The first thing we should get out of the way is that some horses suffer from allergies or medical disorders therefore should never be given some types of treats. Before you even think about treats, your equine vet needs to have cleared your horse to avoid hurting his health in any way. That said, giving your horse hand treats is not something that we would recommend for the following reasons:
Your horse is not a dog. He is not preconditioned to think of food as a sign of affection. In fact, eating is not even their first instinct- breeding is. When a dog finds food, he may bring it back as a sign of affection for the owner. But a horse would never do the same, not even for a mare that has just delivered. This means that when you give your horse treats as a sign of affection you are only doing it to feel good about yourself and not really looking at the other side of the equation. Your horse just enjoys the treat and learns to expect more. He doesn't think 'wow, my owner likes me!' With time, he would learn to see you as a machine that dispenses treats.
People have been seriously hurt when hand feeding treats to their horse. Fingertips bitten off, a badly injured arm or even other body parts brought to harm. It does not mean that the horse is aggressive. He is just spoiled and may nudge a bit to get their share of equine treats. Take for instance, a person who gets their fingertip chewed off. The horse was just reaching for her treat and by bad luck the finger happened to be in the way. We cannot really blame the horse!
Although we have mentioned that food is never really the first need for a horse, just like any other animal (or person) they can be trained to respond to treats. The problem is that the horse would not be responding to you but to the treat. You would slowly be turning your large equine friend into an obedient dog that knows only the treats that he gets after maybe performing a trick.
Let's not entirely weigh on to the negative side, because you can perfectly use treats to reinforce good behavior. This way, you can be able to raise a well mannered horse since they will know what gets them their treat from the owner. Not all horses develop bad habits after learning to expect treats, some are perfectly fine.
You are however advised to only get used to giving your horse treats when there are no other people coming around. This can be a bit impossible since the vet would have to visit sometime, or other visitors who could get harassed by the animal. When the horse sees that he/she is not getting anything from this new person they may nip or nudge and get fussy. The horse has a giant head and jaws that could cause some serious damage!
What can I do if my horse is already spoiled?
You must be very strict to discourage this behavior. A stern ' NO' with you moving away from the horse for a few minutes every time he starts to nudge would train him not to ask for the treats but to wait until they are given.
Ensure that the treats are healthy for the horse. Carrots, apples or store bought molasses should be fine. Avoid toxic substances like raw potatoes, cauliflower, broccoli, cabbage or chocolate. Not that anyone is ever eager to share their chocolate...
The treat should also be placed away from you. Distance is important as it teaches your horse not to nip at your limbs when he expects treats. Your horse could eat them from the ground or from a bucket as a safety measure that would save you and any other person handling the horse in the future.
With all these said, we all have different things that we look at when bonding with our horses. Some are for sport, riding etc. You cannot really point at one method of training and say it is the right one. If treats work well, then by all means go ahead albeit in moderation. We are however against pain or violence, since the horse is an animal that is capable of understanding commands. There are other methods for discouraging negative behavior other than being cruel.