If you just want cattle to eat brush you may try the corrientes. I've one seen a few but the ones I saw were eating just about anything. However, they don't look like beef cattle to me. Besides they are really ugly and scrawny.
I also recently read an article on Beef Mag that said you can train your cows to eat weeks if you mix a bit of the weeds with some sweet feed. Supposedly, they will learn to eat it. I've yet to try it on my cows.