Scientifically, it's kinda dumb not to allow it.  The stuff doesn't leave the digestive system and it reduces fly pressure so you don't need all kinds of pesticides to keep the girls happy.  I used to use Vigilante boluses, but that stuff will kill your developing dung beetles.  I think ClariFly is the same drug, I'd personally stay away from it.  IGR won't hurt your dung beetles.  I have good face fly  control with the Fly Killer Kover on my mineral feeder, but it doesn't help with horn flys on the back.  IGR won't help with flace flys so I think it will be a good combo.  The horn flys on the back of your cows looked pretty similar to what I started seeing with mine this summer.  Hopefully you'll at least have the option of using it.