My friend is trying an experiment at her barn. Some horses are getting about half a shot glass of apple cider vinegar in their morning and afternoon feed, others are not. We walked through the barn the other day and noticed that not only did her mare not have hoards of flies around her (she had been receiving vinegar for about two weeks), but they were not swarming the manure in the stall, while the horse directly across the aisle was covered in them as well as the poo (and she had not gotten any). Proof that it works??? Hmmm! I told her to start giving my boy some too!