some people swear by it. I dont, I didnt see any difference at all. The best fly control I have found is Farnams "Bite Free" its about 20 bucks a bottle, but you dont have to spray it on everyday and it works on ticks too. Which is nice because we live in the south. I have heard really great things about those fly predators too. But I havent tried them yet. Best thing to do is pick up the poo as soon as you can and put it far far away from the barn.....