Natural fly control, has many aspects and you may want to consider the following
1 - Manure and Sanitation Management - You will have less fly problems if you either remove all manure every 7 days to break their cycle or leave manure undisturbed for a season removing only in the fall after frost!
2 - Fly Parasites & Predators - Biological fly control in your manure. These tiny insect use your flies as a breeding chamber, thus killing your flies.
3 - Beneficial Nematodes - Biological fly control of larvae (maggots) in the manure pile.
4 - Sticky Fly Traps - To trap as many adults as possible in the barn.
5 - Traps - To help control breeding adults near & around barns and manure.
6 - Bird Houses - Purple Martins, Tree Swallows etc. consume large numbers of flies.