This needs to be a multi pronged attack. First level and most important must be Manure control, it needs to be spread out and dried. Picking it up and throwing it in a pile doesnt do any good you are just creating a large breeding area instead of lots of little ones. A chain drag harrow works well , drag the field every couple days. What works even better is chickens, feed your horses a textured feed or add a small handful of whole barely grain to the feed. The grain passes through and the chickens come running as soon as the poo hits the ground. Scatter it every where to get the seeds.
Second line is spalding lab fly predators. Swear by them they are well worth the money. They work even better if you can get your neighbors using them.
Third, fly traps,,, but the right kind of fly traps, those scented add water lure type traps do just that LURE flys. Yeh they get full of flys and you think they arer working but they don't. They are drawing in flys from other areas, and the flys they are drawing in are not the ankle biting ones we have problems with , you need these traps, Starbar Bite Free Stable Fly Trap - Horse.com
they work great hammer a stake in the ground and put them about 2 foot off the groud in an open sunny area near the barn, I have a line of three or four. The catch the biting flys and some of the deer flys as well, be advised you may catch a bird so don't freek out just unstick it and let it go.