I use fly predators for my horses, don't even need to spray them unless I take them on a trail ride somewhere outside the pasture or something. Fly predators are shipped to you, they're smaller than gnats and don't bite you or your horses and are actually a species of wasp. You place the larvae on horse droppings in your pasture and when they mature they lay eggs on fly maggots and their larvae eats the maggots. No flies in my pasture at all! Great alternative to fly spray, cause I have yet to find a product that keeps the bigger varieties at bay.