Equine Reproductive Physiology and Breeding Soundness Examination

At a Glance

Reproductive Physiology of the Mare

Estrous Cycle Endocrinology and Phases

The mare is a seasonally polyestrous breeder, with reproductive activity governed by photoperiod and hypothalamic neuropeptide signaling. Gonadotropin-releasing hormone (GnRH) neurons in the arcuate nucleus of the hypothalamus drive the reproductive axis, with kisspeptin neurons playing a modulatory role. Research characterizing the relationship between GnRH, kisspeptin, and RFamide-related peptide 3 (RFRP-3) neurons in the equine hypothalamus across the estrous cycle and anovulatory seasons found that GnRH cells are observed primarily in the arcuate nucleus, with very few labeled cells identified in the pre-optic area. Kisspeptin cells are identified primarily in the arcuate nucleus, with a small number of cells observed dorsal to the arcuate nucleus surrounding the third ventricle. The mean number of kisspeptin cells varies between animals and typically shows no pattern associated with season or stage of estrous cycle, but a seasonal difference is identified in the arcuate nucleus population. Small numbers of RFRP-3 cells are observed in the arcuate nucleus, ventromedial hypothalamus, and dorsomedial hypothalamus. The mean number of RFRP-3 cells appears higher in pre-ovulatory animals compared to all other stages. These interactions suggest the possibility of an ultra-short loop feedback system between these three peptides, and changes in RFRP-3 neurons suggest a possible role in the regulation of reproduction in the horse [14].

The estrous cycle averages 21 to 22 days. Estrus (sexual receptivity) lasts 5 to 7 days, during which the mare will stand for the stallion. Ovulation occurs approximately 24 to 48 hours before the end of estrus. Diestrus (luteal phase) lasts 14 to 16 days, characterized by progesterone dominance and behavioral non-receptivity. The transition between breeding and non-breeding seasons involves altered hypothalamic signaling, with mares entering anestrus during short day lengths.

Follicular Development and Ovulation

Follicular waves occur throughout the estrous cycle, with the dominant follicle emerging during late diestrus and growing rapidly during estrus. Transrectal ultrasonography allows visualization of follicular size, number, and echogenicity. A preovulatory follicle typically reaches 35 to 45 mm in diameter. Ovulation is characterized by collapse of the follicle, appearance of a corpus hemorrhagicum, and subsequent formation of the corpus luteum.

The phase of the estrous cycle at the time of ovum pick-up (OPU) affects oocyte recovery and embryo production outcomes. A retrospective study examining factors contributing to the success of equine in vitro embryo production by intracytoplasmic sperm injection (ICSI) found that most parameters evaluated were superior in follicular phase mares, with or without a dominant follicle, compared to luteal phase mares [12].

Luteal Function and Progesterone

The corpus luteum produces progesterone, which maintains pregnancy and suppresses estrus. Progesterone concentrations rise within 24 to 48 hours after ovulation and remain elevated throughout diestrus. If pregnancy does not occur, prostaglandin F2alpha from the endometrium causes luteolysis around day 14 to 16 post-ovulation, allowing a new follicular wave to emerge.

Endometrial Physiology and Pathology

The equine endometrium undergoes cyclic changes in response to steroid hormones. Lysophosphatidic acid (LPA) and its receptors (LPAR1-4) are present in the equine endometrium and may play a role in endometrial function. Research investigating LPA as a regulator of endometrial connective tissue growth factor and prostaglandin secretion during the estrous cycle and endometrosis found that endometrial LPA concentration is higher in the mid-luteal phase compared to the follicular phase in category I (normal) endometrium. There is an alteration in endometrial concentrations of LPA and LPAR1-4 protein abundance in the follicular phase at different stages of endometrosis. Additionally, LPA increases the secretion of PGE2 from category I endometrium in both phases of the estrous cycle. The effect of LPA on the secretion of connective tissue growth factor and PGF2alpha from endometrial tissue is altered depending on different stages of endometrosis [13].

Endometrosis is a chronic degenerative condition described as endometrial fibrosis that forms in the stroma, under the basement membrane, and around the endometrial glands. This condition can impair fertility by disrupting the endometrial environment necessary for embryo survival and placentation.

