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Another option is to mix small amounts of apple cider vinegar with the animal's drinking water; consuming this solution is known to change the blood pH of the horse, thus keeping horseflies away. Cider vinegar, coupled with essential oils, is also used in home sprays as the sharp odor may repeal horseflies.
Mix the ingredients in a 32 ounce spray bottle. Shake well before every use and spray the horse. The Results. Like recipe #1, above, this homemade horse fly spray worked well around the barn and house, but was of no help in the pasture for deer flies. How Long It Lasted. Also like recipe #1, above, this fly spray lasted between 45 minutes and an hour.