The females have large, piercing-sucking mouth parts which inflict painful wounds. The males are harmless flower feeders. Biting is usually confined to the back with only a few flies on a cow at one time. They attack cattle only during the day and are most active on hot days. See deer fly.
Good sanitation is required if house and stable fly population are to be kept at a minimum.
Weekly cleanout of calf pens and box stalls is a must. Don't forget the accumulated feed, especially under drinking cups. Flies can and will breed in manure that accumulates in exercise yards around feed bunks, fence lines and any place not trampled by cattle. If cattle are kept in the exercise yard during the summer, you will have more fly problems than if cattle are pastured away from farm buildings manure handling and feed storage facilities will require special attention. Health and milk quality control officials specify how often gutters should be cleaned.