Bull infertility represents a significant economic concern in beef and dairy production systems. Since a single bull is expected to breed 25-50 cows per breeding season, subfertility or infertility in even one bull can dramatically impact herd...
Overview and Clinical Importance
Bull infertility represents a significant economic concern in beef and dairy production systems. Since a single bull is expected to breed 25-50 cows per breeding season, subfertility or infertility in even one bull can dramatically impact herd reproductive efficiency. The Breeding Soundness Examination (BSE) is the cornerstone of bull fertility assessment and a critical skill for veterinary practitioners.
This study guide covers male infertility in bulls including the complete BSE protocol, causes of infertility, preputial diseases, and testicular pathology. Understanding these concepts is essential for NAVLE success and clinical practice.
| Structure |
Function |
Clinical Relevance |
| Testes |
Sperm production (spermatogenesis), testosterone synthesis |
Scrotal location required for temperature 4-5 degrees C below body temperature; size correlates with sperm production |
| Epididymis |
Sperm maturation and storage; transport from testis to vas deferens |
Blockage causes infertility; epididectomy used for teaser bull preparation |
| Seminal Vesicles |
Secrete seminal fluid (largest volume contributor); activate sperm motility |
Palpated per rectum during BSE; vesiculitis is most common accessory gland pathology |
| Sigmoid Flexure |
S-shaped bend in penis; maintains penis within sheath when not breeding |
Most common site of penile hematoma (broken penis); held by retractor penis muscles |
| Prepuce/Sheath |
Protective covering for penis; everts during breeding |
Bos indicus breeds have longer, more pendulous sheath with higher injury risk |
Bull Reproductive Anatomy Review
Understanding normal anatomy is essential for performing a thorough BSE and identifying abnormalities.
Key Anatomical Structures
| Component |
Key Assessment Points |
| 1. Physical Examination |
Body Condition: BCS 5-6 recommended at turnout; too thin = low stamina, too fat = poor thermoregulation
Vision: Essential for detecting estrus; evaluate for cancer eye, pinkeye, cataracts
Feet and Legs: Evaluate for lameness, sickle hocks, post legs; mobility critical for range breeding
Teeth: Important in older bulls for adequate nutrition |
| 2. Internal Exam (per rectum) |
Accessory Sex Glands: Palpate seminal vesicles (paired, lobular, 4-5 inches), ampullae, prostate
Inguinal Rings: Check for enlargement (hernia risk)
Abnormal Findings: Enlarged, firm, or asymmetric vesicles suggest vesiculitis |
| 3. External Genital Exam |
Scrotal Palpation: Testes should be firm and resilient (not soft or hard); symmetric; freely movable
Epididymides: Head, body, tail palpable; check for swelling, fibrosis, or segmental aplasia
Spermatic Cord: Evaluate for varicocele or torsion
Penis and Prepuce: Extend fully; check for lacerations, fibropapillomas, persistent frenulum, deviations |
| 4. Scrotal Circumference |
Measured at widest point with testicles pushed to bottom of scrotum
Correlates with sperm production capacity and age at puberty in daughters
Minimum standards vary by age (see table below) |
| 5. Semen Evaluation |
Collection: Electroejaculation most common in field settings
Motility: Greater than or equal to 30% individual progressive motility required
Morphology: Greater than or equal to 70% morphologically normal sperm required |
Bull Breeding Soundness Examination (BSE)
The Society for Theriogenology (SFT) has established standardized guidelines for evaluating bull breeding potential. The BSE should be performed 30-60 days before the breeding season to allow time for retesting or replacement of unsatisfactory bulls.
High-YieldThe BSE does NOT evaluate libido or serving capacity. A bull classified as a 'satisfactory potential breeder' has met minimum standards but is not guaranteed to achieve high pregnancy rates. The BSE is a snapshot of fertility potential on the day of examination.
Components of the BSE
Minimum Scrotal Circumference by Age (Society for Theriogenology Standards)
NAVLE TipRemember the BSE minimum standards with '30-70-34': 30% minimum motility, 70% minimum normal morphology, and 34 cm minimum scrotal circumference for bulls 2 years and older.
BSE Classification
| Age (months) |
Minimum Scrotal Circumference (cm) |
| Less than 15 months |
30 cm |
| 15 to less than 18 months |
31 cm |
| 18 to less than 21 months |
32 cm |
| 21 to less than 24 months |
33 cm |
| 24 months and older |
34 cm |
Sperm Morphology Assessment
Evaluation of sperm morphology is one of the most important components of the BSE. The spermatogenic cycle in bulls is approximately 61 days, with an additional 10-11 days for epididymal transit. Therefore, sperm abnormalities often reflect insults that occurred 2-3 months prior to examination.
