Scrub the buckets (or waterers.) We don’t have automatic waterers, we just use two buckets in each stall. But either way, at most barns I’ve been in they maybe give the buckets a rinse and then fill them back up. Even after one day, they start to feel slimy, and the flies love it. Horses drop bits of grain and hay in there every day. So we actually scrub the buckets out before refilling with clean water.
Manure gets dumped in a trailer and removed once a week. Luckily for me, my dad composts all of ours on his cow farm. A muck pile close to the barn invites way too many flies. And spreading it is a great option, but it needs to be far away from the “action” of the barn.

We have a Shoofly automatic system in the barn. I purposefully didn’t put this as first on the list, because while it’s great, the other things are more important. We schedule ours to go off only a few times a day, when activity is least in the barn, because I don’t want people or horses getting sprayed with that too much.
The aisle is swept or blown regularly. Same idea here – if there’s nothing for them to eat, they’ll fly somewhere else. Little bit of manure, junk from picking out their feet, tiny bits of feed dropped, all of these are heaven for flies! So it’s got to be clean.
Small paddocks are picked and big pastures are chain dragged regularly.
I’ve tried fly predators, feed supplements, hanging bags/strips, every fly spray ever made, pretty much all the things. What actually works is the thing most people don’t want to do/don’t have time to do, which is really keep it clean. Sorry if you were looking for an easier answer. 😜
– Eliza Sydnor Romm