The rumen in a calf is not fully developed until a couple of weeks after birth.

I think your info is off a little.  We don't introduce hay until 4 months of age since it takes 4 months for the rumen to develop sufficient capacity to maximize the potential of hay.  At 6 weeks of age, the rumen is developed enough  to wean the calf and sustain and grow the calf on grain.  I you feed hay you will be holding back the calf until it reaches 4 months of age and grows the rumen large enough to support the hay intake necessary to maximize growth.  It is all about energy intake and you can barely get enough dry matter intake in a 6 week old calf to satisfy the maintenance needs on hay alone, let alone grow the calf.