You might also try pyrethrin. Pyrethrins are natural insecticides produced by certain species of the chrysanthemum plant. You can normally buy a 10% solution of this at local feed stores pretty cheap. Dilute with water and you can "safely" spray most anything or anybody.
Provided you don't get a lot of rain, a good coat of this on a cow will give you 3 - 4 weeks with no flies.