fly spray

Has anyone had success with Fly Formula by Feedmark, or any other in feed supplement for flies? The only thing I found on this before was archived in 2011, so.. My mare recently had to get eosinophilic granulomas (proteins lumps) injected, which were thought to be the result of a reaction to...

Does anyone use this product? A few people have recommended. I have a yearling who is getting bitten to death. I need a fly spray which works well! I do have to get some sun block to as he burns! I messaged this brand and they said horses over 3 but it seems more or less like a natural blend...