Equine Breeding Soundness Examination of the Stallion

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• The Equine Breeding Soundness Examination (BSE) is a multi-faceted assessment integrating historical data (per-cycle pregnancy rates, foaling rates), physical examination (conformation, lameness), reproductive tract evaluation, libido testing, and semen collection with laboratory analysis.
• Routine semen analysis evaluates gel-free volume, sperm concentration, total motility, progressive motility, morphology, and viability, with minimum standards provided by the Society for Theriogenology serving as reference thresholds for classifying a stallion as a satisfactory breeder.
• The BSE is a screening tool with limited predictive value for subsequent fertility; it reliably identifies stallions with obvious subfertility but may fail to detect individuals with functional sperm defects like impaired acrosomal exocytosis or sperm-bound antisperm antibodies.
• Advanced testing, including acrosome reaction assays (e.g., using calcium ionophore) and flow cytometric detection of antisperm antibodies (IgG, IgA), is indicated for stallions with a history of unexplained subfertility despite normal conventional semen parameters.
• Common errors in BSE technique include reliance on a single ejaculate, failure to distinguish primary from secondary spermatozoal abnormalities, inaccurate morphology assessment, and neglecting to integrate historical and physical findings with semen data.
• Classification as satisfactory, questionable, or unsatisfactory breeder requires a holistic interpretation of all findings, acknowledging that fertility is influenced by mare factors, management, and breeding technique, and the BSE represents a snapshot in time.

The breeding soundness examination (BSE) of the stallion is a structured clinical assessment that integrates history, physical examination, reproductive tract evaluation, libido and mating ability testing, and semen collection with laboratory analysis. This article provides a practical procedural guide for the practicing veterinarian performing stallion BSEs, with emphasis on collection technique, semen evaluation methodology, and interpretation of results against published standards. The examination serves two distinct purposes: identifying stallions with the potential for satisfactory fertility and diagnosing specific causes of subfertility in stallions with a known breeding history. The Society for Theriogenology publishes professional resources and guidelines that inform current BSE practice Society for Theriogenology resources.

A complete BSE should include historical data on the stallion's medical and breeding career, observation of breeding behavior, collection and evaluation of semen, and testing for infectious or contagious disease Evaluation of the stallion for breeding soundness. The examiner must recognize that the routine BSE has limited predictive value for subsequent fertility. It reliably identifies stallions with obvious reasons for subfertility but fails to detect some seriously subfertile animals, particularly those with normal conventional semen parameters Capacitation, acrosome function and chromatin structure in stallion sperm. This limitation shapes the clinical approach: the BSE is a screening tool, and stallions with unexplained subfertility require advanced diagnostic testing beyond the standard examination.

At a Glance

Historical Assessment

The breeding history is the single most informative component of the BSE. Obtain records of mares bred per season, cycles per pregnancy, per-cycle pregnancy rates, and foaling rates. A stallion with a per-cycle pregnancy rate below 30 to 40 percent across a reasonable number of cycles warrants investigation even if conventional semen parameters appear satisfactory. Document the number of seasons the stallion has bred, the age at which breeding began, and any changes in fertility over time. Also record management factors such as breeding frequency, whether semen is used fresh, cooled, or frozen, and the fertility of the mare population, since mare factors substantially influence apparent stallion fertility Capacitation, acrosome function and chromatin structure in stallion sperm.

Medical history should include prior illness, injury, or surgery involving the reproductive tract, episodes of colic or trauma, and any medications administered during breeding seasons. Castration status must be confirmed, as cryptorchid or hemicastrated animals may present with ambiguous histories. Ask specifically about previous BSE results, semen quality trends across seasons, and any prior diagnosis of testicular degeneration, ejaculatory dysfunction, or venereal disease.

Physical Examination

Perform a complete physical examination with attention to body condition, musculoskeletal soundness, and cardiovascular fitness. Stallions with hindlimb lameness, back pain, or neurologic deficits may be unable to mount, achieve intromission, or thrust effectively. Examine the eyes and confirm normal vision, since visual impairment can reduce libido or breeding efficiency.