Vaginal Microbiota

The normal bacterial microbiota of the equine vagina changes throughout the estrous cycle. A study examining bacteria in the healthy equine vagina during the estrous cycle sampled the cranial portion of the vagina on day 0 (ovulation), day 3, day 7, and day 14. Bacterial growth was highest on day 3 and 7, representing the beginning and middle of diestrus. The dominant bacteria were Escherichia coli and Streptococcus zooepidemicus. Escherichia coli was especially dominant in maiden mares compared to mares that had foaled. An increase in bacterial diversity throughout the estrous cycle was observed, being highest on day 14. These results suggest that there are changes in the bacterial microbiota of the mare vagina throughout the normal estrous cycle [15].

Understanding the normal vaginal microbiota provides the background to conditions and interventions that might cause the microbiota to change, such as introduction of semen, treatment with antibiotics, or discharge from an unhealthy uterus.

Reproductive Physiology of the Stallion

Spermatogenesis and Endocrine Control

Spermatogenesis in the stallion occurs within the seminiferous tubules of the testes and takes approximately 57 days. The process is regulated by follicle-stimulating hormone (FSH), luteinizing hormone (LH), and testosterone. The hypothalamic-pituitary-gonadal axis maintains continuous reproductive function in the stallion, unlike the seasonal cyclicity of the mare, although some seasonal variation in semen quality occurs.

Testicular Anatomy and Function

The testes produce spermatozoa and testosterone. Testicular size correlates with daily sperm output and total sperm production. Ultrasonographic evaluation of the testis, epididymis, and spermatic cord of the stallion allows assessment of parenchymal echogenicity, detection of abnormalities such as tumors or degeneration, and measurement of testicular dimensions [10].

The epididymis serves as the site of sperm maturation and storage. The spermatic cord contains the ductus deferens, testicular artery and vein, lymphatic vessels, and nerves.

Accessory Sex Glands

The stallion has three pairs of accessory sex glands: the ampullae, seminal vesicles, and bulbourethral glands. The prostate gland is also present. These glands contribute to the seminal plasma, which provides nutrients, buffers, and transport medium for spermatozoa. The gel fraction of the ejaculate is produced primarily by the seminal vesicles.

Breeding Soundness Examination in Mares

History and Signalment

A complete breeding soundness examination begins with obtaining a thorough history, including age, breed, reproductive history (number of foals, breeding method, previous pregnancy losses), medical history (colic surgery, laminitis, systemic illness), and current management (nutrition, vaccination, deworming). The mare's intended use (natural cover, artificial insemination, embryo transfer) influences the examination protocol.

Physical Examination

A general physical examination assesses body condition score, conformation, and overall health. Conformation may provide clues to performance and soundness, although the relationship between conformation and reproductive soundness is not always direct [8]. Specific attention is given to the perineal conformation, including the vulvar seal, angle of the vulva, and the presence of pneumovagina or urovagina.

Reproductive Tract Examination

External Genitalia

The vulva, clitoris, and perineal region are examined for abnormalities such as lacerations, tumors, discharge, or poor conformation. The Caslick index (ratio of vulvar length to vulvar slope) can help predict susceptibility to pneumovagina.

Transrectal Palpation and Ultrasonography

Transrectal palpation and ultrasonography evaluate the uterus, cervix, and ovaries. The uterus is assessed for size, symmetry, tone, and presence of fluid. Ultrasonography can detect endometrial cysts, intraluminal fluid, and abnormalities of the uterine wall. The ovaries are examined for follicular activity, presence of corpora lutea, and ovarian abnormalities such as tumors or hematomas.

Endometrial Cytology and Culture

Endometrial cytology is performed using a guarded swab or cytology brush to collect samples from the endometrium. Samples are stained and examined for inflammatory cells, particularly neutrophils. The presence of neutrophils indicates endometritis. Endometrial culture identifies bacterial pathogens and guides antimicrobial therapy. Samples should be collected during estrus when the cervix is relaxed and the uterus is less susceptible to contamination.

Endometrial Biopsy

Endometrial biopsy provides histologic evaluation of the endometrium. The biopsy is classified according to the Kenney and Doig system (categories I, IIA, IIB, III) based on the degree of fibrosis, inflammation, and glandular changes. Category I indicates a normal endometrium with good prognosis for fertility. Category III indicates severe degenerative changes with poor prognosis. Categories IIA and IIB represent intermediate stages.

