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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.
Basic Recipe. Make a homemade horsefly repellent by mixing a pint of white vinegar with 2 ounces baby oil and a squirt of dish soap in a spray bottle. Apply to skin or clothing when outdoors near areas of horsefly activity.