Stallion Reproductive Evaluation

Courtesy of America’s Horse Daily

It’s always a good idea to have a reproductive evaluation performed on your stallion so you’ll know what to expect and can identify potential problems.

Reproductive evaluations are typically performed on stallions for three reasons:

  • Estimate future reproductive potential
  • Evaluate prior to purchase
  • Determine causes of poor reproductive performance

The evaluation should encompass general health of the horse, behavior, mating ability, genital examination, semen evaluation and other parameters. In most cases, the examination can be performed in one day. Here are the important components of a stallion reproductive evaluation:

Identification. A stallion should be properly and positively identified prior to evaluation. Registration name and number, age and breed are recorded.

Reproductive history. A complete breeding history should be provided, including previous per-cycle and seasonal pregnancy rates, number and type of mares bred per season (maiden, open, wet, etc.), breeding method used (live cover, pasture breeding, artificial insemination with fresh, cooled or frozen semen), and results of previous reproductive evaluations, medical conditions and vaccination history. It is also important to record the intended future use of the stallion and the number of mares anticipated for the next season.

Physical exam. A general physical examination is performed and back, hock, lameness or neurologic problems are identified, along with other medical conditions.

External genitalia exam. The penis, prepuce, sheath, scrotum and scrotal contents are evaluated. Daily sperm production potential is estimated from total testicular volume, calculated from testicular measurement. Spermatogenic efficiency can be determined by comparing the actual number of sperm produced each day (based on semen collection), with the calculated daily sperm production rate based on testicular volume.

Bacterial culture. Microbiological culture swabs from the urethral fossa, urethra, shaft of the penis and prepuce are collected prior to washing the stallion. A second swab from the urethra is obtained after semen collection, and the semen itself is often cultured.

Libido and mating ability. Libido when teased to a mare in estrus and ability to gain and maintain an erection are recorded. The ability to mount a mare or breeding phantom, insert the penis, thrust and ejaculate completely are all noted.

Semen evaluation. In a standard reproductive evaluation, semen is collected twice, one hour apart. Assessment of each ejaculate includes evaluation of: 

1. semen quality

2. semen pH

3. volume of gel-free semen

4. sperm concentration

5. total number of sperm in the ejaculate

6. percentage of progressively motile sperm

7. percentage of morphologically normal sperm

8. the number of progressively motile, morphologically normal sperm. 

It may be beneficial to determine daily sperm output from a stallion. Semen must be collected daily for a minimum of five to seven days to deplete the extragonadal sperm reserves. Thereafter, the total number of sperm collected each day will be relatively constant and represents the number of sperm produced each day. Daily sperm output can be used to estimate the number of mares that may be bred per day.

Other tests. If a standard evaluation does not identify a cause of subfertility or infertility, additional diagnostic tests may be required. Interpretation of the results should take into account the number of mares to be bred and the type of horse-breeding program to be used. Accepted values for semen analysis are: 

1. a minimum of 60 percent progressively motile spermatozoa and 60 percent morphologically normal spermatozoa in each ejaculate collected

2. a minimum of 1 billion progressively motile, morphologically normal spermatozoa in the second of two ejaculates collected one hour apart after sexual rest for one week at any time of the year. 

The presence of an adequate number of motile spermatozoa in an ejaculate is no guarantee of fertility. The final criteria upon which to assess true stallion fertility are conception and foaling rates in mares bred.