I saw this topic in Paul's daily-ish email and thought I would throw in my two cents. I even created an account just to post this, but I have been wanting to make one for a long time.
This is by no means a long-term solution, like breeding fly resistance out of the cow, but it could work short-term on localized areas. You could try Four Thieves Vinegar. There are various recipes out there, but the one I use is:
2 quarts of apple cider vinegar
2 tablespoons each, dried rosemary, sage, lavender, wormwood, peppermint, and 2 tablespoons of fresh, minced garlic.
You will need 2- glass quart mason jars with lids. Divide the herbs and garlic evenly into the jars, and slowly pour the vinegar over the herbs.
Cap the jars and steep the Four Thieves mixture in a cool, dark area for 30 days.
Strain out the used herbs and add them to a compost heap. Return the strained liquid to the jars and cap them.
Store the jars in a cool, dark place. Properly stored, the Four Thieves will last for years.
Use it to repel fleas, ticks, flies, crawling insects. Dampen a washcloth with water and add 2 tablespoons of Four Thieves to the cloth.
Wipe down horses, CATTLE, dogs, goats, to repel fleas and flies.
Four Thieves is good for spraying socks and pants while outdoors or camping.
Fill a spray bottle with 1/2 cup of the Four Thieves mixture, add warm water, shake well, and spray exposed areas like arms and legs to repel biting insects.
Four Thieves can be used on mosquito bites to prevent itching. Spray the mixture around window sills and picnic tables.
Spray 30 minutes on grassy areas before cookouts and outdoor events.
Taken from : http://thehomesteadsurvival.com/legend-thieves-vinegar/#.U6DdOiiMUYJ
People report Four Thieves working to keep flies off their harvested deer and wild game while they are hanging up in the garage and butchering it. It's worth a try and, if it doesn't work for your cattle, you could use it as a cleaning solution (dilute it 1:1 with water), liniment, or bug spray (as stated above) for yourself