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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.
Fly sprays are basically ineffective when it comes to horse flies. The best control is obtained by trapping the female flies before they have the opportunity to bite/reproduce. This will reduce the population and make the area around your barn and pasture a much more enjoyable place for both you and your animals.