I read that those big horse flies breed in/near water. We have a fairly large watershed lake across the road and a lagoon type sewage system. Any water bred flies/mosquitoes are impossible to deal with other than with a repellent on the horse.
I mix up a sort of non-chemical daily fly spray that has ACV and Avon's Skin So Soft mixed up with water. I also add a few drops of citronella, pennyroyal and tea tree oil. I keep it in a large pump type bottle sprayer. It doesn't last long so again its a daily application.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France