It does make it interesting and pondersome that so many horses have picked up this bad habit at the barn at the same time..... Maybe talk to the other owners of the horses that have also started biting to see if you can come up with any one changing factor that all the horses have had -- if you don't think it is a worker, anyway.
As far as fixing the biting .... let's think about what horses do to each other in the wild. What does the stallion do to the mare that gets "out of line" in his herd? He either kicks her or bites her. Done. End of story. Discipline.
So, that's virtually what I employ with any sort of dangerous behavior including biting, kicking, etc.
1) You need to really pay attention and watch your horse. You know he is biting now so really look for the signs he's about to, so that you can deliver quick discipline.
2) If he tries to bite you (I don't care if he even gets you or not. If he starts to and tries to, he needs to be shown that is not acceptable), you need to make him think his world is going to end for the next 3 seconds. And you must react within 3 seconds or he will not connect the discipline to the action of bitin. This is why its important to be watching him so you can act right away if he tries to bite.
3) I have no problem punching or smacking a horse in the mouth, or hitting them with whatever I have in my hand (no sharp objects of course
). If you bite me buddy, you can be sure "the world is going to end". A quick, swift discipline to the face only when it is warranted will not create a head shy horse. And hitting on the neck or shoulder doesn't mean a darn thing to the horse and they certainly won't correspond it to the bite. They are biting from the mouth, so you need to "bite back" at their mouth.
4) So you need to react within 3 seconds of the bite, and you need to carry it out for a full 3 seconds. No more; no less. Charge at their head, punch/slap/smack them in the mouth, yell, wave your arms --- just do whatever you need to do to make that horse think they are going to die in that 3 seconds.
5) After 3 seconds of discipline, go back to what you were doing as if nothing has happened.
Now, I certainly don't promote beating the horse with a metal bat for a full 3 seconds ..... but a couple hits in the mouth and voice/body language is plenty to make them think "uh-oh ... that was the wrong thing to do".
This method works for me.
None of my horses bite.
None of my horses are face or head shy.
I hand feed treats to my horses all the time.
They know their respect boundaries, and they know not to cross them.
Like I said, that's what works for me and my horses. You may find something different that works for you. The water bottle is interesting and if it works, fine. As long as the dangerous behavior stops!
We have a little mare quite a few years ago that was a bit grabber. So every time she'd try to get the shank of the bit in her mouth to play with it, the rider would reach up and grab her left ear (that's usually the side she grabbed the bit). She hated
that. And promptly stopped playing with the bit because it always resulted in her ear getting grabbed.