Uterine Culture and Sensitivity

A guarded uterine swab or low-volume uterine lavage is collected for aerobic bacterial culture and antimicrobial sensitivity testing. Common pathogens include Streptococcus equi subsp. zooepidemicus, Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Fungal culture may be indicated in cases of persistent endometritis.

Hormonal Evaluation

Hormonal assays may be indicated in mares with abnormal cyclicity or suspected endocrine disorders. Progesterone measurement confirms luteal function and can be used to monitor the estrous cycle. Estradiol measurement may be used to assess follicular activity. Equine chorionic gonadotropin (eCG) measurement is used for pregnancy diagnosis between 40 and 120 days of gestation.

Breeding Soundness Examination Protocol

A systematic approach to breeding soundness evaluation of the broodmare includes the following components: history, general physical examination, reproductive tract examination (external genitalia, transrectal palpation and ultrasonography, endometrial cytology and culture, endometrial biopsy), and hormonal evaluation when indicated [16].

Breeding Soundness Examination in Stallions

History and Signalment

The stallion's history includes age, breed, breeding history (number of mares bred, conception rates, previous fertility problems), medical history (testicular trauma, illness, surgery), and management (nutrition, exercise, libido). The intended use (natural cover, artificial insemination, cooled or frozen semen) determines the examination protocol.

Physical Examination

A general physical examination assesses body condition, conformation, and musculoskeletal health. Conformation may provide clues to performance and soundness, and lameness can affect breeding ability [8]. The external genitalia are examined for abnormalities of the penis, prepuce, and scrotum.

Testicular Examination

Palpation and Measurement

The testes are palpated for size, consistency, and symmetry. Testicular measurements (length, width, height) are obtained using calipers or ultrasonography. Testicular volume is calculated using the formula for an ellipsoid: volume = length x width x height x 0.5236. Total scrotal width is measured at the widest point of the scrotum.

Ultrasonography

Ultrasonographic evaluation of the testis, epididymis, and spermatic cord allows assessment of parenchymal echogenicity, detection of abnormalities such as testicular tumors, varicoceles, or hydroceles, and measurement of testicular dimensions [10].

Semen Collection and Evaluation

Collection Method

Semen is collected using an artificial vagina with a mare in estrus or a phantom mount. The collection technique should minimize contamination and stress. Two ejaculates collected one hour apart are recommended for a complete evaluation, as the second ejaculate often provides a more representative sample of daily sperm output.

Macroscopic Evaluation

The ejaculate is evaluated for volume, color, and consistency. Normal semen is opaque, white to cream-colored, and has a characteristic odor. Abnormal color (yellow, green, red, brown) may indicate urine contamination, infection, or blood.

Microscopic Evaluation

Sperm motility is assessed immediately after collection. Total motility and progressive motility are evaluated subjectively or using computer-assisted semen analysis (CASA). Sperm concentration is determined using a hemocytometer or spectrophotometer. Total sperm number per ejaculate is calculated as concentration x volume.

Sperm morphology is evaluated using stained smears. The percentage of morphologically normal spermatozoa is determined. Abnormalities are classified as head, midpiece, or tail defects. The presence of proximal cytoplasmic droplets indicates immaturity.

Additional Tests

Sperm viability (live/dead ratio) can be assessed using eosin-nigrosin staining. The hypoosmotic swelling test evaluates sperm membrane integrity. Acrosome integrity can be assessed using fluorescent staining. These tests provide additional information about sperm function.

Behavioral Evaluation

Libido and mating behavior are assessed during semen collection. The stallion should show interest in the mare or phantom, achieve erection, and ejaculate within a reasonable time. Behavioral abnormalities such as lack of libido, failure to achieve erection, or ejaculatory dysfunction may indicate underlying problems.

Breeding Soundness Examination Classification

The evaluation of the stallion for breeding soundness involves a systematic assessment of reproductive anatomy, semen quality, and behavior. The classification system categorizes stallions as satisfactory, questionable, or unsatisfactory based on the results of the examination [6].

