My take on yearling bulls is that they can handle a bit of feed. In my situation they are weaned in early autumn and while there is plenty of grass stockpiled over winter it is very low quality so feed is essential to get them looking like a usefull bull in spring. I have been using yearlings myself for over 10 years, I have been renting one of my yearlings to my neighbour for about 8 years and I usually let him have him on the 1st of Sept. which in my opinion is too early as the grasses are slow coming on and it is tough on them but the bull always seems to deal with it, becomes very fit looking but holds his condition surprisingly well. I let my neighbour have one I may use myself the following year and have the genomics and parent verification done on him so that if he jumps the fence and comes in with my cows as he did last year I know the sire of the resulting calf. I have also sold quite a few yearling bulls and some have had a workload greater than I would have liked but they seem to deal with it.
I think feed to develop them and then putting them to work while they are still growing helps with their development and soundness. Bulls kept and fed and then sold as 2 yr olds as virgin bulls on the other hand I think is detrimental to their development especially if they are pushed to get to those high weights to become the heaviest in the sale.
Ken