As much as I would love to keep the flies away from my horses, I'm not sharing my garlic!
I've heard some anecdotal evidence that it helps with flies, and a couple other things, though for the life of me, I can't think what they were right now. But I've also heard just as much anecdotal evidence that it doesn't do anything for them.
So my position is, I'm not going to feed it, especially in light of the damage that it COULD cause. Plus, like I said, I don't want to share my garlic.
Learning never stops