If you want to get a fly problem under control try
Fly Parasites. My wife and I had a feed lot for about 10 years before returning to Houston, we would have up to 30 bulls on the 5 acre lot and the flys would get atrocious in the summer until we found the site that is linked
here. You buy a bag of pupa/eggs from them, I always bought 2, and put them where the problem is, about a week after placing them there wasn't a fly to be found of any type and we never saw the predators, they are very small. No poison nor traps, we also use to buy
preying mantis egg sacks from them as well to put around the house to control grass hoppers, crickets and wood roaches, works like a champ. Anytime I can avoid chemicals and poisons I do.