Research has shown it is best to keep them at the same growth curve. You don't want them gaining 2.5 - 3#/head/day then put them on a grass diet, gaining 1#/hd/day.
That effects their marbling. Calves raised on the dam, weaned and fed to keep growth consistent will marble better than an up and down diet.
If you plan on feeding to finish, why yoyo their diet from grain to roughage back to grain?
Like Joe said, every farm and circumstances are different.
If your goal is simply to finish out at 1200#, continue feeding grain. They need about a 14% protein diet at that weight. About 12% by 800#. 3% of their BW is a good goal.
As Joe mentioned, I feed whole shell corn and a protein pellet. Yes, you will see the corn in the manure. WSC is about 8% less efficient than all the fancy processed ways you can buy corn. So, if you can buy WSC for 8% less, you are better off. I would use WSC even if I didn't save $ because it's healthier. Less chance of accidosis and calves like granular feed better than powdery feed. The whole reason feed mills came up with sweet feed, was to make ground feed more palatable to cattle. Cattle don't like eating dust.
I take weaned steers, start them at 1% bw with corn and protein. Each week keep increasing amount until at full feed, twice a day.
I have Simmentals, so probably a bit heavier. But, I finish at around 1200# at 12 to 13 months old and grade choice.
