A lot of people do feed their bucks alfalfa with no problems. Some people says it causes them to get urinary stones. That only happens if their calcium/phosphorus ratio is way off.
I just feed my bucks a timothy hay. In fact, everyone gets the same hay. The bucks get a little grain, though that is mostly to make them go running for their stalls rather than a big need for it. They also get free choice loose minerals (everyone does).