The reproductive examination proceeds systematically. Inspect the penis and prepuce for lesions, including squamous cell carcinoma, habronemiasis, trauma, or persistent frenulum. Palpate the testes and epididymides for size, symmetry, consistency, and the presence of masses. Normal testes are firm but slightly compressible. Soft or small testes suggest degeneration or hypoplasia. Measure scrotal width with calipers at the widest point, and record individual testicular length, width, and height. Total scrotal width in mature stallions typically ranges from 8 to 12 cm, and values below 8 cm warrant concern. Testicular size correlates with daily sperm output and therefore with the number of mares a stallion can service per season.

Palpate the spermatic cords and inguinal rings to detect hernia, varicocele, or other abnormalities. Rectal palpation of the internal genitalia, including the ampullae, vesicular glands, and prostate, is indicated when ejaculatory dysfunction or hematospermia is suspected. Ultrasonography of the testes and accessory glands provides additional diagnostic information when palpable abnormalities are detected.

Libido and Mating Ability

Assess libido by exposing the stallion to a teaser mare in estrus or to a mount dummy. Record the time from exposure to erection, the time from erection to mounting, and the time from mounting to ejaculation. Normal stallions typically achieve ejaculation within 5 to 10 minutes of exposure. Failure to achieve erection, mounting, or ejaculation within a reasonable period may indicate psychologic inhibition, pain, or neurologic dysfunction. Distinguish between lack of libido and inability to copulate. A stallion with normal libido but an inability to achieve intromission may have penile or preputial pathology, while a stallion with poor libido may require behavioral modification or endocrine evaluation.

Observe the ejaculatory response, including the characteriztic tail flag and urethral pulsations. Confirm that ejaculation has occurred by examining the collection vessel for semen. Some stallions mount and thrust without ejaculating, and this must be recognized as ejaculatory dysfunction instead of normal behavior.

Semen Collection

Semen collection should follow a complete physical examination and assessment of libido. A breeding phantom or a mare in estrus may serve as the mount. The choice depends on stallion experience and temperament. A phantom is preferred for routine collections because it reduces the risk of injury to both stallion and handler and allows more consistent collection conditions.

An artificial vagina (AV) is the standard collection device. Several models exist, including the Missouri, Colorado, and CSU designs. The AV should be prepared with a clean liner, a non-spermicidal lubricant, and water at a temperature between 45 and 50 °C at the time of filling. The internal temperature at collection should approximate 42 to 44 °C. Stallions vary in preference, and the operator should adjust temperature and pressure within safe limits to achieve a normal ejaculatory response.

A breeding phantom should be sturdy and placed on non-slip footing. The stallion is led to the phantom, and the penis is washed with warm water and dried before collection. Soap or disinfectant residues can contaminate the ejaculate and should be avoided. The AV is held at the side of the phantom, angled downward approximately 30 to 45 degrees, and the penis is directed into the open end as the stallion mounts. The ejaculate is collected into a receptacle attached to the AV, which is often fitted with a filter to remove the gel fraction and debris.

A nose twitch or sedation is occasionally needed for stallions that are difficult to handle. Sedation should be used with caution because it can alter ejaculatory quality and libido. If sedation is required, the drugs used and the dose should be recorded in the examination report.

The collection area should be quiet and free of distractions. The same handler and routine should be used for repeated collections to reduce variability. Two ejaculates collected approximately one hour apart are recommended for a complete evaluation, because the second ejaculate often provides a more representative sample of daily sperm output in stallions with sperm-rich extragonadal reserves.

Semen Evaluation

Semen should be evaluated immediately after collection. The gel fraction is removed and the gel-free volume is recorded. The sample is kept at approximately 37 °C during evaluation to preserve motility. A calibrated spectrophotometer or a hemocytometer is used to determine sperm concentration. Total sperm number is calculated as concentration multiplied by gel-free volume.

Motility is assessed by light microscopy using a warmed stage. A small drop of semen is placed on a slide, covered with a coverslip, and examined at 200 to 400x magnification. Total motility and progressive motility are estimated subjectively, or measured objectively with computer-assisted sperm analysis (CASA) if available. CASA provides more repeatable measurements but requires careful calibration and standardization of settings.

