There's been a few questions come through lately regarding badly behaved stallions and whether gelding will fix the problems. So below is a summary of what the research on gelding and behavior has turned up.
Stallions are very dominant by nature, and full of energy. Nipping, rearing, prancing, calling and other high jinx and horsing around are normal behavior. Geldings are generally more placid & predictable and much easier to handle and this is why many male horses are gelded. The gelding tends to be more suitable for a wider range of today's equine activities than the stallion. Although the stallion's extra testosterone enables him to perform with more muscle definition, energy and flashiness, it also tends to distract him from the task at hand with mating and defence impulses. In times past, only stallions were considered suitable for real riders. That is, riders that went to war on horseback, where it was important that the horse would join in the battle with hoofs and teeth. Enough said!
While your colt is little and cute, it can be tempting to want to keep him entire. It's when he has turned into a full blown display of horse testosterone with special handling and housing requirements that castration looks attractive. The reality is that only the very best horses should be kept entire for breeding. If you are worried about the procedure, be reassured that gelding is the most common surgical procedure performed on horses.
So how will castration affect your colts behavior? That depends on how old he is and what he has already learned. Gelding a colt will remove the underlying drives for unwanted behaviors but it will probably not stop these behaviors if these undesirable habits have already formed. Once bad manners have become established, it is a matter of re-training. Clearly it is much easier to geld before your colt starts acting out than after.
A horse goes through puberty between 18 and 24 months. There is a difference in how your colt will develop which depends on when he is gelded. A colt gelded before puberty is much less likely to develop the mating related behaviors of the stallion. He will end up taller by up to 10cm or 4 inches than if he were left entire. The younger a colt is gelded the more likely he is to end up with finer features, less muscle mass and a thinner neck than as a stallion. Furthermore, the younger a colt is gelded, the easier he may be to handle and if gelded very young the incision required is very small and can be sutured closed and very little scarring results.
For horses gelded late, or after puberty, there is no way to 'turn back time' and remove any stallion-like body shape or mating related behavior. And if he has learned he can get his way with people by being aggressive, gelding will not cause him to unlearn this, only retraining will.
When to geld depends on the horse in question. Young colts that become obsessed with mares or their privates, or who start to develop the cresty, thick stallion neck are better to be gelded sooner. Some colts will show very little interest in mares and no muscular thickening and can be left (behavior notwithstanding) until older if so wished.
After the operation, it will take some time for your colt to forget what he was. It can take a month. It can even take 6 months. When his testosterone levels drop, so will his stallion-like behavior. His metabolism will slow down and he will require less food and more exercise to maintain condition.
After gelding, some horses will retain all of the mating behaviors. These are known as 'proud cut'. While it used to be thought that this was due to a failed surgery, todays surgical skill level suggests that other factors are at work. It may be that the adrenal gland (near the kidney) is producing excess testosterone. It may be that gelding happened well after the mating behaviors became established. Other bad horse-human manners are the result of bad training.
.........And brilliantly written by Temple Grandin of the Department of Animal Science at Colorado State University
explaining how herd dynamics in grazing animals are learned:
Castration will reduce aggression in adult animals and, if done at a young age, mostly eliminate it. In grazing animals, an orphan male raised away from its own species may be imprinted to people and think he is a person. The resulting behavior is cute in a young animal, but when the male becomes fully mature he can be dangerous. At full maturity lie may turn on his caretakers to prove that lie is now the dominant male in the herd. Raising young bull calves in a social group helps prevent aggression toward people. Young bulls and stallions must learn they are not people. Orphaned male grazing animals should be either castrated or placed in a social group with their own kind by 6 weeks of age. When they grow up with their own kind they learn who they are, and any aggression is more likely to be directed toward their own kind. The male aggression problem is not due to the animal being tame. It is due to mistaken identity. Social behavior in grazing animals has to be learned. Grazing animals must learn the normal give and take of social behavior. Horses or cattle that are reared alone will often be vicious fighters when mixed with other animals. A young stud colt reared alone may constantly fight other horses because he has never learned that once he has become dominant he doesn't need to keep fighting. Stallions will be easier to manage when they mature, if they are reared as young colts in a pasture full of other adult horses.
The [behavioral] complications increase as stallions are kept in confined and unnatural conditions of isolation. Isolation tends to produce psychological aberrations in the stallion, with an associated reduction in the degree to which behavior can be predicted. At this point other horses, animals or people may be wrongly perceived as a threat, with the result being that they are driven forcibly from the area - through or over gates and fences if necessary. Confinement by itself can have a powerful effect, with some stallions showing a degree of tolerance and others far less so.