Common Sperm Abnormalities
High-YieldThe distal midpiece reflex (DMR) is the MOST COMMON sperm abnormality seen in bull ejaculates. It is a compensable defect (sperm swim backwards and cannot fertilize) and is transient after stress/heat events. However, knobbed acrosome and Dag defect are HERITABLE and affected bulls should be culled.
| Classification |
Criteria |
| Satisfactory Potential Breeder |
Passes physical exam, meets minimum SC for age, greater than or equal to 30% motility, greater than or equal to 70% normal morphology |
| Classification Deferred |
Marginal results or temporary condition; recommend retest in 2-6 weeks (e.g., young bull still maturing, recent illness, heat stress) |
| Unsatisfactory Potential Breeder |
Fails to meet minimum standards with poor prognosis for improvement (e.g., heritable defects, permanent injury, severe degeneration) |
Preputial Disease and Injuries
Preputial injuries are a common cause of bull infertility and can result in significant economic loss. Bos indicus breeds (Brahman, Brahman crosses) are predisposed due to their longer, more pendulous sheath. Polled Bos taurus breeds (Angus, Polled Hereford) have a rudimentary caudal preputial muscle, predisposing them to preputial prolapse.
Common Preputial Conditions
NAVLE TipFor NAVLE, remember: Bos INDICUS = prolapse after Injury (longer sheath); Bos TAURUS = phimosis (retracts and scars). Preputial length must be at least 1.5 times the free portion of the penis for successful breeding after surgery.
| Abnormality |
Location |
Origin/Cause |
Prognosis |
| Proximal Cytoplasmic Droplet |
Head/Midpiece junction |
Testicular origin; indicates incomplete sperm maturation |
Good in young bulls (immaturity); guarded in older bulls (degeneration) |
| Distal Midpiece Reflex (DMR) |
Tail bent at distal midpiece |
Epididymal origin; most common tail defect; appears 4-16 days after stress/heat |
Good; transient; recovery within 16 days; up to 25-30% tolerated |
| Nuclear Vacuoles |
Head (crater/diadem defect) |
Testicular; oxidative stress; DNA damage; spermatogenic insult |
Guarded to poor; may indicate serious spermatogenic disruption |
| Pyriform Head |
Head shape (pear-shaped) |
Testicular; abnormal spermiogenesis |
Variable; depends on percentage and other defects present |
| Detached/Loose Heads |
Head separated from tail |
Can be artifact (handling); or testicular/epididymal pathology |
Depends on cause; repeat sample if suspected artifact |
| Knobbed Acrosome |
Acrosomal cap |
Heritable defect (recessive); affects zona pellucida penetration |
Poor; heritable; cull from breeding |
| Dag Defect |
Coiled/kinked tail |
Heritable defect; causes sterility when greater than 50% |
Poor; heritable; cull from breeding |
Penile Conditions
Penile Hematoma (Broken Penis)
Penile hematoma results from rupture of the tunica albuginea of the corpus cavernosum penis. This occurs when the erect penis strikes the cow's pelvis during a mistimed breeding lunge.
- Location: Almost always at the dorsal aspect of the sigmoid flexure
- Clinical Signs: Symmetrical swelling dorsal to penis, anterior to scrotum; reluctance to mount
- Diagnosis: Palpation, ultrasonography
- Treatment: Medical (hydrotherapy, NSAIDs, antibiotics, 60-90 days rest) or surgical (evacuate clot, repair tunica if early)
- Prognosis: 50-75% recovery with treatment; surgical success 70% if performed early (within 5-7 days)
Complications of Penile Hematoma
- Abscessation (hematoma is excellent culture medium)
- Adhesions between peripenile layers
- Vascular shunts (prevent erection)
- Dorsal nerve damage (loss of glans sensation)
- Recurrence in subsequent breeding season
High-YieldHereford and Polled Hereford bulls are overrepresented in penile hematoma cases. Bulls require at least 60 days of COMPLETE sexual rest after treatment. Younger, inexperienced bulls are more commonly affected.
