We had an almost identical situation last year. A apple orchard that had been untouched for 15 years. We couldn't walk from one end to the other with multi-flora roses, wild blackberry, grass and weeks. Today, it's all gone and the property looks like we mow it every week, except for the thistle. We have a 5 foot browse line and if we duck can actually see more than 100 feet now.
Make sure they are well fed to start so they don't inadvertently eat something poison, and if it's not too difficult to set up some temporary cross fencing I'd keep them in a section until they mow it down short, or else they'll skip some weeds and things that they could eat but won't because they'll just graze the good stuff. We intentionally overstocked the property to get it down more quickly, and will only now keep the easy keepers.
If the fencing is good enough some goats may help with what the cattle don't eat.