The effect of laboratory methods on semen analysis and breeding soundness examination classification in stallions has been evaluated. Different laboratory methods can affect the results of semen analysis and the classification of stallions as satisfactory or unsatisfactory [7].

Approaches to Interpretation

Approaches to breeding soundness examination and interpretation of results vary among practitioners. Standardized protocols and reference values help ensure consistency in evaluation and classification [17].

Pregnancy Diagnosis in Mares

Transrectal Ultrasonography

Transrectal ultrasonography is the most common method for early pregnancy diagnosis. The embryonic vesicle can be detected as early as 9 to 10 days post-ovulation. The vesicle appears as a fluid-filled, anechoic structure within the uterine lumen. The embryonic heartbeat can be detected around day 24 to 26.

Pregnancy diagnosis at 14 to 16 days post-ovulation allows detection of the embryonic vesicle and assessment of its size, shape, and location. The vesicle should be spherical and located in the uterine body or base of a horn. Irregular shape or abnormal location may indicate a compromised pregnancy.

Hormone Assays

Equine chorionic gonadotropin (eCG) is produced by the endometrial cups and can be detected in maternal blood between 40 and 120 days of gestation. eCG measurement is used for pregnancy diagnosis during this period. Progesterone measurement can confirm luteal function but does not differentiate between pregnant and non-pregnant mares during the luteal phase.

Transrectal Palpation

Transrectal palpation can detect uterine enlargement and fetal presence after 30 to 35 days of gestation. The fetal bulge and placentation can be palpated. This method is less sensitive than ultrasonography for early pregnancy diagnosis.

Transabdominal Ultrasonography

Transabdominal ultrasonography is used for later pregnancy diagnosis and fetal assessment. The fetal heart rate, movement, and anatomy can be evaluated. This method is useful for monitoring high-risk pregnancies and detecting fetal abnormalities.

Factors Affecting Reproductive Success

Mare Age and Breed

Mare age affects reproductive performance. A retrospective study examining factors contributing to the success of equine in vitro embryo production by ICSI found that mares between 6 and 15 years old were superior to other age groups in most parameters evaluated, including the average number of blastocysts per ovum pick-up session. The impact of age was similar when evaluated within two breeds, American Quarter Horse and Warmblood mares [12].

Breed also has an important effect on most parameters evaluated, including number of oocytes recovered, blastocysts produced per ovum pick-up, and blastocyst rates. The study evaluated American Quarter Horse, Warmblood, Crioulo, Lusitano, and Mangalarga breeds [12].

Season

The overall impact of season on equine in vitro embryo production is less pronounced than age and breed. The only statistically significant difference found was a higher rate of oocyte maturation during the summer season [12].

Endometrial Health

Endometrial health is critical for pregnancy establishment and maintenance. Endometrosis, or endometrial fibrosis, can impair fertility by disrupting the endometrial environment. The role of lysophosphatidic acid in the development of tissue fibrosis varies depending on the organ, and its profibrotic role in mare endometrosis remains unclear. Endometrial LPA concentration is higher in the mid-luteal phase compared to the follicular phase in normal endometrium, and there is an alteration in endometrial concentrations of LPA and LPAR1-4 protein abundance in the follicular phase at different stages of endometrosis [13].

Semen Quality

Semen quality is a major determinant of fertility. Sperm motility, morphology, and concentration are evaluated during breeding soundness examination. The effect of laboratory methods on semen analysis and breeding soundness examination classification in stallions has been evaluated, highlighting the importance of standardized protocols [7].

Practical Implementation of Breeding Soundness Examination

Mare BSE Protocol

1. Obtain history and signalment
2. Perform general physical examination
3. Examine external genitalia and perineal conformation
4. Perform transrectal palpation and ultrasonography of reproductive tract
5. Collect endometrial cytology and culture samples during estrus
6. Perform endometrial biopsy if indicated
7. Evaluate hormonal status if indicated
8. Document findings and provide recommendations

Stallion BSE Protocol

1. Obtain history and signalment
2. Perform general physical examination
3. Examine external genitalia and scrotum
4. Measure testicular dimensions and calculate volume
5. Perform ultrasonographic evaluation of testes, epididymis, and spermatic cord
6. Collect two ejaculates one hour apart
7. Evaluate semen for volume, concentration, motility, and morphology
8. Assess libido and mating behavior
9. Document findings and provide classification

