A “proud cut” gelding is a castrated male horse that continues to behave like a stallion, mounting mares, acting aggressively toward other horses, or displaying other stud-like behavior. The term has been used for generations among horse owners, but it rests on a misconception. Veterinary science has shown that the condition people call “proud cut” doesn’t actually exist the way most horsemen think it does.
Where the Term Comes From
For many years, horse owners believed that if a veterinarian didn’t remove every bit of tissue attached to the testicle during castration, particularly the epididymis (a coiled structure sitting alongside each testicle), the leftover tissue would produce enough hormones to keep a gelding acting like a stallion. A horse castrated this way was said to have been “proud cut,” implying the vet didn’t cut enough.
This belief has been disproven for over three decades. The epididymis does not produce the hormones responsible for stallion behavior. If both testicles are fully removed, the horse is correctly castrated regardless of whether the epididymis stays behind. Some people have also pointed to the adrenal glands as a possible source of male hormones strong enough to drive stud behavior, but research has shown this explanation doesn’t hold up either.
Why Some Geldings Still Act Like Stallions
If leftover tissue isn’t the cause, why do so many geldings display stallion-like behavior? The answer usually comes down to one of two explanations: learned behavior or, less commonly, a hidden retained testicle.
Learned behavior is by far the more common reason. A large study of over 1,200 geldings submitted for hormonal testing because of stallion-like behavior found that roughly 80% had testosterone levels well below the range that would indicate retained testicular tissue. In other words, four out of five “proud cut” geldings had no hormonal reason to act the way they did. Their behavior was behavioral, not medical.
Research on the timing of castration reinforces this. A study comparing horses castrated before age two with those castrated after age three found that 20% to 30% of the early-castrated group still displayed stallion-like sexual or aggressive behavior as adults. That rate was statistically no different from the group castrated later in life, after years of practicing stallion behavior. This means the behavior can emerge regardless of whether the horse ever had a chance to learn it. Veterinary behaviorists attribute this to innate social behavior, essentially normal horse-to-horse interactions that owners interpret as stallion-like.
Even when castration is specifically performed to eliminate problem behavior in mature stallions, it only resolves sexual behavior and aggression toward people about 60% to 70% of the time. Aggression toward other horses drops in only about 40% of cases. Castration changes the hormonal landscape, but it doesn’t erase behavioral patterns the horse has already established.
When a Retained Testicle Is the Real Problem
The one situation where a gelding’s stallion behavior genuinely has a hormonal cause is cryptorchidism, a condition where one or both testicles never descended into the scrotum. If a horse was castrated but only one testicle had dropped, the undescended one may still be inside the abdomen or inguinal canal, quietly producing testosterone. This horse isn’t technically a gelding at all. He’s a partially castrated cryptorchid.
In the study of 1,200-plus geldings, a small percentage did show hormonal markers consistent with retained testicular tissue. About 4.3% had elevated levels of a hormone called anti-Müllerian hormone (AMH), which is produced almost exclusively by testicular cells. These cases point to actual retained tissue that a previous castration missed.
How Vets Confirm Retained Testicular Tissue
If your gelding’s behavior is extreme enough to raise suspicion, a blood test can settle the question. True geldings have testosterone concentrations below 0.19 nmol/L. A cryptorchid horse with retained testicular tissue will typically show levels above 0.30 nmol/L.
When a simple blood draw isn’t conclusive, vets can use a stimulation test. The horse receives an injection of a hormone that triggers testosterone production, and a second blood sample is drawn two hours later. If functional testicular tissue is present, testosterone levels will spike. A gelding with no testicular tissue will show little to no response. This test is considered highly reliable for distinguishing a true gelding from a cryptorchid.
AMH testing offers another angle. Because AMH is produced specifically by testicular cells and not by the adrenal glands or other tissues, it can confirm whether testicular tissue exists even in cases where testosterone results are borderline.
Removing a Retained Testicle
If testing confirms retained testicular tissue, surgical removal is the standard treatment. The traditional approach is an open surgery, which research suggests actually performs well compared to newer alternatives. A study comparing open surgery with laparoscopic (minimally invasive) surgery found that the laparoscopic approach required longer surgical preparation, longer time under anesthesia, and resulted in more postoperative complications. However, laparoscopy can be useful when the location of the retained testicle is unknown beforehand or when a previous open surgery failed to locate it.
After successful removal, testosterone levels drop and stallion-like behavior driven by hormones will diminish over the following weeks. Behavior that was learned rather than hormonally driven may persist and require training-based management.
Managing a Gelding With Stud-Like Behavior
For the majority of so-called proud cut geldings whose blood work comes back normal, the path forward is behavioral rather than medical. These horses aren’t hormonally abnormal. They’re displaying behavior that falls within the normal range of horse social interaction, or they’ve developed habits that are self-reinforcing.
Turning out a behaviorally studdy gelding with mares often makes the problem worse, as it gives the horse opportunities to practice and reinforce mounting and herding behavior. Keeping such geldings with other geldings, providing consistent groundwork, and working with a trainer experienced in stallion-like behavior can all help. The key insight is that once hormones are ruled out, you’re dealing with a training problem, not a surgical one.