Other Penile Conditions
| Condition |
Clinical Signs/Diagnosis |
Treatment/Prognosis |
| Preputial Prolapse (Acute) |
Everted preputial mucosa protruding from sheath
Edematous, inflamed tissue
Common in Bos indicus after breeding injury |
Treatment: Massage (15-20 min), reduce edema, reduce prolapse, retain with tape/tube or purse-string
Prognosis: Good if treated early; 60 days sexual rest |
| Preputial Prolapse (Chronic) |
Severe tissue damage, necrosis, fibrosis
May not be reducible
Often result of neglect |
Treatment: Hydrotherapy, Epsom salt soaks, debridement; surgery if fibrosis/stenosis
Prognosis: Guarded to poor; may require posthioplasty or circumcision |
| Preputial Laceration |
Occurs during breeding; longitudinal tear on ventral prepuce
Bos taurus: retracts into sheath, may develop phimosis
Bos indicus: develops prolapse |
Treatment: Second intention healing; prevent abscess/adhesion; 60 days rest
Prognosis: 86% success with surgery |
| Phimosis |
Inability to extend penis through preputial orifice
Due to stenosis from scarring (healed laceration) |
Treatment: Surgical scar revision; allow granulation before surgery
Prognosis: 63% success rate |
| Paraphimosis |
Inability to retract penis into sheath
Exposed penis becomes damaged, edematous |
Treatment: Reduce edema, replace penis; may require amputation if severe
Prognosis: Poor for breeding |
| Penile Fibropapilloma (Warts) |
Bovine papillomavirus; bulls 1-3 years old
Pedunculated masses, usually near glans penis
Scant hemorrhage from sheath after breeding |
Treatment: Surgical excision (standing or lateral recumbency); catheterize urethra
Prognosis: Good; may recur; 4 weeks healing before service |
| Preputial Abscess |
Located midway between preputial orifice and scrotum
Result of laceration/infection |
Treatment: Drainage, antibiotics
Prognosis: Poor; bulls seldom return to breeding |
Testicular Pathology
Testicular Degeneration
Testicular degeneration is the diffuse disintegration of seminiferous tubule structure and function, resulting in decreased sperm production, reduced scrotal circumference, and increased sperm abnormalities.
Causes of Testicular Degeneration
Clinical Signs and Diagnosis
- Decreased scrotal circumference (compare to previous BSE records)
- Soft, flaccid testes on palpation (normal = firm, resilient)
- Decreased sperm concentration
- Increased percentage of abnormal sperm (proximal droplets, pyriform heads, nuclear vacuoles)
- Ultrasonography may show increased echogenicity (fibrosis) in severe cases
- Progressive sequence of sperm abnormalities appears over weeks following insult
Prognosis and Recovery
Recovery is possible if the cause is removed and damage is not severe. Allow 3-4 months (one spermatogenic cycle plus epididymal transit) for potential recovery. Severe or prolonged insults may result in permanent fibrosis and irreversible infertility.
High-YieldOBESITY is the LEADING CAUSE of testicular degeneration in bulls. Fat accumulates in the scrotum, disrupting thermoregulation. Even a 1 degree C increase in scrotal temperature for 10 days can affect sperm production for 4-5 months!
Testicular Hypoplasia
Testicular hypoplasia is congenital underdevelopment of one or both testes due to reduced germ cell populations. It is heritable in some breeds (Swedish Highland, Belgian Blue). The left testis is more commonly affected.
- Clinical Signs: Small, soft testes; reduced scrotal circumference; low sperm concentration
- Diagnosis: Small SC from young age; fails to increase with maturity
- Prognosis: Poor; no treatment; heritable so cull from breeding
Seminal Vesiculitis (Vesicular Adenitis)
Seminal vesiculitis is the most common inflammatory condition of the bull reproductive tract. It is frequently seen in young peripubertal bulls and is associated with close confinement and high-energy diets.
NAVLE TipSeminal vesiculitis is often discovered during routine pre-sale BSE of yearling bulls. White blood cells (pus) in the semen smear is a key finding. Affected bulls are classified as 'unsatisfactory' until resolved.
| Condition |
Description |
Treatment/Prognosis |
| Persistent Frenulum |
Connective tissue band between penis and prepuce; prevents full extension; may cause ventral deviation |
Simple transection; excellent prognosis; often discovered at BSE |
| Penile Deviation |
Ventral, lateral, or spiral deviation during erection; may be congenital or acquired; prevents intromission |
Fascia lata implant surgery for correction; variable success |
| Hair Ring |
Accumulated hair encircling penis; occurs in young bulls during homosexual mounting; can cause urethral fistula |
Remove hair ring; check for urethral damage |
| Category |
Examples |
| Thermal Disruption |
Obesity (fat in scrotum), scrotal dermatitis, fever, heat stress, frostbite, orchitis |
| Nutritional |
Obesity (LEADING CAUSE), protein deficiency, poor body condition |
| Trauma/Vascular |
Testicular torsion, scrotal trauma, antisperm antibody formation |
| Infectious |
Systemic illness, orchitis, brucellosis |
| Iatrogenic/Toxic |
Anabolic steroids, growth implants, gossypol toxicity |
| Age-Related |
Idiopathic degeneration in older bulls (greater than 5 years) |
| Feature |
Details |
| Etiology |
Trueperella (Arcanobacterium) pyogenes most common; also Histophilus somni, Chlamydia, gram-negative anaerobes |
| Clinical Signs |
Usually NO outward signs; rare: arched back, pain on defecation/rectal exam |
| Diagnosis |
Rectal palpation (enlarged, firm, asymmetric vesicles); WBCs/pus in semen smear; ultrasonography |
| Treatment |
Systemic antibiotics (tilmicosin, tulathromycin achieve gland concentration); intraglandular injection; surgical vesiculectomy in valuable young bulls |
| Prognosis |
Guarded; medical therapy often unsuccessful; surgery successful in young bulls but not in adults |