A Breeding Soundness Exam (BSE) aims to evaluate your stallion’s overall health, reproductive anatomy, ability to perform in the breeding shed, and semen quality. This exam ensures your stallion is free of disease and defects that may hinder their breeding performance.

The results of a breeding soundness exam can help you make informed decisions about whether your stallion is fit to breed, as well as how many mares he can cover. Your veterinarian will help you interpret the results of your exam to verify that your stallion can achieve an acceptable conception rate.

Addressing issues early can prevent poor performance during the breeding season, and avoid exacerbating pre-existing health or reproductive issues.

While a BSE is an important tool to assess readiness, the most reliable estimate of a stallion’s fertility is the pregnancy rate he achieves when bred to sound mares in good breeding condition.

Read on to learn everything you need to know about the process, purpose, and benefits of a breeding soundness exam for stallions, as well as possible findings and their interpretation.

Breeding Soundness Exams for Stallions

Breeding soundness exams for stallions help breeders estimate their horse’s reproductive potential and diagnose or address poor reproductive performance. [1] These exams may also be included in a routine pre-purchase examination prior to sale of a high value stallion. [1]

A BSE includes an assessment of your stallion’s general health, sexual behavior and performance, reproductive organs, and semen characteristics. [1] From these factors, stallions are classified as satisfactory, questionable, or unsatisfactory breeding prospects. [1]

Satisfactory breeders are projected to achieve a minimum 75% success rate assuming they cover 40 mares by live cover or 120 mares by artificial insemination. [1]

When to Perform a Stallion BSE

Stallion BSEs are commonly used to evaluate horses in the following contexts: [3]

  • Pre-Purchase or after a recent purchase
  • Prior to the breeding season
  • In response to suspected fertility issues
  • In consideration of a young stallion’s breeding prospects

Timely conception is very important when it comes to running a viable breeding program. In a study of Thoroughbred farms in Kentucky, mares needed to produce live foals in six out of seven years for the operation to remain profitable. [2] Ensuring a stallion can reliably impregnate mares is essential for viable breeding operations.

Reproductive Anatomy of Stallions

Possessing basic knowledge of stallion reproductive anatomy is important for understanding and interpreting the results of a breeding soundness exam. Anatomical abnormalities can impact fertility and performance, and should be carefully considered when planning a breeding program.

A stallion’s reproductive anatomy includes both internal and external structures that contribute to breeding ability. Knowing what to expect in terms of size, shape, and characteristics of these structures makes it easier to understand your veterinarian’s assessment of anatomical soundness and how these findings can impact breeding performance.

Reproductive Tract of the StallionIllustration:

External Reproductive Anatomy

A stallion’s external anatomy includes the penis, prepuce, scrotum, testicles and epididymides. Together, these organs make up the bulk of a stallion’s reproductive anatomy.

Penis and Prepuce

A stallion’s penis has three parts including: [3]

  • The root: Also called the bulb, is the part of the penis closest to the stallion’s body
  • The body: Also called the shaft, is the main part of the penis
  • The glans: The enlarged part of the penis at the free end

The prepuce, also commonly called the sheath, is the double fold of skin that houses the penis when it is not erect.

Scrotum

The scrotum is the pouch of skin that contains the testicles. It plays an important role in maintaining a normal testicular temperature. [3]

Testicles and Epididymides

A stallion’s testicles are located inside the scrotum, and typically measure 8 – 12 cm long, 6 – 7 cm high, and around 5 cm wide. [3] Important components of the testicles include: [3]

  • Seminiferous tubules: where sperm cells are made
  • Leydig cells: the primary site of testosterone production

The epididymides are a pair of elongated, coiled tubes located at the back of the testicles. They are responsible for transporting, maturing, and storing sperm cells produced in the testes.

The epididymides consist of a head, body, and tail. The head is attached at the top, interior portion of the testicles, and the tail is located at the tail of the testicle. [3]

Internal Anatomy

The main internal reproductive organs of stallions are the accessory sex glands. These include: [3]

  • The vesicular glands
  • The prostate gland
  • The bulbourethral glands

These accessory sex glands contribute to the production of seminal fluid, also known as semen. [3] Seminal fluid serves as the medium for sperm transport and contains nutrients, enzymes, and other substances that support and protect sperm during their journey through the female reproductive tract.

