Our situation may be a little different than some as our bulls spend more "down time" during the year than they do breeding in the pasture - usually 4 months in the pasture and 8 months away from the cows as we like to keep our calving window fairly tight in the spring and like to do the bulk of our calving in March and April. For the most part during his down time all he'll get is grass in the lots we put him in to graze until the winter when we start to put hay out then he maybe gets a few pounds of cracked corn with the cows close to calving in the maternity lot early in the year during calving season. That's enough down time he should be able to recover without needing to put him on a feed ration program as you don't want to put too much condition on your bulls as it could affect their fertility if they get too fat. The only times we maybe supplement a little extra feed to a bull after pulling him from the pasture is when using a yearling or 2 year old as sometimes a younger bull can come out of the pasture looking a little rough, especially if it's a hot and dry summer plus the fescue in our summer pasture can be rough on all our cattle in general some years. But in general we try to let our bulls gain back the weight they lost during breeding season mostly by grazing and hay in the late fall and winter.
Now when holding over weaned bulls to develop for sale or to use ourselves as yearlings we will separate them off from the rest of the calves around Dec. 1 and put them on their own feed program where much like Black and Good says he does, we start them at around 1-1.5% of their body weight and gradually work them up to 2% on a feed ration and free choice hay then start to back them off that ration closer to breeding season. It's a fine line to walk as the saying is "fat sells" but I don't think you should get too carried away with trying to get to a certain weight at a mature age rather than what kind of condition the bull is carrying. A fat bull may look good but a leaner bull may be more fertile and sound for breeding season. Just like our replacement heifers, we back them down on feed as it gets warmer out and usually then only let them have hay and whatever grass is starting to grow and turn them out with the mature cows a few weeks before breeding season so they don't get too fat and get their fighting out of the way before we have to move them for the summer.