I feed my horses a prepared garlic powder through Springtime Inc. Work great. No flies, ticks ect.. I didn't even need to put up any fly strips at the barn. The horses love the garlic powder in their feed too. Will stop feeding it after the snow flies, and ticks won't be an issue. The no flies is just an added bonus for me.
I even used fly predators for years, and they were effective, but nothing like the garlic powder.