Go to Arbico.com they have larvae that will eat flies before they hatch, you provide them with the acreage you want to be covered and the area you're in and they send it to you monthly. I used them for a few years for my horses this was fabulous it really reduced the fly population:
http://www.arbico-organics.com/
Also how about fly mask they are normally sold for horses they come in a multitude of size so you should find a size that fits your cow. Try Jeffers catalogue online
http://www.jefferspet.com/fly-masks/camid/EQU/cc/2251/c2c/sc/ 
Et voila
Sylvia