Records and Measurements

Common Failure Patterns

Professional Escalation Criteria

Refer to a veterinary theriogenologist or equine reproduction specialist when:

• Mare fails to cycle despite appropriate management
• Mare has repeated early pregnancy loss
• Endometrial biopsy shows category IIB or III changes
• Stallion has persistent poor semen quality
• Stallion has testicular abnormalities detected on ultrasonography
• Breeding soundness examination results are inconclusive
• Advanced reproductive technologies (ICSI, embryo transfer) are being considered

Welfare and Safety Considerations

Mare Welfare

Breeding soundness examination procedures should be performed with minimal stress to the mare. Transrectal palpation and ultrasonography require restraint and may cause discomfort. Endometrial biopsy should be performed during estrus when the cervix is relaxed. Proper hygiene and aseptic technique reduce the risk of infection.

Stallion Welfare

Semen collection should be performed in a safe environment with experienced handlers. The stallion should be accustomed to the collection procedure. Proper restraint and handling techniques reduce the risk of injury to the stallion and handlers.

Biosecurity

Breeding soundness examination equipment should be cleaned and disinfected between animals. Gloves should be changed between examinations. Isolation of animals with suspected infectious diseases reduces the risk of transmission.

Regulatory Considerations

Breeding soundness examination protocols may be required by breed registries for stallion licensing. Semen collection and processing for artificial insemination must comply with regulations regarding disease testing and semen quality standards. The World Organisation for Animal Health provides guidelines for animal health and welfare in breeding programs [5].

Practical Decision Framework for Managing Subfertile Mares and Stallions

Systematic Triage of Breeding Failure

When a mare or stallion fails to achieve pregnancy after three to four breeding cycles, a structured decision framework helps identify the primary cause and guide intervention. The framework follows a sequential evaluation of mare factors, stallion factors, and management factors, with clear escalation criteria at each step.

Step 1: Confirm Breeding Management Accuracy

Before pursuing diagnostic testing, verify that breeding timing and technique are correct. Record the following for each cycle:

If breeding management records are incomplete, implement a standardized recording system before proceeding to advanced diagnostics. The American Association of Equine Practitioners provides owner resources for maintaining breeding records [1].

Step 2: Mare-Focused Evaluation Pathway

For mares that fail to conceive after three cycles with confirmed breeding timing, initiate the following sequential evaluation:

Phase A: Cycle Regularity Assessment (Cycles 1-3 failure)

• Perform transrectal ultrasonography every 2-3 days during estrus to confirm ovulation
• Measure progesterone at day 7 post-ovulation to confirm luteal function (target >4 ng/mL)
• Document follicular development patterns: dominant follicle size at ovulation, presence of multiple follicles, ovulation failure (luteinized unruptured follicle)

If the mare fails to ovulate or has irregular cycles, evaluate for:

• Ovarian abnormalities (tumors, hematomas, anovulatory follicles)
• Endocrine dysfunction (equine Cushing's disease, thyroid disorders)
• Seasonal transition issues (early or late breeding season)

Phase B: Uterine Health Assessment (Cycles 4-6 failure)

• Perform endometrial cytology during estrus using a guarded swab
• Collect endometrial culture with antimicrobial sensitivity
• Evaluate for persistent mating-induced endometritis (PMIE): ultrasound for uterine fluid accumulation at 24-48 hours post-breeding

The Merck Veterinary Manual provides detailed protocols for endometrial sampling and interpretation [2].

Phase C: Endometrial Biopsy (Persistent failure beyond 6 cycles)

• Schedule biopsy during estrus when the cervix is relaxed
• Submit for histopathologic evaluation using the Kenney and Doig classification system
• Interpret results with prognosis categories:
  • Category I: Good prognosis, expected foaling rate >80%
  • Category IIA: Fair prognosis, expected foaling rate 50-80%
  • Category IIB: Guarded prognosis, expected foaling rate 10-50%
  • Category III: Poor prognosis, expected foaling rate <10%

Step 3: Stallion-Focused Evaluation Pathway

When multiple mares fail to conceive to a particular stallion, or when a stallion's per-cycle conception rate drops below 50%, initiate stallion evaluation.

