Others are covering this on one of the other forums I'm on.
Seems folks in Australia and Texas are using ACV (Apple Cider Vinegar). They say that you must get the "natural" stuff with the "mother" in it - not the processed kind you'd find in Food Lion, IGA, Piggly Wiggly, Safeway or Wal-Mart. I have no idea where to find it that way - but haven't really searched yet... I guess Natural Food stores? They said that each individual full size horse got about 1/2 cup at each feeding. Some also put It in their water tanks or buckets - after going thru a period of getting the horses used to it so they don't refuse to drink at all.
I have used ACV from Wal-Mart and have seen a difference in my ponies. It's been a while - I have a lot that I feed individually and I'm not ready to start adding other liquids right now... BUT then again, we too have started having problems with flies, mosquitos and bot flies with all of our rain. I DO like using a Swat type product I make myself - using Vaseline and fly spray concentrate. I take the extra large container and add 1 oz of my fly spray concentrate. Then I apply the "ointment" to their faces, ears, chests and bellies as needed. Usually don't need to do this everyday. Of course, if they rub on you - it's very greasy and will stain clothes, LOL!