The cows get fly tags on the ear, just because. There are rubs between pastures. About every three weeks I spray their bellies in a chute because the rubs don't get their stomachs. I did have treated minerals but when I went to replentish, the feed store was out.
The mineral treatments don't do much good for me since we have such high mineral content in the soil here. Only a select few cows touch the minerals.
There are farms around me (all sides) who do not treat for flies, so it is a bad situation. This spring was one of the worst times I have ever seen here.