There is a popular controversy among people who are trying to teach their horses to do things. We know that being rewarded on any level is positive reinforcement. Some believe that giving treats to horses, though, makes them nippy and behave badly, which I find can also be true.The horse is a creature of habit. Although some say that they have really little intelligence, I find this to be absolutely untrue. Horses have personalities and capacities for learning that varies from horse to horse. Some horses learn quickly and readily without much trouble. Others must be forced to perform the same action over and over and over, with very little retention.
I personally have this view: When offering treats as a reward for a requested behavior, you must also discipline if any negative behavior is exhibited.
My horse, Poncho, a paint who has just turned 4 is a perfect example. He is gelded, so there is no hormonal issue thrown into the mix here. I am currently teaching him to shake hands with both forelegs. When I began this adventure, I did not offer any treat at all, but simply asked him to pick up his front leg on the side of his body at which I was currently standing. At the same time I was asking with a snap of my finger, I said "shake" and touched the outside of his fetlock with my boot. As SOON as he even so much as moved his foot to comply, I stopped asking and petted him and stroked his neck and said "good boy". As a creature of habit, I was soon able to just say the phrase "shake" and snap my fingers and he would start to pick up his foot. At the point where he would pick it up a couple of inches, I would reach down as soon as he lifted it and pick it up in my hand and praise him.
Now, the next step was to simply say "Poncho, shake" and have him willingly lift his foot to me and place it in my hand. From the very first successful attempt, I began giving him a reward (treat), which encouraged him to do this EVERY TIME I requested it.
The problem lies in him ANTICIPATING the treat and reaching for my hand with his teeth. When he does this, I smack him on the nose to discourage this behavior and say "NO!" The use of treats is not the problem in training. The problem is the lack of discipline when the horse exhibits a negative behavior relative to offering or expecting a treat.