Calves need to drink their mother's milk but also should eat grass as they grow. As soon as calves are naturally weaned, it's grass time in the pasture until they are ready to be butchered. Whether calves should be slaughtered for meat is another matter. Until then, they should eat what they naturally want.
Grain-only diets for veal cows are inhumane. A cow naturally eats grass, and to deprive it of ANY of such nourishment can only be an offense against nature or at least against the cow's natural state. It has been shown that grass keeps the cows healthier, whereas when grain is fed, the cows suffer from illnesses in the gastrointestinal tract. Perhaps some grain is acceptable, however for the sake of the animal, sickness should be avoided if it can be done so at any reasonable cost. Since grass is not expensive, some of it should be fed to the cows.