Horses are expensive but these are my favorite products to use!
ABSOLUTE MUST HAVE if you have horses that are easily irritated or allergic to flies! Best product I have found!
Farnam Endure Horse Fly Spray, 128 oz
This is a great option if you have mouthy horses. Ours kept pulling the regular heater out by the cord and then we would have to break ice in the mornings! This also keeps the water at a great temperature!