Phase A: Basic Semen Assessment

• Collect two ejaculates one hour apart
• Evaluate immediately for:
  • Total motility: target >60%
  • Progressive motility: target >50%
  • Sperm concentration: target >100 million/mL
  • Total sperm per ejaculate: target >2 billion
  • Morphology: target >60% normal spermatozoa

The effect of laboratory methods on semen analysis and breeding soundness examination classification has been documented, emphasizing that standardized protocols are essential for accurate interpretation [7].

Phase B: Advanced Semen Testing (if basic parameters are borderline)

• Perform computer-assisted semen analysis (CASA) for objective motility assessment
• Evaluate sperm membrane integrity using hypoosmotic swelling test
• Assess acrosome integrity using fluorescent staining
• Consider sperm chromatin structure assay for DNA integrity

Phase C: Testicular and Physical Examination

• Measure testicular dimensions using calipers or ultrasonography
• Calculate testicular volume: length x width x height x 0.5236
• Total scrotal width: target >8 cm in mature stallions
• Ultrasonographic evaluation of the testis, epididymis, and spermatic cord for parenchymal abnormalities [10]

Step 4: Management Factor Assessment

Evaluate environmental and management factors that may affect fertility:

Record System for Breeding Management

Essential Records for Mares

Maintain a breeding record for each mare that includes:

Cycle Tracking Sheet

• Date of estrus onset
• Daily follicle measurements (mm) with ultrasound
• Ovulation date and time
• Breeding dates and method (natural cover, AI, cooled semen, frozen semen)
• Semen source and quality parameters
• Post-breeding uterine fluid assessment (24 and 48 hours)
• Pregnancy diagnosis results (14-16 day ultrasound)
• Any treatments administered (oxytocin, antibiotics, uterine lavage)

Annual Reproductive History

• Number of cycles bred
• Number of ovulations
• Number of pregnancies
• Number of live foals
• Pregnancy loss rate
• Complications (dystocia, retained placenta, endometritis)

Essential Records for Stallions

Maintain a breeding record for each stallion that includes:

Semen Collection Log

• Date and time of collection
• Collection method (artificial vagina, phantom, live mare)
• Ejaculate volume (mL)
• Color and consistency
• Sperm concentration (million/mL)
• Total sperm per ejaculate (billion)
• Total motility (%)
• Progressive motility (%)
• Morphology (% normal)
• Any abnormalities noted

Breeding Performance Summary

• Number of mares bred per season
• Number of cycles per pregnancy
• Per-cycle conception rate
• Seasonal conception rate
• Number of live foals produced

Troubleshooting Common Breeding Problems

Problem: Mare Fails to Show Estrus

Possible Causes and Diagnostic Steps

1. Silent estrus: Progesterone measurement to confirm luteal phase vs. follicular phase. If progesterone is low (<1 ng/mL) and mare is not showing estrus, consider behavioral evaluation.

2. Anestrus: Evaluate season, photoperiod, body condition. Measure progesterone to confirm absence of luteal activity. Consider GnRH or hCG therapy under veterinary guidance.

3. Ovarian abnormalities: Ultrasonography to identify granulosa cell tumors, ovarian hematomas, or anovulatory follicles. The American College of Veterinary Internal Medicine provides guidelines for endocrine evaluation [3].

4. Pregnancy: Rule out existing pregnancy with ultrasound before initiating any treatment.

Problem: Mare Has Uterine Fluid Post-Breeding

Immediate Assessment

• Ultrasound at 24 hours post-breeding
• Measure fluid depth and echogenicity
• Assess cervical relaxation

Management Protocol

• Administer oxytocin (10-20 IU IV or IM) to promote uterine clearance
• Perform uterine lavage with sterile saline if fluid persists beyond 48 hours
• Consider ecbolic therapy (oxytocin, prostaglandin F2alpha) under veterinary supervision

Chronic Cases

• Evaluate perineal conformation for pneumovagina
• Consider Caslick procedure for vulvar conformation defects
• Perform endometrial culture and sensitivity for persistent infections

Problem: Stallion Has Poor Libido

Immediate Assessment

• Evaluate breeding environment (quiet, familiar location)
• Assess mare stimulus (use estrus mare with good behavioral signs)
• Review training history and handling practices

