I have 7, one month old calves on 3 nurse cows that are already eating some grain. They are going in the feeder with their mommas, and after I turn the cows out, I put grain in there so they can pick at it when they want. This evening I turned the cows out, got a couple lbs of "sweet feed", textured type, and put it in the feeders and 3 calves went right in to get some. I haven't put any in their mouths, but again, they are mimicking the cows. They have some real "bladey" nice orchard grass hay and a couple will pull a little out and chew on it. Since they are on the nurse cows, they don't get the "hay-belly" but I like them to have it if they want it. I have found that there is a fine line to tread with high grain intake and no hay. I have seen some scour type manure and calves will sometimes actually go off feed. I believe it is acidosis, but I am not a nutritionist. I think the little bit of hay they consume helps to keep their manure firmer and gut bacteria more balanced.
Again, I am not a nutritionist. I don't believe in pushing the hay with little or no grain. I also think they tend to not get so bored with some hay to chew on, especially in small pens. But I like the way the calves look when they get milk a little longer, and have watched over the years the different baby calf rearing techniques of farmers. Milk replacer is expensive, but I am willing to spend a little more to see the calf looking sleek and well rounded. Most of the registered dairy farmers that I milk test for, that do some showing, do keep their calves on milk for up to 8-12 weeks. I also look at our beef cows, and figure that if mother nature provides for that calf to be able to nurse 5-8 months, then my feeding milk for 8 plus weeks while getting a bottle calf going good on grain isn't all wrong.