Why do some geldings act like stallions?

When a gelding continues to act like a stallion (or stud), such as sniffing or mounting mares, squealing, kicking and fighting, they're referred to as "proud cut".

It is a popular belief that both testicles were not removed when the horse was gelded. This is certainly possible but not common. A vet would be able to verify this for you. A male horse that still has one testicle left is often referred to as a "jig".

If the horse still has one testicle left, that one testicle will continue to produce testosterone, which would certainly explain his behavior.

There is another possible explanation. The adrenal glands produce a small amount of testosterone as well, so it's also possible that the horse has overactive (called hyperactive) adrenal glands. A vet should be able to advise you on what you should do.

Sometimes this behavior is simply because the colt was gelded after puberty. When a colt is gelded after 1- 1/2 to 2 years old, (especially if they were allowed to breed during that time), they often continue to act like a stallion. In short, they "didn't get the memo" that they are no longer a "stud". :D

What age should you geld (or cut) a stud colt?

It is possible to geld a stud colt shortly after birth but that is not popular nor recommended.

Normally, colts are gelded between 6 months and 2 years of age.

Some colts will begin showing stallion traits at an early age and begin harassing mares or generally making a nuisance of themselves by 6 to 8 months of age. Other colts may not show any signs of wanting to mate until 2 years old.

It's basically a personal choice that depends on your philosophy regarding horse care and training, what you want out of your horse or what you plan to do with them, the individual horses physical and emotional condition, the laws of your state regarding stallions or rules of your boarding facility (if they're boarded), etc.

Whenever you geld your colt, be sure to ask your vet about a tetanus vaccine or booster.