Start With Water
Thoroughly clean out a spray bottle and rinse it many times. You might eventually spray water around the horse's face, so make sure no residue remains of whatever may have been in the bottle you're using.
Fill the spray bottle with warm water and turn the nozzle to the soft mist setting. You might have handy a two-liter soda bottle, also filled with warm water, to refill your sprayer.
Begin by positioning the horse and asking him to drop his head. Then spray one or two quick sprays in the air away from the horse, about at the level of his shoulder. Pet the horse and stand quietly a moment, letting him know that's all you wanted him to do.
If the horse startles, as he probably will if he worries about fly spray, then concentrate on asking him to drop his head, to move his hips or whatever exercise you choose. You want to make the exercise more important than the spray. You're not going to discipline the horse or fault him for getting upset about the spray - just build his confidence that he can stand quietly, even when you're spraying into the air.
If the horse pulls back, he'll put tension on the rein or lead. Continue to hold that tension until he brings his head forward or drops it slightly, giving to the pressure. If he steps back or pulls hard, you may have to move his hips to get him to quit pulling and to give to the pressure. If that happens, then forget about spraying until you can get the horse to give to pressure easily.
Continue to work with the horse until you can spray the air without the horse objecting. Then position the horse and turn the sprayer toward him. Spray twice into the air (as you've been doing), then once onto his shoulder, then once into the air, as before. The idea is that the spray is gone before the horse can react.
Remember, though, the focus is on obeying your cues, not the spray. So you have to give a cue first. It's normal for people to pay attention to spraying and react when the horse moves away. Be sure that the horse feels rewarded for obeying your cues.
If you've told him to drop his head, release the rein, but don't take advantage of his good nature by spraying him endlessly. A few sprays at a time teaches him you're not using the cues, and particularly the "head down" or " calm down " cue, as a means of trickery.
Build on this process until you can eventually spray the horse's legs, then body, then neck. When the horse is comfortable with all of that, you can spray the air beside his head.
In real life, you're not going to spray the horse's head with fly spray. You'll likely apply it with a cloth so that you don't get spray in his eyes. But for the purposes of this lesson, you can continue to work with the horse until you can spray his head with water. That will come in handy should you want to let water run over his head, such as when rinsing after a bath.
Don't expect your horse to be a robot, anymore than you would if you were the one getting sprayed. You want to build his confidence that the spray won't hurt him. So you have to find a balance between being a taskmaster and having him obey your cues.