7 Steps for a Stallion Breeding Soundness Exam

The typical breeding soundness exam for a stallion includes properly identifying the horse, collecting information on his breeding and health history, testing for venereal diseases, a general physical examination, external and internal reproductive exams, and semen collection and evaluation.

This comprehensive exam helps your veterinarian identify any underlying problems impacting your horse’s reproductive health or fertility. This information enables you and your veterinarian to make informed decisions about your stallion’s suitability for breeding.

1) Stallion Identification

The first step of a BSE is documenting some basic identification about your stallion. This step is important to facilitate proper alignment of the breeding soundness exam with your stallion in the future.

Identification includes standard information about the stallion such as their registered name, registration number, age and breed. [1] Other information collected includes:

  • Coat color
  • Markings
  • Lip tattoos
  • Microchip information
  • Brands
  • Scars

A photograph attached to the breeding soundness exam can also provide a quick and easy reference showing which horse it pertains to. [1]

2) Health and Breeding History

Next, your stallion’s health and breeding history have to be reviewed. It’s important to identify any past health concerns and assess if they might impact breeding performance.

Health History

Details such as pre-existing medical conditions, vaccination history (with special attention to the vaccine against Equine Arteritis Virus) and other health factors are reviewed. [1]

While the BSE also includes a routine physical examination to shed light on these issues, your veterinarian needs a complete health history to fully assess any risks to your stallion’s breeding performance.

Breeding History

Providing the results of any previous breeding soundness exams serves as a reference for prior diagnoses. [1]

Your stallion’s breeding history also includes the number of mares they have bred per season, and whether the mares were maidens, barren, or foaling. Provide any known information on conception and foaling rates, as they are good indications of your stallion’s past fertility. [1]

The type of breeding your stallion participates in is also a factor for consideration. Whether he is used for hand breeding, pasture breeding, or artificial insemination all provide valuable information for your BSE. If your stallion is used for artificial insemination, it’s also important to note whether the semen is administered as fresh, cooled, or frozen. [1]

Intended Use

Information such as the number of anticipated breedings for the upcoming season, as well as the method of breeding round out the picture so your veterinarian can advise you on best breeding management practices. [1] Following a BSE, your veterinarian can work with you to optimize breeding frequency based on results of the exam.

For example, your veterinarian may be able to estimate the number of mares a stallion can cover in a season. Research shows that reduced semen quality in stallions can occur in response to either too many or too few matings. Optimizing this schedule can lead to higher breeding success. [4]

3) Venereal Disease Testing

Stallions should be free of venereal diseases to be considered for breeding. These are diseases that spread between horses through the act of mating, and include: [1]

To test your stallion for venereal diseases, your veterinarian will collect swabs of the following areas prior to washing: [1]

  • Urethral fossa
  • Urethra
  • Penis shaft
  • Prepuce

Samples from the urethra are taken after arousal to facilitate collection of pre-ejaculatory seminal fluid. [1] A second sample is taken from the urethra after ejaculation to facilitate semen culture. Positive results from this sample indicate infection of the urethra, accessory sex glands, or epididymis. [1]

4) General Physical Examination

Your stallion’s body condition score is verified during a BSE to ensure they are in good condition. A body condition score of 4 or less on the 9-point Henneke scale is considered poor and may negatively impact semen quality. [3]

If your stallion’s body condition is low, your veterinarian may opt to conduct their BSE again after he has gained weight and is in adequate condition. This exam is typically conducted after your stallion has been in good condition for 60 days to ensure all the sperm present have matured since reaching ideal body condition. [3]

Health Conditions

BSEs include a routine physical exam to rule out significant medical issues such as fever or colic. If your stallion has experienced these issues recently, you may want to re-evaluate their breeding soundness with a follow-up exam before assigning their breeding prospects classification. [1]

