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So in the case of stallion-like behavior in a gelding, the first important question is: Both testicles were fully removed at the time of castration? A fairly common scenario is that one of the testicles is not in the scrotum at the time of castration, so it is not removed. It can be out of easy reach high in the inguinal area, or it can be all the way up in the abdomen. It would require some diagnostics to locate a remaining testicle and more complicated surgery to remove it, and so it is just left in many cases. The usual plan is to wait and see how the "cryptorchid" gelding does.

A less-common finding is that a small piece of testicle is inadvertently left. This "remnant" can still produce enough male hormones to drive sexual behavior. For your gelding, neither of these are likely the case, since you owned the horse at the time and attended the castration.

Another reason complete castrates show stallion-like behavior is that no matter how old or experienced colts are when castrated, there is a wide range of retained stallion-type sexual and aggressive behavior.

Decisions to explore the possibility of a remaining testicle or remnant are usually based on the intensity of sexual and inter-male aggressive behavior rather than the number of elements of stallion-like behavior. It's easy to explore whether a purported gelding has remaining testicular tissue. Your veterinarian can do a blood test that involves taking a baseline blood sample, administering hCG (a hormone that will stimulate any testicular tissue to release androgens), then taking a second blood sample. While it is not 100% accurate in ruling out the presence of remaining testicular tissue, this simple test catches most cases. And if a hidden testicle or remnant that is producing androgens is found and removed, the problem stallion-like behaviors usually, but not always, subside within a few weeks after surgical removal.