lining.

Why it matters:

• Creates a hostile environment for sperm and embryos

• Can be acute, chronic, or subclinical

Risk Factors:

• Older age

• Poor uterine clearance

• Previous foaling complications

Delayed Uterine Clearance

What it is: Failure to remove post-breeding inflammation, fluid, or debris.

Common in:

• Older mares

• Mares with poor uterine tone or conformation issues

Ovarian Disorders

Examples:

• Granulosa cell tumors

• Hormonal imbalances

Possible Signs:

• Irregular or absent cycles

• Stallion-like behavior

Cervical and Conformational Abnormalities

Poor vulvar or cervical conformation can allow air or contaminants into the reproductive tract, increasing infection risk.

Common Causes of Infertility in Stallions

Testicular Degeneration

What it is: Loss of normal sperm-producing tissue.

Contributing Factors:

• Age

• Illness or fever

• Trauma

Thermal Stress

Why it matters: Sperm production is highly sensitive to heat.

Sources of heat stress:

• Fever

• Excessive insulation

• Environmental heat

Infection or Inflammation

• Seminal vesiculitis

• Orchitis or epididymitis

These conditions can reduce sperm quality and motility.

Hormonal Imbalances

Endocrine disorders may affect libido, sperm production, or ejaculation.

Subclinical Infertility: When Nothing Looks Wrong

One of the most frustrating aspects of equine reproduction is that many infertile horses show no outward signs of illness.

Examples include:

• Mares cycling normally but failing to conceive

• Stallions with normal libido but poor semen quality

This is why veterinary reproductive evaluation is essential.

Diagnosis: Finding the Limiting Factor

Veterinarians may recommend:

• Reproductive ultrasound

• Uterine culture and cytology (mares)

• Semen evaluation (stallions)

• Hormonal testing

• Endometrial biopsy (mares)

Identifying the specific cause allows targeted treatment rather than trial-and-error breeding.

Treatment and Management

Medical Treatment

• Antibiotics or uterine therapy when indicated

• Hormonal support

• Anti-inflammatory strategies

Management Adjustments

• Optimizing breeding timing

• Post-breeding uterine care in mares

• Temperature and stress management in stallions

Age and Expectations

Fertility naturally declines with age. Understanding realistic expectations helps guide decisions about:

• Breeding frequency

• Assisted reproductive techniques

• Long-term breeding plans

When to Call Your Veterinarian

• Failure to conceive after multiple cycles

• Irregular or absent estrous cycles

• Changes in breeding behavior

• Known history of reproductive disease

Early evaluation saves time, expense, and frustration.