First off I want to let you know that I raise cattle and have been around them my entire life. So based on that experience I can tell you it is not going to be easy. The cheapest way is going to be an electric fence. You may need a solar powered fencer. Any scent that will repel cattle is going to scare deer to much to use the food plot.

I have got electric, barbed wire, board fence and woven wire around my pastures and I can tell you if they ever get a taste of your food plot, they will try all kinds of tricks to get back to it.

sorry not to be of much help here, maybe some these other guys will have some ideas.