Sodfarmer... it's been proven that flies are responsible for a large portion of mastitis cases in young heifers, especially prior to breeding. I don't remember the statistics, but I did read an article where researchers tied fly tags to the tails of pastured open dairy heifers, and it significantly reduced the cases of mastitis compared to a control group without fly tags. Something like 20% in one group vs. 80% in the other group.