Treats (or anything fed from the hand) are one of the worst ideas that has happened to the horse industry. Few folks do well at observing their horse, even less recognize what is happening if they do observe.
Couple that with trying to teach folks to teach their horses to respect the human's space, then confuse the horse by showing it that it can come into the human space and nibble, and the treats are a recipe for an really unhealthy relationship.
It is no surprise that biting horses were so rare 50 years ago, and and horses respectful of a human's space are so rare today.
Now, don't be whacking at him with your hand or riding crop. That will teach him to duck and dodge your hand. You do NOT want your hands to be associated with punishment. You need them for handling your horse.
Don't wait to correct the horse for nibbling, because you are too little too late. Correct him with an elbow or forearm for getting into your space. A pretty good bump to the nose, then forget it. It's over. No grudges. If he does it again a few seconds later, a firmer bump to the nose with the elbow or forearm. Then forget it. It's over. Be consistent, be fair, and be insistent. Think like the lead horse. Escalate if you need.
Dumas' Grrrl observed that lots of clients here are young gals. Okay, if you are a young gal, just where would you let your prom date touch you when he comes to pick you up, as your mom and/or dad are waiting and watching? That's right. And that's what your standard for your horse should be. Respectfully waiting on her little highness at an appropriate respectful distance. You gotta insist on respect, and know respect, before you can expect the horse to accept that he must show respect. (Like that?
Okay Dumas Grrl,
Yes, we do get kudos for raising and training horses with willing manners based on respect. I agree with your comments.
Thanks for your comments.
Author of "Back Country Trail Basics"
From Western Horseman
2003 winner of AHP Best Educational Series.