Morphology is evaluated on a fixed, stained smear. Eosin-nigrosin stain is commonly used because it also allows assessment of viability. At least 100 spermatozoa should be classified under oil immersion. Sperm are categorized as normal or abnormal, and abnormalities are further classified by region: head, midpiece, and tail. The percentage of morphologically normal sperm is a key criterion in the breeding soundness examination.

The Society for Theriogenology has published minimum standards for stallion semen quality, and these are widely used as reference thresholds in clinical practice Society for Theriogenology resources. A stallion meeting these thresholds is classified as a satisfactory breeder. The following table summarizes the reference ranges commonly applied in a standard breeding soundness examination.

These thresholds are guidelines, not absolute predictors of fertility. A stallion that falls below one or more thresholds may still be fertile, and a stallion that meets all thresholds may still be subfertile. The breeding soundness examination is most useful for identifying stallions with clear reasons for subfertility, and it has limited power to predict subsequent fertility in the field Capacitation, acrosome function and chromatin structure in stallion sperm.

Interpretation of Semen Findings

Interpretation requires integration of semen data with the history, physical examination, and libido assessment. A single poor ejaculate does not justify a diagnosis of infertility. Repeat collection is indicated when the first ejaculate is abnormal, because collection technique, sexual preparation, and environmental factors can all affect semen quality.

Low total sperm number is the most common abnormality in stallions presented for breeding soundness examination. Causes include small testicular size, advanced age, recent illness or fever, exogenous steroid administration, and incomplete ejaculation. Testicular dimensions should be measured with calipers and compared with the total sperm output. A stallion with small testes and low sperm output has a poor prognosis for improvement.

Poor motility can result from cold shock, contamination of the AV with water or lubricant, or prolonged interval between collection and evaluation. If motility is low, the sample should be re-evaluated after a short incubation at 37 °C. Persistent low motility in a properly collected sample warrants further investigation, including assessment of sperm viability and morphology.

High percentages of morphologically abnormal sperm may indicate testicular dysfunction, epididymal disease, or recent thermal insult. The specific abnormality pattern can provide clues. For example, a high proportion of proximal droplets suggests incomplete maturation, often associated with frequent ejaculation or epididymal dysfunction. A high proportion of detached heads may indicate sample handling trauma or testicular degeneration.

Advanced Semen Testing

Standard semen evaluation does not identify all causes of subfertility. Some stallions with normal motility, morphology, and concentration have severely reduced fertility. Acrosomal dysfunction is one recognized cause. In impaired acrosomal exocytosis, sperm fail to undergo the acrosome reaction required for fertilization, despite otherwise normal semen quality The role of impaired acrosomal exocytosis (IAE) in stallion subfertility. This condition has been confirmed in Thoroughbred stallions and is associated with a susceptibility genotype in the FKBP6 gene Severe subfertility in Thoroughbred stallions positive for the Impaired Acrosome Reaction susceptible gene (FKBP6) in Argentina.

The acrosome reaction test is performed by incubating sperm with a calcium ionophore and assessing the percentage of sperm that undergo acrosomal exocytosis. A reduced response compared with a fertile control stallion supports a diagnosis of impaired acrosomal exocytosis. This test is not part of a routine breeding soundness examination but should be considered when a stallion has a history of unexplained subfertility and normal conventional semen parameters.

Antisperm antibodies have also been associated with poor semen quality in stallions. Flow cytometric detection of IgG and IgA bound to sperm has shown that IgA binding is higher in non-satisfactory breeders, and IgG-positive stallions have lower motility, concentration, and total sperm numbers Equine sperm-bound antisperm antibodies are associated with poor semen quality. Testing for antisperm antibodies is not routine but may be useful in stallions with unexplained poor semen quality or a history of genital tract inflammation.

Documentation and Classification

The breeding soundness examination should be documented in a structured format that includes the history, physical examination findings, libido assessment, collection details, semen parameters, and any advanced testing performed. The report should state the classification of the stallion as a satisfactory, questionable, or unsatisfactory breeder, with the basis for that classification clearly explained.

