Problem with the anti-bloat stuff is it needs to be in them BEFORE you turn out. At least a week before. Doesn't sound like you have a bloat problem, though. One thing, don't buy bloat blocks. Cattle can't get enough of anything in a block. They get tired of licking and leave before they get what they need. Many companies, ours included, will not make blocks because of this. Loose is much, much better. (Sorry, Jersey Lilly. Don't mean to go against what you said, but if you used bloat blocks and they worked, you most likely didn't have a problem anyway.)
The hay or straw will also provide dry matter, which is probably lacking.
Can you get by feeding 200 lb. calves just forage? They don't have much of a rumen at this point and I would think you would need to supplement them with something. I don't know that for a fact, understand. It just stands to reason they would need something they could eat less of that had more nutrition in it. As I say, I'm not sure about this. It will be interesing to see what others have to say. 200 lbs. isn't very big so they probably aren't very old. Yep, I would think they need more than just forage until they get some age on them. FWIW