One home remedy to keep flies off of horses is to place garlic powder in the animal's feed or water. The sulfur that occurs naturally in garlic helps repel flies, as does the strong odor it produces. Garlic also helps create an alkaline pH in the body that repels flies. This is because flies are drawn to acidic environments.
Studies have shown that feeding horses too much garlic can be harmful, but using smaller amounts is considered safe. For this reason, it is recommended that horses be given specially designed equine garlic supplement instead of cloves of fresh garlic.
Another solution involves distilled white vinegar or regular apple cider vinegar mixed with water in a 1:1 ratio and used as a spray to repel flies. Apple cider vinegar can also be added to the animal's food or drinking water. When adding the vinegar to food, use 2 to 4 ounces daily. When adding it to water, use 1 cup of vinegar for every 20 gallons of water. The use of vinegar is similar to the use of garlic, as it also creates an alkaline pH in the horse's body.
Essential oils, such as citronella, geranium, eucalyptus and lemongrass, can be diluted with water, oil or lotion and sprayed over the horse's skin. Flies are repelled because they find these scents unpleasant. Other essential oils, including thyme, lavender, tea tree, pine and clove, can also be used.