Breeding Soundness Examination of Stallions

Initial Steps & History

• Primary Objective: To evaluate the stallion's reproductive potential and determine if it can be used for natural service or artificial insemination.

• Detailed History: Must include prior breeding results, nutrition, vaccination status, disease history, and previous drug use, such as anabolic steroids.

• Follow-up: Stallions recently retired from racing may require re-evaluation in 3–6 months if they fail an initial BSE due to prior drug use.

Genital Organ Evaluation

• External Genitalia:

    ◦ Scrotum/Testes: Evaluated for elasticity and firmness; 180° rotation is common and usually not clinically significant.

    ◦ Ultrasonography: Used to measure individual testes and check for normal echoic appearance (e.g., the central vein) or pathologies like varicose veins.

    ◦ Penis: Examined during washing for lesions like squamous cell carcinoma, sarcoids, or "the bean" (hardened smegma) in the urethral diverticulum.

• Internal Genitalia (Per Rectum):

    ◦ Evaluation of the ampullae, vesicular glands, prostate, and internal inguinal rings.

• Daily Sperm Output (DSO) Estimation:

    ◦ Calculated using testicular volume or scrotal width (which should be > 8–9 cm for a satisfactory result).

Behavioral Assessment

• Libido: Evaluated by exposing the stallion to an estrous mare; evidenced by vocalization, pawing, and the flehmen response.

• Breeding Ability: Observation of normal mounting, copulatory thrusts (typically 3–5), and ejaculatory jets (5–7), often confirmed by "tail flagging".

V. Microbiology & Swabs

• Swab Locations: Pre-wash penile skin, pre-ejaculate urethra, and post-ejaculate urethra.

• Pathogens of Concern: Identification of Pseudomonas aeruginosa, Klebsiella pneumoniae, or Taylorella equigenitalis (contagious equine metritis) which can cause endometritis in mares.

Semen Collection & Analysis

• Method: Typically collected using an Artificial Vagina (AV) with water at ~42°–50°C.

• Frequency: At least two ejaculates collected 1 hour apart.

• Semen Quality Thresholds (Satisfactory):

    ◦ Total Motility: ≥ 65%

    ◦ Progressive Motility: ≥ 50%

    ◦ Morphologically Normal: ≥ 50%

• Sperm Numbers: After sexual rest, the first ejaculate should have ≥ 8–10 billion sperm, and the second should have ≥ 4 billion.

Ancillary Sperm Function Tests

• CASA: Computer-assisted analysis for objective motion and head shape assessment.

• DNA/Chromatin Integrity: The Sperm Chromatin Structure Assay (SCSA) assesses DNA stability and is moderately correlated with fertility.

• Acrosomal Responsiveness: Challenges the sperm's ability to undergo acrosomal exocytosis, useful for stallions with normal motility but poor fertility.