I feed a very scientifically measured 4-5 glugs from a full gallon jug.
It is about 60cc once a day.
I had one horse who was exceptionally attractive to biting flies. In the spring when the blackflies come out the horses all get a thick welt at their neck and chest junction where the flies can get through the hair as the horses graze. My guy would get two or three welts. ACV started at least ten days before blackfly season meant he’d get one welt like a normal horse. I don’t think it is a cure all, but if it will reduce bites by a percentage I may not notice, but the horse appreciates it.
I don’t notice a difference with either of my current horses. Except last summer one of them was having eating issues and didn’t get his daily ACV. He was bothered and bitten more than my other horse who did get his ACV, and usually it is the other way around.