A good mineral and good grass hay if they don't have anything to pick around on if its only going to be a couple of weeks. No use in spending anything that you don't have to now. If they back up some, you'll get it back pretty quick on grass. This assumes that its your grass and not a gain deal. Also assumes that you're not talking about a very long ride for them. Just my opinion, but costly inputs such as those out of a sack remain on the wrong side of the balance sheet until they're hanging in a cold place.
I'm only responding again to bump this back up to the top for you. I expect that everybody who posts here has a different opinion about what to do in your situation. Sometimes we do things differently with different sets of calves in the same year. Maybe one of the members will have a better idea that will fit your program better. But with just two weeks till turnout, I'd stick with the grass hay and not change rumen activity. They'll catch up on protein when they put their heads down in green. But let's bump it up and see what some of the folks from the Plains have to say.