Flies are easier to keep under control if you can eliminate or reduce the things that attract them to begin with. Starting with adequate and effective manuer managment. Eggs are passed into the soil through manuer which then hatch and the larve are then ingested while grazing. Eggs, larve seem to adhere closer to the ground ~ so don't cut pasture too close to the ground, that way horses aren't grazing next to the ground and lessening the likelyhood of them taking in larve. Spreading manuer over the whole pasture contaminates the whole pasture to eggs/larve.
Look into ways to compost manuer if possible or dispose of in other ways .. ie hauling it away or better yet, having someone else haul it off
Chickens are said to be great bug and tick getters..