Not sure if you have cedar but ticks HATE cedar so that is a good natural fly repellent.
I also use bats and ducks. Bats eat their weight in bugs per night so they really knock down the population quickly. A few
bat houses placed advantageously as they are very finicky where their homes are placed may bode well for you. Ducks do well eating bugs and insects as well. Together they do two things, get insects both during the day and during the night. Unfortunately bats fly so they get the bugs flying higher, and ducks are limited to the first two feet of the ground so insets down low at night are still an issue, and bugs up higher during the day is a problem, but for me and my sheep, the duck/bat strategy has worked to some degree. It is better than nothing.
I am assuming you are placing
DE upon the soil where the cows are treading, or lime to break down the manure? That goes a long ways as well.