The testicles of adult males have a dual function: they produce both sperm as well as testosterone (male sex hormone). As a result of castration (the operative removal of the testicles), both functions cease to exist: castrates are infertile as there is no production of sperm and the drive to reproduce disappears because the production of testosterone stops.

Spermatic cord
A castration involves the removal of both testicles in their entirety, each with the accompanying epididymis. The spermatic cord is severed above the testicle during the procedure. So the tube for transporting the sperm will be cut off and the gelding (castrated stallion) indeed can no longer impregnate mares. However, if even a small piece of the testicle remains behind in the animal’s scrotum, it will continue to produce testosterone. And that hormone is not transported via the spermatic cord, but it is introduced right into the bloodstream. Which means that a stallion that is not completely castrated will continue to have the desire to cover mares. And even though a mare will not become in foal when covered, such an over-eager animal amidst other animals causes problems, for example at the riding association or a show jumping competition.

Jump horse
A young stallion saddle horse is rejected during the studbook inspection as a stud-horse. Yet both his sire and both grandfathers are renowned jump horses. Moreover, the stallion’s mother has won multiple prizes at local jumping competitions. Which is why the owner wants to make a second attempt next year at getting this stallion approved as a stud-horse. Should the animal meanwhile achieve appealing results in jumping competitions, then that will certainly increase his chances considerably. Therefore he will not be castrated for the time being.

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stallion saddle horse

Show jumping competition
Everything goes according to plan during the first few competitions: he is a first-rate jumper and causes no problems amidst the other horses. But that all changes when one of the participating mares is in heat. The stallion is suddenly unmanageable and wants to cover the mare. That alone is cause for a lot of commotion in the bustle of the event with large crowds and hundreds of participating horses. But to make matters worse, a horsewoman is sitting horseback on the mare when the stallion attempts to mount her. Bellowing on his hind legs with an erection, he charges the mare and the horsewoman gets the shock of her life. Fortunately, all ends well. But things have since gone completely wrong with the stallion: he has woken up and is no longer manageable amidst the other horses. We set up an appointment for a castration.

Castration in standing position
His scrotum contains two testicles as thick as a man's clenched fists. He is given an injection with a sedative and a twitch on the upper lip. He is then administered a local anaesthetic by means of injections in the testicles and the scrotum. Pricking the horse high between its rear legs is the riskiest part of the surgery. I then disinfect the skin, wash my hands, put on sterile gloves and proceed to cut. The first testicle drops downward attached to the spermatic cord. Using one hand, I push the surrounding tissue upwards and with my other hand I apply a solid crushing forceps (écraseur) around the spermatic cord. It cracks in the steel jaws when I subsequently use both hands to forcefully squeeze the forceps. So it is essential to ensure that the area is sufficiently numbed: not only for the sake of animal welfare, but also in light of the safety of the person performing the castration. Because he is meanwhile within the range of the animal’s rear legs. The forceful crushing followed by tying the spermatic cord is essential in preventing bleedings following the amputation of the testicle.
Next the same procedure is followed on the other side and then I check that both of the amputated testicles are complete. The wounds in the scrotum are not sutured so as to prevent damming caused by wound discharge. The horse is administered painkillers and antibiotics; which is repeated in the two days to come.

Liability
A few months later, however, the gelding is still behaving like a stallion: he not only mounts mares every chance he gets, but he also actually covers them. The mares cannot become in foal as a result, but behaviour of this kind will lead to problems should the animal be sold. Such a gelding will be indicated as ‘incorrectly castrated’ which is a ‘hidden defect’. The seller will be forced to take the horse back, refund the purchase price and must cover any consequential losses. That bill will then undoubtedly be presented to the vet who carried out the surgery. He will be lawfully liable for any and all damages if he failed to perform the operation in accordance with the generally accepted norms of the trade.

Hormone stimulation
If the castration is carried out properly, then the testosterone level in the blood will quickly drop from the stallion level to virtually zero. That will not happen if a small piece of testicle tissue remains behind during the operation. The results of blood tests for the presence of male sex hormone will then be dubious: the blood level will drop, to be sure, only not to zero. There is a hormone-based trick that can be used to determine whether testicle tissue was left behind in the body of the animal after a castration. The first step involves taking a blood sample. After that, a good amount of HCG (Human Chorionic Gonadotropin) is injected into the bloodstream of the animal. If there is any testicle tissue left behind in the body, then the HCG will stimulate that tissue to produce testosterone. So if a gelding was castrated incompletely and still has a small piece of testicle left, then its testosterone level in the blood will increase following the injection. A second blood sample is taken a few hours after the HCG injection. Laboratory testing of the two samples will demonstrate whether there is an increase in the testosterone level or not.

Acquired behaviour
The testosterone level in this gelding is virtually zero in both blood samples; there is no increase whatsoever. And so the animal has been properly castrated. His behaviour can be explained by the prolonged presence of male sex hormone in his blood due to the postponed castration. That left traces in the animal’s brain and has perhaps permanently influenced its behaviour. Stallions that are castrated before puberty do not develop that macho-behaviour. Puberty in horses begins around the age of two. And so a stallion foal that is not eligible for breeding purposes is best castrated before it reaches the age of two: meaning as a yearling.

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castration of a yearling in a standing position

Body size
In the horse world however, there is a misunderstanding that stallions that are castrated at an early age will lag behind in size later on. The opposite is true: males that are castrated at an early age develop into larger adult males. This is because testosterone inhibits the growth of bones in terms of their length. The production of testosterone does not get underway if the horse is castrated as a yearling. This means that the growth of the bones is not inhibited and stallions that are castrated at a young age are ultimately larger than stallions that are not castrated or are castrated at a later age. Moreover, the surgery has fewer complications in young animals and they recover from surgery more quickly.

Conclusion
There are advantages to having a stallion castrated at a young age: the gelding lacks the pranks that are typical of stallions, it grows to a larger size and there are fewer complications further to the surgery. These advantages were known as early on as in ancient China: the young males were castrated at a young age prior to serving as eunuch of the imperial court.