Medical Evaluation

• Physical examination for musculoskeletal pain
• Testosterone measurement (normal range 1-3 ng/mL)
• Thyroid function assessment if indicated

Management Interventions

• Reduce breeding frequency
• Provide positive reinforcement training
• Consider GnRH therapy under veterinary guidance

Problem: Stallion Has Poor Semen Quality

Immediate Assessment

• Repeat collection after 24-48 hours rest
• Evaluate for testicular swelling, heat, or pain
• Check for fever or systemic illness

Diagnostic Workup

• Testicular ultrasonography for parenchymal abnormalities [10]
• Bacterial culture of semen if pyospermia present
• Hormonal evaluation (testosterone, LH, FSH)

Management Interventions

• Reduce breeding frequency to allow sperm reserves to recover
• Provide cooling and shade during hot weather
• Evaluate nutrition for adequate selenium, vitamin E, and zinc

Professional Escalation Criteria

Refer to a board-certified theriogenologist or equine reproduction specialist when:

Mare Escalation Criteria

• Failure to conceive after 6 cycles with confirmed ovulation and breeding timing
• Repeated early pregnancy loss (3 or more losses before 60 days)
• Endometrial biopsy showing category IIB or III changes
• Persistent endometritis unresponsive to standard therapy
• Ovarian abnormalities requiring surgical intervention
• Need for advanced reproductive technologies (ICSI, embryo transfer, oocyte transfer)

Stallion Escalation Criteria

• Persistent poor semen quality (total motility <30%, progressive motility <20%)
• Testicular abnormalities detected on ultrasonography (tumors, degeneration, varicocele)
• Complete ejaculatory failure
• Behavioral problems unresponsive to management changes
• Need for semen freezing or export

Breeding Soundness Examination Classification The evaluation of the stallion for breeding soundness involves a systematic assessment that categorizes stallions as satisfactory, questionable, or unsatisfactory based on the results [6]. When results are borderline or inconclusive, referral to a specialist for repeat evaluation is warranted.

Common Failure Patterns and Interventions

Welfare and Safety Context

Breeding soundness examinations and reproductive management procedures should prioritize animal welfare. The World Organisation for Animal Health provides guidelines for animal health and welfare in breeding programs, emphasizing the importance of minimizing stress and pain during reproductive procedures [5].

Mare Welfare Considerations

• Perform transrectal examinations gently with adequate lubrication
• Limit duration of examinations to minimize discomfort
• Use sedation when necessary for fractious mares
• Provide appropriate pain management for surgical procedures (Caslick, ovariectomy)

Stallion Welfare Considerations

• Ensure safe handling facilities with experienced personnel
• Allow stallion to become accustomed to collection procedures
• Monitor for signs of stress or discomfort during collection
• Provide adequate turnout and social contact

Biosecurity Considerations

• Change gloves between examinations
• Disinfect equipment between animals
• Isolate animals with suspected infectious reproductive diseases
• Follow quarantine protocols for new arrivals

Records and Measurements Summary

This decision framework provides a systematic approach to managing subfertile mares and stallions, with clear recording requirements, troubleshooting protocols, and escalation criteria. Implementation of these protocols can improve breeding efficiency and reduce the economic impact of reproductive failure.

Frequently Asked Questions

What is the normal length of the mare estrous cycle?

The normal equine estrous cycle averages 21 to 22 days. Estrus (sexual receptivity) lasts 5 to 7 days, and diestrus (luteal phase) lasts 14 to 16 days. Ovulation occurs approximately 24 to 48 hours before the end of estrus. The cycle is regulated by hypothalamic neuropeptides including GnRH, kisspeptin, and RFRP-3, with seasonal variation influenced by photoperiod [14].

When should a breeding soundness examination be performed on a mare?

A breeding soundness examination should be performed before the breeding season, ideally during estrus when the cervix is relaxed and the uterus is less susceptible to contamination. The examination includes history, physical examination, reproductive tract evaluation, endometrial cytology and culture, and endometrial biopsy when indicated. Mares with a history of reproductive problems should be examined well in advance of the intended breeding date [16].

What does a stallion breeding soundness examination include?