As a part of the general wellness exam, stallions should also be tested for equine arteritis virus, equine infectious anemia, and contagious equine metritis to ensure they do not pass these diseases to mares. [1]

Breeding Performance & Conformation

To assess whether a stallion is physically capable of breeding, issues relating to vision, heart and lung function, and locomotion are evaluated. [5] Your veterinarian will also check to make sure your stallion is able to gain an erection, mount, penetrate, thrust, and ejaculate. [1]

This exam rules out the presence of musculoskeletal issues that may hinder breeding such as: [1][3]

Conformation faults or other conditions that cause pain can result in psychological stress and lead to unwillingness to breed. [3] Stallions should be addressed early to prevent issues during the breeding season.

Heritable Conditions

If heritable health problems are evident or identified on genetic tests, they should be considered when choosing whether or not to breed. [1] Heritable conditions are health issues or disorders that can be passed down from parent to offspring through their genetic material.

Heritable conditions that stallions can pass on to their offspring include, but are not limited to: [3]

Stallions with heritable conditions should not be used for breeding or should have their mating partners carefully chosen to decrease the risk of producing foals with these conditions.

5) External Reproductive Exam

A more focused examination of your stallion’s external reproductive organs including the penis, prepuce, sheath, scrotum, testicles, and epididymides is an essential component of a BSE.

Penis and Prepuce

Evaluation of the penis and prepuce is typically conducted when the penis is dropped, such as during washing, or after erection. [1][3] When the penis is flaccid, it is typically around 50 cm long and between 2.5 and 6 cm in diameter. [3]

During this part of the exam, your veterinarian checks to ensure the penis is a normal size and shape, and that there are no lesions or other defects present. [1][3]

Lesions on the penis can indicate past or present health issues such as: [5]

Testes and Epididymides

The testes and epididymides are often evaluated after semen collection, as stallions are typically calmer at that stage of the evaluation. This check is performed to confirm both testes are present and fully descended. [1]

To perform this part of the exam, the testes are palpated, examining for general characteristics such as: [1][3][5]

  • Presence of both testicles
  • Size
  • Symmetry
  • Position
  • Smoothness
  • Turgidity
  • Resilience on palpation
  • Free movement within the scrotum

The position of testicles can be an important factor for breeding soundness. If a testicle is rotated more than 180 degrees, it may indicate the possibility of more serious issues such as a twisted spermatic cord (spermatic cord torsion), which hinders blood flow to the testes. [5]

Your veterinarian also uses this check to make sure the epididymis can be palpated at the caudal (bottom) pole of each testicle. [1]

Total scrotal width is measured at the widest point of the testicles using calipers. This measure may range from 9 – 13 cm. Larger testes tend to have greater sperm output. [3]

It’s possible to roughly calculate the volume of the testicles by measuring the length, width, and height of the testicles using an ultrasound machine. Testicle size is measured to predict daily sperm production. [1]

It’s important to note that while testicle size has been shown to correlate to sperm output, it may not be a good indication of sperm quality. [6] Sperm quality should be assessed in addition to scrotal width or testicle volume when estimating the number of mares a stallion can cover.

Ultrasound examination of the testicles can also be helpful to identify defects such as lesions on the testes or fluid accumulation in the scrotum. [1]

6) Internal Reproductive Exam

Transrectal examination of internal reproductive components is not a routine part of breeding soundness examinations for stallions. However, if your stallion has a history of reproductive issues, has abnormal findings in their physical examination, or shows abnormal semen characteristics, your veterinarian may opt to perform an internal exam. [1]

Internal reproductive exams are typically conducted after semen collection, when stallions are calmer. [1] However, the accessory sex glands may be more palpable before ejaculation, so some practitioners may opt to palpate before collection. [3]

Although uncommon, transrectal ultrasound examination can also help to identify lesions located on internal reproductive organs. [5]

7) Semen Collection and Evaluation

Semen collection and evaluation to predict fertility is an important component of the breeding exam in stallions.

