I don't raise bulls but I have been custom feeding yearling bulls for a few years.  They get the same ration that is limit fed to my yearling heifers but they get all they want to eat.  It is 93% forage and 7% grain. They have been gaining somewhere around 2.5# per day and the most corn they ever get is less than 4# a day.  Seems like they have the ability to keep gaining on grass instead of stalling out like fat bulls do.  I'm no grass fed guru but it seems like the sensible way to develop bulls.