The Society for Theriogenology classification system is widely used in North America. A satisfactory breeder meets the minimum thresholds for semen quality, has normal genital anatomy, and demonstrates adequate libido and mating ability. A questionable breeder has marginal semen quality or minor physical abnormalities that may affect fertility. An unsatisfactory breeder has clear abnormalities that are expected to impair fertility.

The classification should be accompanied by a narrative interpretation. The referring veterinarian or owner needs to understand that the breeding soundness examination is a snapshot of the stallion's reproductive function at one point in time. Fertility in the field is influenced by mare fertility, management, and breeding technique, and the examination cannot guarantee pregnancy rates Evaluation of the stallion for breeding soundness.

Recognized Complications and Failure Modes

The most consequential failure mode in stallion BSE is the false reassurance provided by a satisfactory classification. A stallion may pass conventional semen evaluation yet remain severely subfertile because standard assessment does not interrogate fertilising capacity at the molecular level. Impaired acrosomal exocytosis, a condition in which sperm fail to undergo the acrosome reaction required for fertilisation, has been documented in stallions with otherwise normal motility, morphology, viability, and DNA quality. Retrospective analysis of 1,128 stallions evaluated at a teaching hospital identified 21 with this condition, all of which had subfertility unexplained by conventional BSE findings. The diagnosis requires specialised testing, either flow cytometric assessment of acrosomal exocytosis after calcium ionophore challenge or genetic testing for the FKBP6 susceptibility genotype. The clinical hallmark is a history of persistently low pregnancy rates despite satisfactory semen parameters, and the condition has been confirmed as a cause of subfertility in Thoroughbred stallions. Detection is only possible when the examiner maintains a low threshold for advanced testing in the face of a history that contradicts the conventional findings.

Sperm-bound antisperm antibodies represent a second failure mode that escapes routine detection. Flow cytometric evaluation of 21 stallions demonstrated that IgA-bound spermatozoa were more prevalent in non-satisfactory than satisfactory breeders, and IgG-positive stallions had significantly lower total sperm motility, concentration, and total numbers. Importantly, IgA binding was higher in the non-breeding season, so a single sample collected outside the breeding season may overestimate the severity of antibody-mediated impairment. Routine light microscopy does not detect antibody binding, and the examiner must request flow cytometry specifically when poor semen quality coexists with otherwise unexplained subfertility.

Collection-related complications include ejaculatory failure, incomplete ejaculation, and contamination of the ejaculate with urine or extender. Incomplete ejaculation is detected by comparing the gel-free volume and sperm concentration against the expected values for the individual stallion, and by observing the tail-end of the ejaculate for the characteriztic sperm-rich fraction. Urine contamination is suggested by a yellow discolouration, a pungent odour, and the presence of uric acid crystals on microscopic examination. Extender contamination occurs when the artificial vagina is not rinsed adequately between collections or when lubricant enters the collection cone.

Common Errors in Examination Technique

The most frequent error is the single-ejaculate evaluation. Stallion semen quality varies with ejaculation frequency, season, and sexual preparation, and a single collection may misclassify a stallion as unsatisfactory when he is simply under-collected. Two ejaculates collected one hour apart, after a period of sexual stimulation, provide a more reliable basis for classification. A second common error is the failure to distinguish primary from secondary spermatozoal abnormalities. The examiner must categorise defects by their site of origin, because testicular and epididymal pathology carries a different prognosis than abnormalities arising from collection technique or prolonged sexual rest.

Inexperienced examiners frequently overestimate sperm morphology by evaluating too few cells or by failing to use a proper differential interference contrast preparation. A minimum of 100 to 200 spermatozoa should be assessed per sample, and the examiner should be aware that phase-contrast microscopy can obscure subtle acrosomal defects. The use of an inappropriate stain, or no stain at all, further compromises accuracy. Corrective action includes standardizing the staining protocol, calibrating the counting chamber, and seeking a second opinion when morphology findings are borderline.

