The Professional Guide to Stallion Semen Analysis

Clinical Standards for Equine Breeding Soundness (EBSE)

The Professional Guide to Stallion Semen Analysis: Clinical Standards for Equine Breeding Soundness (EBSE)

This comprehensive guide provides veterinary professionals and breeders with the clinical standards and methodologies required for performing a complete Equine Breeding Soundness Evaluation (EBSE) and semen analysis.

Chapter 1: Stallion Selection and Reproductive Behavior

The evaluation of a stallion begins with his genetic pedigree and a thorough assessment of his reproductive behavior. In the equine industry, a stallion must possess a strong, controllable libido. We evaluate the stallion's response to a "tease mare," looking for immediate interest, vocalization, and penile erection. Unlike other species, stallions are highly sensitive to their environment; stress or poor handling in the breeding shed can lead to psychological inhibitions that manifest as a refusal to mount.

Chapter 2: Physical Examination and Athletic Soundness

Stallions are reproductive athletes. To successfully mount and ejaculate, they require immense strength in their hindquarters and a stable lumbosacral spine. The physical examination focuses on identifying any musculoskeletal issues, such as arthritis, hock pain, or previous tendon injuries, that might prevent the stallion from supporting his weight during a mount. Chronic pain is a significant suppressor of libido and can lead to aggressive or defensive behavior.

Chapter 3: Examination of Scrotum and Testicular Integrity

The stallion's testes are oriented horizontally in a relatively tight scrotum. During clinical palpation, we assess symmetry, size, and consistency. Healthy equine testes should be smooth, firm, and resilient—similar to the feel of a ripe plum. We pay close attention to Testicular Tone; softness is often an early indicator of testicular degeneration, which is common in older stallions or those recovering from systemic illness.

Chapter 4: Scrotal Width and Predicted Sperm Output

In equine andrology, Total Scrotal Width (TSW) is the most repeatable and reliable objective measurement. We use calipers to measure the width of both testes together at their widest point. TSW is a direct predictor of the stallion's Daily Sperm Output (DSO). For a mature stallion (over 4 years old), we expect a minimum TSW of 8.0 cm, with elite sires often exceeding 10.0 cm.

Chapter 5: Internal Reproductive Glands (The Accessory Glands)

The internal examination of the stallion is performed via transrectal palpation and ultrasound. We focus on the prostate, seminal vesicles (vesicular glands), and ampullae. Stallions produce a significant amount of seminal fluid, and the vesicular glands are responsible for the "gel fraction" of the ejaculate. We look for any signs of Seminal Vesiculitis, which introduces inflammatory cells (pus) into the semen, killing sperm and reducing fertility.

Chapter 6: Penile Anatomy and Hygiene

The stallion's penis is a large, musculocavernous organ that requires regular hygienic maintenance. We examine the Glans Penis and the Urethral Process for any signs of trauma, warts (papillomas), or SCC (Squamous Cell Carcinoma). A major focus is the Preputial Diverticulum (The Bean); we check for the accumulation of "smegma" (dried secretions and debris) which can harbor bacteria and cause irritation.

Chapter 7: Semen Collection and Temperature Sensitivity

Semen collection in stallions is performed using an Artificial Vagina (AV)—typically the Missouri or Colorado models. The AV must be carefully prepared to match the stallion's preference for Temperature (45°C–50°C) and Pressure. If the AV is too cold, the stallion will not ejaculate; if it is too hot, the sperm will be instantly killed (thermal shock).

Chapter 8: Sperm Quality — The Cooled-Shipped Standard

This is the scientific heart of equine andrology. Because most sport horse breeding relies on transport, we evaluate semen not just for immediate motility, but for Longevity.
  • Progressive Motility: The SFT standard for a "Satisfactory" stallion is >60% progressive motility in the raw ejaculate.
  • Cooled Motility: We dilute the semen with a specialized Extender and test the motility again at 24 and 48 hours (stored at 4°C). If the motility drops below 30% after 24 hours, the stallion is considered a "poor cooler."

Chapter 9: Sperm Morphology and Cellular Integrity

Morphology is assessed using Eosin-Nigrosin staining under 1000x oil-immersion microscopy. We count at least 100 to 200 cells, looking for head defects (pyriform, microcephalic) and tail defects (coiled, bent). The SFT standard requires >70% normal morphology.

Chapter 10: Venereal Disease and Biosecurity

The final chapter addresses the biosecurity risks inherent in the equine industry. We monitor for Contagious Equine Metritis (CEM), a highly contagious bacterial disease that can be carried asymptomatically by the stallion. We also test for Equine Viral Arteritis (EVA), which can be shed in the semen and cause abortions in pregnant mares.

The "Thermal Integrity" Advantage for Stallions

One of the most critical engineering features of the SQA-6100VET for the equine industry is its internal, real-time heating stage.
  • The Challenge: Stallion sperm are exceptionally sensitive to "Cold Shock." In a typical lab, the room temperature is ~20°C, while the stallion's body is ~38°C. This 18-degree drop causes the sperm plasma membrane to transition from a fluid to a crystalline state, resulting in a sudden, permanent loss of motility.
  • The Advantage: The SQA-6100VET features an integrated heating stage that maintains a constant 37°C–38°C environment throughout the analysis.
  • The Result: This ensures that the stallion's progressive motility is measured under "life-like" conditions. Without this built-in heater, a fertile stallion could be incorrectly labeled as sub-fertile simply because his sperm "crashed" on a cold microscope slide. For high-value thoroughbred and sport horse sires, this feature is the ultimate insurance policy for diagnostic accuracy.

Conclusion: The Path to Precision

Equine Breeding Soundness Evaluation (EBSE) is a comprehensive assessment that combines physical examination, behavioral evaluation, and precise semen analysis to determine a stallion's reproductive fitness. The integration of modern CASA technology, such as the SQA-6100VET, provides the thermal control and objective measurement capabilities essential for accurate evaluation of equine semen. By adhering to SFT standards and maintaining strict biosecurity protocols, veterinary professionals can ensure that stallions are evaluated with the highest level of diagnostic accuracy, protecting both the genetic investment and the health of the equine breeding industry.

Disclaimer: This guide is for educational purposes. All clinical diagnoses should be made by qualified veterinary professionals in accordance with local regulations.

References & Technical Resources