It's not a secret that farms - and all of their products and byproducts tend to attract flies. Horse flies, black flies, biting flies - there are many different kinds of flies. And yes, the flies are irritating and just gross. The old fashioned fly tape works to a degree and the hanging bag fly traps also work - but only on adult flies - there's nothing that's non-chemical that stops the generations of new flies and their offspring.
I've seen the advertisements for Fly Predators in various catalogs but am always wary of ads. I asked our vet if he had any info on them - if anyone he knows uses them, likes or dislikes, etc. - and he said that they got Fly Predators a few years ago and they made a huge difference the first year. So if the vet recommends with no vetted interest then it might be worth a try.
How does it work? The company - after however much research - figures out how many fly predators - insects that specifically eat fly larvae (only certain kinds of flies - but the usual fly suspects on farms) - to send you based on the number and kinds of animals you have, how large a space you have, where you live (seasonality/climate) and what kinds of buildings you have. The company tells you how many Fly Predators you should have per month. At first, it seems a bit pricey and in a strange way, luxurious - to have insects shipped to you on a monthly basis for about 8 months of the year - but the immediate difference in the fly population has been startling. It's almost as if we're missing the flies - something uncanny is going on if there're not flies bothering you when you pick eggs from the coop or restack hay that's been knocked over. The Fly Predators are truly money well spent. Yes, if you visit, you'll see some of those hanging fly traps outside to catch the random adult flies (who must come in from someone else's farm... ).