Going back to read this - I would think carefully about the horses that are getting “free choice hay” and concentrate and are still underweight.
How much hay is “free choice hay” - are they being fed together or separately? Do they tend to finish the hay or not?
Is it really “free choice” - meaning as much as they could possibly eat, or is it “hay during the day” (aka hay being put in stalls xx times/day.)
If you really have horses (perhaps older) that are getting as much hay as they could possibly eat, plus concentrate and are still underweight - they may need 6+ lbs of concentrate 2x a day. (It’s not really that much, e.g. for senior horses that can’t chew hay well.)
The key point would be making sure they are getting the right kind of concentrate, not necessarily the amount.
And yes, it might be expensive. Raising your rates might be necessary.