add whole grain or textured feed to their diet, then get some chickens. Free eggs and they spread the manure so no fly breeding areas. Some of the feed through fly controls are hogwash, the dielectic something or other earth ones. Basically a fancy word for sharp crystals, but the fly eggs get laid after the horse poops and once it hits the ground the poop aint moving around so it wont be cutting up any fly eggs. The concept of feed through fly control is to put poison on the manure. Id rather have chickens spread it out as soon as it hits the ground. Also go with the shiney glue traps put low to the ground to catch biting fly, not the add water fly baits. All they do is lure non biting house flys in.