A stallion breeding soundness examination includes history, physical examination, testicular palpation and measurement, ultrasonographic evaluation of the testes and epididymis, semen collection and analysis (volume, concentration, motility, morphology), and behavioral assessment. Two ejaculates collected one hour apart are recommended for a complete evaluation. The stallion is classified as satisfactory, questionable, or unsatisfactory based on the results [6].

How is pregnancy diagnosed in mares?

Pregnancy is most commonly diagnosed by transrectal ultrasonography, which can detect the embryonic vesicle as early as 9 to 10 days post-ovulation. The embryonic heartbeat can be detected around day 24 to 26. Equine chorionic gonadotropin (eCG) measurement can confirm pregnancy between 40 and 120 days of gestation. Transrectal palpation can detect uterine enlargement after 30 to 35 days.

What factors affect equine in vitro embryo production success?

Factors affecting equine in vitro embryo production by ICSI include mare age, breed, season, and phase of the estrous cycle at the time of ovum pick-up. Mares between 6 and 15 years old have superior outcomes compared to other age groups. Breed has an important effect on oocyte recovery and blastocyst production. Oocyte maturation rates are higher during summer, and follicular phase mares have superior outcomes compared to luteal phase mares [12].

What is endometrosis in mares?

Endometrosis is a chronic degenerative condition of the equine endometrium, described as endometrial fibrosis that forms in the stroma, under the basement membrane, and around the endometrial glands. The condition is classified according to the Kenney and Doig system (categories I, IIA, IIB, III) based on the degree of fibrosis, inflammation, and glandular changes. Endometrosis can impair fertility by disrupting the endometrial environment necessary for embryo survival and placentation [13].

What bacteria are normally found in the healthy equine vagina?

The dominant bacteria in the healthy equine vagina are Escherichia coli and Streptococcus zooepidemicus. Escherichia coli is especially dominant in maiden mares compared to mares that have foaled. Bacterial growth is highest on day 3 and 7 of the estrous cycle (beginning and middle of diestrus), and bacterial diversity increases throughout the cycle, being highest on day 14 [15].

How does season affect equine reproduction?

Season affects equine reproduction through photoperiod regulation of hypothalamic neuropeptide signaling. Mares are seasonally polyestrous, with reproductive activity during long day lengths and anestrus during short day lengths. The overall impact of season on equine in vitro embryo production is less pronounced than age and breed, but oocyte maturation rates are higher during summer [12]. Stallions show some seasonal variation in semen quality, but they remain reproductively active year-round.

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References and Further Reading

• aaep.org
• www.merckvetmanual.com
• www.acvim.org
• Merck Veterinary Manual. Merck Veterinary Manual.
• Animal Health and Welfare. World Organisation for Animal Health.
• Evaluation of the stallion for breeding soundness.. The Veterinary clinics of North America. Equine practice, 1992.
• Evaluation of the effect of laboratory methods on semen analysis and breeding soundness examination (BSE) classification in stallions.. Theriogenology, 2020.
• Equine conformation: clues to performance and soundness?. Equine veterinary journal, 2006.
• Scapulohumeral arthrodesis in miniature horses.. Veterinary surgery : VS, 2003.
• Ultrasonographic evaluation of the testis, epididymis, and spermatic cord of the stallion.. The Veterinary clinics of North America. Equine practice, 1992.
• The reliability and forensic soundness of the equine shin circumference measurement in living animals versus post-mortem examination.. Veterinaria italiana, 2018.
• Successful equine in vitro embryo production by ICSI - effect of season, mares' age, breed, and phase of the estrous cycle on embryo production.. Theriogenology, 2024.
• Lysophosphatidic acid as a regulator of endometrial connective tissue growth factor and prostaglandin secretion during estrous cycle and endometrosis in the mare. BMC Veterinary Research, 2020.
• Characterizing the relationship between gonadotropin releasing hormone (GnRH), kisspeptin, and RFamide related peptide 3 (RFRP-3) neurons in the equine hypothalamus across the estrous cycle and in the anovulatory seasons.. Theriogenology, 2024.
• Bacteria in the healthy equine vagina during the estrous cycle.. Theriogenology, 2023.
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This article is educational and is not a substitute for veterinary diagnosis or treatment. Contact a veterinarian for advice about an individual animal.