Acquired stallion infertility is quite rare, with the exception of age, trauma to the genitalia, and fever. Stallion fertility must be based on the pregnancy rates of healthy mares. There is no test or series of tests that guarantee a stallion to be fertile, though many types of infertility can be defined with a careful exam.

See also: Paralysis of the Penis

• History: having a history of settling mares is the most valuable piece of information available.
• Physical exam with particular attention paid to the testicles, penis and external inguinal rings.
• Rectal exam to palpate internal reproductive structures. An experienced ultrasound operator should be able to gather even more useful information.
• Attempted collection, evaluate ejaculation and semen samples.
  • In the case of a stallion who is being examined for ejaculatory failure, particular attention should be paid to whether the stallion seems to ejaculate. Tail flagging, feeling the pulsitile urethral contractions on the bottom of the penis, and the appearance of satiety on the stallions part, are all indications of ejaculation. The rule out lists of problems is a little different for those who do not produce semen though they seem to successfully complete the act from those who fail to complete the copulation.
  • If the stallion is to be bred heavily (greater than one collection every other day for prolonged periods) a series of semen samples should be examined for staying power of the semen's number and quality.