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Use vinegar as a natural fly spray for your horse. You can use 2 cups of vinegar with 1 cup of water and 1 cup of Avon Skin So Soft bath oil mixed with 1 tbsp. of Eucalyptus oil. You can also add a tbsp. or two of citronella oil to this mixture. Shake it well and then spray on your horse.
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.