Stallion Reproductive Assessment
SIVE article in relation to the intervention of Dr. Giovanna Romano
The assessment of an entire male horse (stallion) is a crucial step before entering the breeding season.
As Dr. Giovanna Romano emphasizes, evaluating these animals can be challenging, especially when they are retired athletes. Typically, this assessment takes place after the competitive season, which is not ideal for reproductive evaluations due to the natural decline in hormone levels and sperm production (DSO – Daily Sperm Output) during the fall and winter months, as well as the stress these horses endure during their careers.
These stallions are usually chosen for breeding not for their fertility, but for their performance, morphology, and pedigree, with the hope that they will pass on those desirable traits to their offspring.
Preliminary Evaluation
When evaluating a stallion, it is essential to determine whether the horse will be used for natural breeding, fresh semen production, or frozen semen production. Some foreign Stud Books require early Breeding Soundness Evaluation (BSE) before the horse begins its competitive career, which includes a fertility test with two semen collections taken an hour apart. This test determines which stallions are suitable for reproduction and which may be excluded or castrated.
Before starting the clinical examination, proper identification of the horse must be made through microchip reading and examination of vaccination records. The horse’s medical history plays an important role, as previous illnesses, even minor ones like fever in the past 60 days, or the administration of medications or hormones, can significantly affect sperm production.
General and Specific Examinations
The general physical examination includes a complete review of the horse’s overall health, from its teeth to its hooves, with a focus on any defects or musculoskeletal issues that may affect its ability to breed. In some cases, imaging (e.g., radiology) or specialist consultations may be required.
A critical part of the evaluation is screening for sexually transmitted diseases (STDs), especially when semen is being shipped internationally. Tests typically include monitoring for EVA (Equine Viral Arteritis), AIE (Equine Infectious Anemia), Herpesvirus, CEM (Contagious Equine Metritis), Pseudomonas, and Klebsiella.
Next, the reproductive organ examination includes visual and palpatory assessments of the penis, urethral fossa, prepuce, scrotum, and testes. This should ideally be done while the stallion is excited (by introducing a mare in estrus or ovariectomized), which helps assess libido.
Testicle size is proportional to daily sperm production, and younger stallions may have smaller testes, which increase in size until the age of 6-8 years. If the stallion is tense, sedation with detomidine may be used to relax the animal and ensure proper palpation of the testicles.
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Semen Collection and Evaluation
For semen evaluation, an artificial vagina is used to collect the semen. The sample is then examined for its quantity and quality, including sperm concentration (50-600 million/ml), volume (20-200 ml), total sperm count, morphology (normal vs. abnormal), motility, progressive motility, and other tests like acrosomal reaction, chromatin integrity, and membrane stability.
The goal is to ensure the stallion produces enough viable sperm to fertilize the mare, with a minimum of 300 million sperm in fresh semen and 250 million in frozen semen.
Sperm abnormalities such as malformed heads, defective acrosomes, and coiled tails are noted as undesirable. In some cases, up to 1-2% of germ cells in the sample may be normal. Identifying sperm morphology is facilitated by special staining techniques (e.g., Diff-Quick).
Finally, semen freezing capability is essential to evaluate, as approximately 20% of stallions are not suitable for freezing, which affects their ability to produce frozen semen.