A third error is the failure to integrate the historical data with the physical findings. A stallion with a satisfactory semen evaluation but a history of mounting difficulties, poor libido, or musculoskeletal pain is not a satisfactory breeder, regardless of his sperm output. The BSE is a whole-animal assessment, and the examiner who focuses exclusively on the ejaculate will miss the most common reasons for breeding failure. The Society for Theriogenology provides professional resources that emphasize the integration of history, physical examination, and semen evaluation in the final classification.

Limitations of the Evidence Base

The routine BSE has limited predictive value for subsequent fertility. Because stallions are selected primarily on pedigree, conformation, and athletic performance, fertility is often a secondary consideration, and the average stallion breeds relatively few mares per year. Meaningful fertility data are therefore difficult to obtain, and the BSE generally succeeds only in identifying stallions with obvious reasons for subfertility. It fails to identify some seriously subfertile animals, particularly those with functional sperm defects that do not alter conventional parameters.

Expert opinion still differs on the clinical significance of antisperm antibodies. The association between IgA binding and unsatisfactory breeder classification is documented, but the pathogenic mechanism, the threshold for clinically significant binding, and the response to treatment remain uncertain. Similarly, the prevalence of impaired acrosomal exocytosis outside the Thoroughbred breed is unknown, and the genetic basis has been confirmed only in that breed. The examiner should interpret a negative genetic test as informative but not definitive, and should recognize that the acrosome reaction test requires careful laboratory standardization.

Regulatory requirements for infectious disease testing vary by jurisdiction and by the intended use of the stallion. The World Organization for Animal Health publishes international standards for animal health and trade, and the examiner should consult current national and regional requirements before certifying a stallion for breeding or for movement across borders. These requirements change, and the examiner has a professional obligation to verify the current standards at the time of examination.

Referral and Escalation

Referral for specialist evaluation is warranted when a stallion has a history of unexplained subfertility despite satisfactory conventional findings, when advanced testing is required, or when the examiner lacks the laboratory capacity for flow cytometry or genetic analysis. Specialist centers can perform the acrosome reaction test, antisperm antibody assays, and FKBP6 genotyping, and can provide a second opinion on difficult morphology cases. Referral is also appropriate when the stallion has a suspected testicular tumor, a scrotal hernia, or a behavioral problem that does not respond to standard management.

Regulatory reporting is required when a stallion is diagnosed with a notifiable infectious disease, when a zoonotic agent is identified, or when the examination reveals a condition that affects the safety of handlers or other animals. The examiner should be familiar with the reporting obligations in their jurisdiction and should document all findings in a manner that supports subsequent review. The American Veterinary Medical Association provides practice resources that address professional obligations in diagnostic and reporting contexts.

Frequently Asked Questions

How Should I Proceed When a Stallion Refuses to Mount or Ejaculate During the Examination?

A failure to mount or ejaculate does not automatically classify a stallion as an unsatisfactory breeder. Distinguish between lack of libido and inability to ejaculate. Reassess the environment, handler experience, and collection technique before concluding a physical or behavioral problem exists. A quiet, consistent collection area with a reliable teaser mare often resolves first-attempt failures. If ejaculation still fails, consider a second collection after a short rest or on a different day. Pharmacologic assistance may be appropriate in selected cases, but current formularies must be consulted for product choice and dosing. Document the circumstances of every failed attempt, as this information informs the final classification and any referral recommendation.

What Minimum Equipment Is Required to Perform a Defensible Breeding Soundness Examination?

A defensible examination requires an artificial vagina appropriate for stallion anatomy, a collection dummy or live mount mare, a graduated cylinder or scale for volume measurement, a spectrophotometer or hemocytometer for concentration, a microscope with phase-contrast optics for motility and morphology, and a warm stage or water bath to maintain sample temperature. When a spectrophotometer is unavailable, a hemocytometer provides an acceptable alternative. A microscope with only bright-field optics limits accurate assessment of subtle sperm defects, so referral for advanced evaluation is appropriate when such defects are suspected. The Society for Theriogenology publishes professional resources on examination technique and equipment standards that can guide practice setup.

How Do I Interpret a Breeding Soundness Examination in a Stallion With a History of Subfertility but Normal Routine Semen Parameters?