It’s important to note that semen characteristics fluctuate throughout the year, requiring careful interpretation of results. [7] Differences between fertile and sub-fertile stallions may be more pronounced during the breeding season. Collections should ideally occur during the breeding season to make informed decisions. [8]

Collecting Semen from Stallions

To ensure cleanliness, the stallion’s genitals should be washed with warm clean water following stimulation but prior to semen collection. [3]

Semen is collected with the aid of a mare in heat or a breeding phantom (dummy). An artificial vagina is used to capture the stallion’s ejaculate. [3]

The artificial vagina is filled with water to create the optimal temperature and pressure for the stallion and sperm. It is also lubricated with non-spermicidal lubricant to allow easy penetration. [3]

The stallion’s libido should be assessed during collection, and can be classified on a 0 – 4 scale, with a score of zero meaning severely limited libido and a score of four meaning a high libido. [9] Stallions with a good libido are efficient in the process of mating, while those with a low libido may take some time and convincing before covering mares.

Semen Characteristics

For a breeding soundness exam, semen is collected twice, roughly one hour apart. [1] When assessing the ejaculates, the number of sperm in the second ejaculate is typically half of the number in the first ejaculate.

However, several characteristics should remain similar between the two ejaculates, including: [1]

  • Volume of ejaculate
  • pH of the semen
  • Motility of the sperm

Semen Quality

Before evaluating semen quality, the gel fraction is collected and discarded, leaving the gel-free semen portion of the sample. [1]

After removal of the gel fraction, semen is transferred to a warm graduated cylinder where it is evaluated for color and consistency. Normal semen is typically white and opaque.

Abnormal color and consistency can indicate the presence of debris, blood, urine, or purulent discharge. [1] These observations may indicate contamination of the sample, infection, or other issues with the stallion or sample quality.

Semen pH

The pH level of semen is measured with a pH meter and should fall between 7.2 and 7.7. [1] A high pH (less acidic) may indicate incomplete ejaculation, inflammation, or contamination of the sample with soap or urine. [1][5]

Semen Volume

Gel-free semen volume is estimated using a graduated cylinder and reported in milliliters. [1][3] The volume of ejaculate may vary, but generally falls within 15 – 150 mL. [9]

Excessive teasing of a stallion can result in larger ejaculate volume. However, the total number of sperm does not change in these cases, leading to a lower sperm concentration. [1] Ejaculation volume may also be reduced in the winter compared to the summer. [9]

The volume of gel-free semen is ultimately used to calculate sperm concentration.

Sperm Concentration

The number of sperm per milliliter of ejaculate can be assessed through several different methods including: [1][3]

  • Manual counting using a hemocytometer
  • Spectrophotometer
  • Cell counter

The concentration of sperm is then considered together with total semen volume to calculate the total number of sperm in the ejaculate. The normal concentration range for stallions is 4 – 12 billion sperm per ejaculate. [9]

When accounting for quality, ejaculates should contain at least 1 billion progressively motile, morphologically normal sperm. [1]

Sperm Motility

Because semen is quite concentrated and lends to clumping of sperm cells, raw semen is often diluted in semen extender to evaluate motility. [1] Progressively motile sperm can then be identified as those that are swimming straight and have a normal morphology. [3]

The speed of sperm can be assessed on a scale from 0 – 4, with sperm that are not moving classified as 0, and sperm that are moving rapidly classified as 4. [9] Longevity of sperm motility is an indication of ejaculate quality and can be determined by evaluating motility over time. [1]

The percentage of progressively motile sperm is positively associated with pregnancy rates in mares. [10] There should be a minimum of 60% progressively motile sperm in a semen sample. [1][3]

Sperm Morphology

An eosin-nigrosin stain, also called a “live-dead” stain is commonly used to assess sperm morphology (shape). [1] Morphology should be assessed by a trained technician with the appropriate equipment to avoid misclassification. [11][12]

Common morphological issues of sperm include: [3]

  • Bent tails
  • No heads
  • No tails
  • Large heads
  • Double heads
  • Immature sperm cells

