I think it was in statistics class in college where I first heard the saying, "There's lies, damn lies, and there's statistics." When you have horses you have to modify that a bit to, "There's Flies, Damn Flies and then there's MORE Damn Flies."
I think everyone that's owned a horse has tried multitudes of different sprays, supplements, fly sheets and fly masks. Of course, almost all work, but it truly does depend on how you define work.
Most of the sprays will work to a degree, for a while, but either the spray wears off relatively quickly or fly spray is an acquired taste for the flies and it takes a bit for the word to get out around fly town.
Supplements may work. I don't' know. However, a garlic supplement seems more like your seasoning the horse for the flies not deterring them.
Fly sheets and Fly Masks are more of an armor type of deterrent. However, the horses tend to look like futuristic alien rocks stars with the fly sheets on. Additionally, I don't know how many people I have had to explain, "No we don't blind fold our horses."
Most of our horses are fine with fly masks. However, Bobbie seems to get a form of road rage or "fly mask rage" when he has one on. Drivers seem to feel protected when they're in their little cars, behind their wheels and normally nice folks will flip the bird or swear at you like you've stolen one their young. When Bobby gets his fly mask on, our tough little pony, suddenly is not only tough, he's invincible. He flips the bird to our larger thoroughbreds and our Hanoverian by gnashing at them with his teeth, chasing and terrorizing them as he rules the field with an iron hoof. The other horses bolt around the field with the fear of god in the eyes.
We're going to try the garlic on Bobbie this year.