A stallion with normal motility, morphology, and concentration but persistently low pregnancy rates requires investigation beyond the routine examination. Impaired acrosomal exocytosis is a recognized cause of unexplained subfertility in stallions with otherwise satisfactory semen quality, and the condition has been documented in Thoroughbred stallions carrying a susceptibility genotype in the FKBP6 gene. The acrosome reaction test, using calcium ionophore stimulation, can identify affected individuals. Flow cytometric assessment of sperm-bound antibodies may also be warranted, as antisperm antibody binding has been associated with poor semen quality in some stallions. Referral to a specialised laboratory is recommended when this pattern of unexplained subfertility is encountered, because the routine examination has limited predictive value for identifying these cases.

What Records Should I Maintain From a Stallion Breeding Soundness Examination?

Maintain a complete record that includes the signalment, historical breeding data, physical examination findings, collection conditions, and all raw semen evaluation data. Record the number of mounts, collection time, ejaculate volume, concentration, total sperm number, progressive motility, and morphological classification with the specific defects observed. Include the results of any advanced tests, such as acrosome reaction assessment or DNA integrity assays. Document the classification assigned and the rationale for that classification. These records support future comparisons, medicolegal defense, and communication with referring veterinarians. The AVMA practice resources provide general guidance on medical record content and retention that applies to reproductive evaluations.

How Should I Communicate an Unsatisfactory Classification to the Owner or Trainer?

Present the findings objectively, focusing on the specific parameters that fell below accepted thresholds instead of a general statement about fertility. Explain that the classification reflects the stallion's potential for fertility at the time of examination, not a guarantee of sterility or a prediction of certain pregnancy rates. Discuss the possibility of a second examination after a management change or a period of sexual rest, since a single unsatisfactory evaluation does not permanently condemn a stallion. Provide a written summary that includes the raw data and the classification criteria used. If referral for advanced testing is indicated, explain what additional information those tests could provide and how that information might alter the clinical picture.

How Does the Stallion Breeding Soundness Examination Differ From the Bull Examination?

The fundamental goals are similar, but the practical approach differs in several respects. Stallion semen is typically collected with an artificial vagina and a mount mare or dummy, whereas bull collection often uses electroejaculation or an artificial vagina. Stallion ejaculate volume is larger and more variable than the bull's, and gel fraction removal is a routine step in stallion evaluation. The Society for Theriogenology has published specific classification criteria for bulls, but stallion classification relies more heavily on historical fertility data and clinical judgment because the routine breeding soundness examination has limited use as a predictor of subsequent fertility. Stallion examinations also place greater emphasis on breeding behavior and mating ability, since natural cover is the predominant breeding method in many equine operations.

Related Clinical & Scientific Guides

• Diagnostic Approach to Canine Infertility in the Bitch
• Canine Neonatal Resuscitation: Protocol and Monitoring
• Pregnancy Diagnosis in the Bitch: Methods and Interpretation

References and Further Reading

• Severe subfertility in Thoroughbred stallions positive for the Impaired Acrosome Reaction susceptible gene (FKBP6) in Argentina. 2024.
• Equine sperm-bound antisperm antibodies are associated with poor semen quality.. 2018.
• Evaluation of the stallion for breeding soundness.. 1992.
• The role of impaired acrosomal exocytosis (IAE) in stallion subfertility: A retrospective analysis of the clinical condition, and an update on its diagnosis by high throughput technologies.. 2022.
• Autologous osteochondral grafting (mosaic arthroplasty) for treatment of subchondral cystic lesions in the equine stifle and fetlock joints.. 2004.
• Capacitation, acrosome function and chromatin structure in stallion sperm.. 2005.
• Society for Theriogenology Resources. Society for Theriogenology.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.
• American Veterinary Medical Association Practice Resources. American Veterinary Medical Association.

Related Articles

• Equine Breeding Soundness Examination: Mare Evaluation
• Breeding Soundness Examination of the Canine Male
• Breeding Soundness Examination of Bulls: Standards and Interpretation
• Equine Brucellosis: Diagnosis and Management
• Equine Pregnancy Diagnosis: Methods and Clinical Application

This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.