Betsy Ross at
sprays compost tea on her pasture and on her cattle and it seems to keep the flies away.
Do you have plenty of dung beetles hauling the manure away? That helps.
Joel Salatin, the famous Virginia farmer and farm writer, started culling his animals that had flies. He noticed that some of his animals had more flies than others and some had none at all while the rest were being bothered. It sounds harsh, but if they're all going to slaughter anyway...at least he no longer has flies. First he culled for ability to give unassisted live birth, then for fertility, then for health (including health of offspring), then for disease/parasites, then for flies, then for weight performance. Basically he has his own breed of cattle going. How would you like to have an entire herd with no worms, no flies, good weight gain, and the cows give birth every spring, by themselves, like clockwork?