The number of sperm with these abnormal morphologies is recorded and used to calculate the percentage of normal sperm in the stallion’s semen sample. [3]

The number of morphologically normal sperm is positively correlated to pregnancy rates. [10][13] At least 60% of the sperm in a semen sample should be morphologically normal. [1]

Daily Sperm Output

Daily sperm output is not a component of every breeding soundness exam but can be useful for stallions that are expected to cover a large number of mares. This information can help determine the number of mares that the stallion can cover in a season. [1]

Daily sperm output can be estimated using the second ejaculate collected during a breeding soundness exam. However, a better approximation requires assessment after 5 – 7 days of daily collection. [1]

Collecting semen for five to seven days in a row will deplete the stallion’s sperm reserves, allowing for a more accurate estimation of the number of sperm produced per day. [1] This protocol gives fewer variable results when compared to two semen collections one hour apart on the same day. [14]

For stallions that are expected to cover a large number of mares, it may also be advantageous to assess semen quality along with daily sperm output, rather than during routine breeding soundness exams. [1]

Summary

A breeding soundness exam in stallions assesses factors such as conformation, breeding performance, and reproductive and general health to inform sound breeding decisions.

  • BSEs are performed by veterinarians and include a comprehensive health history, a general physical exam, venereal disease testing, an external reproductive exam, and semen collection and evaluation
  • The results of the BSE indicate if your stallion is fit to breed and inform breeding decisions, such as the number of mares he can cover in a season
  • If reproductive problems are identified during your stallion’s breeding soundness exam, your veterinarian can help you to address them with management or therapeutic interventions to treat underlying conditions

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References

  1. McCue, P.M. Chapter 97: Breeding Soundness Evaluation of the Stallion. Equine Reproductive Procedures. 2014.
  2. Bosh, K.A., et. al. Impact of reproductive efficiency over time and mare financial value on economic returns among Thoroughbred mares in central Kentucky. Equine Veterinary Journal. 2010. View Summary
  3. The Stallion: Breeding Soundness Examination and Reproductive Anatomy. National Cooperative Extension. 2020.
  4. Stout, T. A. E. Prospects for improving the efficiency of Thoroughbred breeding by individual tailoring of stallion mating frequency. Equine Veterinary Journal. 2012. View Summary
  5. Ball, B.A. Diagnostic Methods for Evaluation of Stallion Subfertility: A Review. Journal of Equine Veterinary Science. 2008.
  6. Blanchard, T.L., et. al. Evaluation of Testicular Size and Function In 1–3-Year-Old Stallions. Proceedings of the Annual Convention of the American Association of Equine Practitioners. 2001.
  7. Janett, F., et. al. Seasonal changes in semen quality and freezability in the Warmblood stallion. Theriogenology. 2003. View Summary
  8. Suliman, Y., et. al. Seasonal variations in quantitative and qualitative sperm characteristics in fertile and subfertile stallions. Archives Animal Breeding. 2020. View Summary
  9. Irfan, M., et. al. Effective use of Breeding Soundness Examination to Maximize Stallion Fertility and Progeny Quality. Biological Times. 2023.
  10. Blanchard, T.L., et. al. Relationships between stallion age, book size, number of matings (covers), breeding soundness examination findings, and fertility parameters in 15 Thoroughbred stallions (34 stallion years). Clinical Theriogenology. 2010.
  11. Brito, L.F.C., et. al. Effect of method and clinician on stallion sperm morphology evaluation. Theriogenology. 2011. View Summary
  12. Hernández-Avilés, C., et. al. A matter of agreement: The effect of the technique and evaluator on the analysis of morphologic defects in stallion sperm. Theriogenology. 2023. View Summary
  13. Parlevliet, J.M. and B. Colenbrander. Prediction of first season stallion fertility of 3-year-old Dutch Warmbloods with prebreeding assessment of percentage of morphologically normal live sperm. Equine Veterinary Journal. 1999. View Summary
  14. Köhne, M., et. al. Semen quality evaluation in young stallions – feasibility and comparison of two different protocols. Pferdeheilkunde – Equine Medicine. 2020.