Breeding

The Proud-Cut Gelding

A horse-breeding mystery: Find out if your gelding might actually be a cryptorchid stallion.

You acquired him last winter to be a pasture companion for your mare. He was a 5-year-old horse that supposedly had been gelded as a yearling. The two horses got along well all winter, but when the mare came into season in the spring, the new horse began exhibiting behaviors much more like a stallion than a gelding. Chances are good that your “gelding” is actually a cryptorchid stallion and not a gelding at all. Cyrptorchidism is a developmental defect where one or both testicles do not descend into the scrotum.

Causes of Cryptorchidism

Misconceptions abound as to the cause of stallion-like behavior in a horse that has supposedly been gelded. The term “proud-cut” has been used historically for many of these cases. Traditionally, proud-cut implies that a part of the epididymis (the sperm storage site located adjacent to the testis) was left in the horse at the time of castration.

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Normally, each testis and associated epididymis is removed during castration. However, since the epididymis does not produce testosterone, leaving the structure in a horse would not lead to a continuation of stallion-like behavior. Continual expression of stallion-like behavior following castration surgery in young horses is more likely due to failure to remove a testis. Stallions that are castrated at an older age might retain some stallion-like behaviors. Although the source of testosterone has been removed, some learned behaviors persist.

How Do You Tell?

Hormonal diagnostic tests are available to determine if a horse is a true gelding or a cryptorchid stallion. The most commonly used test is measuring testosterone levels in the blood. Testosterone is produced by cells within the testes. In the absence of testicular tissue, testosterone levels in the blood should be very low (less than 100 pg/ml). Intact stallions with two scrotal testes usually have blood testosterone levels of 500-1,000 pg/ml or higher. Testosterone levels in a cryptorchid stallion are higher than that of a gelding, but usually lower than that of an intact stallion (i.e. 100-500 pg/ml). Unfortunately, evaluation of testosterone levels in a single blood sample might not always be sufficient to differentiate a gelding from a cryptorchid stallion. In those cases, a test called human chorionic gonadotropin stimulation can be used. A blood sample is collected just before administration of hCG. A second blood sample is collected one to two hours later.

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The hCG will cause an increase in testosterone secretion from testicular tissue present in cryptorchid horses. No increase in testosterone will be elicited in true geldings.

What Should You Do?

While cryptorchid testes continue to produce testosterone, sperm production within the cryptorchid testes is inhibited due to the higher temperatures within the scrotum. Consequently, horses with two cryptorchid testes might tease mares, gain an erection, mount and ejaculate, but are infertile. It is generally recommended that the retained testes be removed from cryptorchid stallions. Affected horses can exhibit unpredictable, aggressive or stallion-like behavior and can have a slightly increases risk of medical conditions such as testicular tumors and torsion of the spermatic cord.