I've used feed through fly control. It works. It doesn't get all of them, but as for normal flies, I did see a big difference. Keeping clean is really important, my paddock/shed gets cleaned twice a day.
We're having a mosquito problem paired with a highly allergic horse (hundreds of hairless spots on chest, neck, girth area and face, who bites herself bloody wherever she can reach. The only spray who works longer than a couple of hours is Mosquito Halt. I spray in the evening when its starting to get dark and they're good until next day about mid afternoon.
Horseflies are less of a problem (no breeding grounds, I.e. Woods here. There are two B52-type horseflies coming about lunch time, but they just circle the horses and fly off, rather frustrated