Bulls need a high roughage diet with more protein than fat. If you get bulls too fat, it hurts them in several ways but your primary risk is decreased long-term fertility. In all honesty, you'd probably do just fine feeding them good hay with a a few pounds of cubes a day.
Never feed a bull a finisher ration (i.e. lots of corn). Sometimes we used a developer ration (about half of what we'd feed a steer) plus free access to round bales and grass.
I've asked the question about why does fat hurt and the technical answer is that fat causes the bulls to have a higher internal body temperature, particularly when its deposited in the testicles, which hurts the production of sperm. DL or someone may can elaborate on this, but short answer is too much fat on a bull is very bad.