Sally,
It would probably be great for your gelding and stallion to live together. The younger the colt is introduced to other geldings, the more likely he will assimilate with a herd for the rest of his life. It is easier to introduce a stallion before he begins his breeding career. Of course, you'll have to monitor the situation closely and there will be some sorting out of who's boss, but one will emerge as the leader and the other the follower and then hopefully they will establish a compatible relationship. I would prefer the older gelding to emerge as dominant; that will keep the colt's aggressiveness limited.
It is always possible for a horse to get hurt in the process of assimilating new horses into a herd; so that is a risk you’ll have to be willing to accept by turning him out. Many people chose not to allow a valuable breeding stallion to interact in a herd of any kind, because of the increased risk of injury, even though conception rates are proven to be much higher with pasture breeding programs. Only you can decide whether the reduced risk for the horse is worth the benefit he will gain from being socialized into a herd, which will affect his happiness for the rest of his life. No horse wants to live in solitary confinement.
I do not agree with waiting until the horse is four to saddle train him, especially if you plan to leave him a stallion (which is not a good idea for most amateurs). Given that his urge to breed will be growing stronger as he matures, the more training and performance is imposed on him, the more control you will have over him. I prefer to start all young horses as three year olds, but in the case of a stud colt, I would start him earlier.
As for why a horse responds differently to male and female is not so much gender related as it is related to leadership, confidence and firmness in handling. Women tend to approach horses from a more submissive point of view, trying to make friends and keep the peace. Men usually approach horses with a body language that shows confidence, leadership and a no-fuss-no-muss demeanor. Horses, being better attuned to body language than us humans, will size up a person right away and factor the results into his behavior.
If the horse has had experience with a human that was a "push over," he will recognize that demeanor more quickly and may take it as a green light to take control. Interestingly, if the horse has never been handled by anyone except very confident and in-charge people, he will tend not to size up the human, but just act obediently automatically, as he knows that is expected of him. However, if someone with less confidence begins to handle that horse and the horse is allowed to act impulsively, it's training will completely unravel. At this point, the horse MAY have learned that there are two classes of humans, not male and female, but enforcer or enabler.
This might result in a horse that is like Dr. Jekyl and Mr. Hyde; responding perfectly to one person and a total brat to another. This can also result in a horse that learns to test the water right away when assessing a new human handler and may subtlety invade your space or commit some other transgression. If a correction comes spontaneously from the handler, the horse will shift into "good boy" mode. If the invasion is tolerated or ignored, it is an open invitation for the horse to escalate his behavior and become rude, disrespectful and eventually aggressive.
If your horse is obedient and respects your leadership and authority, he should not be acting aggressively when others are around you. That makes me wonder if he views you as one of his mares, rather than as the boss of him; an object to be protected from invading marauders. If your horse is at any time acting like he is herding you or aggressively defending you, I suggest that you reassess your relationship with the horse and take immediate actions to make him more subordinate to you. There is lots of info about this on my website.
Anytime your horse acts disrespectfully or aggressively toward humans, for any reason, he should be punished immediately with a negative stimuli; swiftly and harshly. This is dangerous behavior. Sometimes with stallions, the stimuli you use must escalate quickly to a lot off pressure or a harsh cue. There must be enough pressure to motivate the horse to change and when it comes to aggressive and reproductive behaviors, it takes a lot of motivation to cause a change.
Using an aversive sound as negative stimuli (yelling or growling) works well with all horses (I call it hissing and spitting at a horse). If the sound is followed up with a physical stimuli (like a spanking or shanking), then the sound stimuli alone will soon more easily motivate the horse to change.
If your horse is subordinate and obedient, he simply should never act that way around humans. With stallions, it is critical from day one that you use very forceful measures to let the horse know that aggressiveness toward humans or other horses will under no circumstances be tolerated. If you plan to use this stallion for any type of performance, this becomes paramount, since he will likely encounter others humans and horses in his regular routine. Horses are very capable of abiding by rules, but they must be consistently enforced.
Please keep in mind that owning a stallion gives you a much greater responsibility than mare and gelding owners have, both legally and ethically. Stallions are much more prone to instinctive behaviors, like reproductive and combative, and as owner, you have to accept the responsibility of providing additional control, confinement and training that comes with owning a stallion. Good luck with this colt; keep him busy.
JG
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