Yeah. Cattle like variety in their diets. A few years ago a determined steer got through the fence protecting some young royal palms and was nibbling fronds. The wife saw it and ran at him screaming, waving her arms wildly, and slapping the 1000 lb beast. The poor guy was terrified and leaped clean over the fence. Ever after that day, even as he grew to be 1800 lb, when my wife was around him you could see him thinking "be careful not to piss off the crazy lady" and he was very well behaved.
Guards around trees are important not only to keep cattle from eating trees but also because they put a lot of weight into their small hoof-prints, and that can damage the roots of many trees. Try to keep the cattle at least 3 to 4 feet away from the trunk.