Good mineral program, and we give our heifers additional grain with the hay.
We feed them a rate of 1% of thier weight,  a good energy feed like ground ear corn with 15% of it distillers.
Don't know what your hay tests at, you have to make the call there.

Best thing IMOP is green grass and sunshine, but around here, that is not an
option in early March.
Heifers have a big task, feed a calf, get in condition to breed back and all
while they are still growing up themselves.

BTW I noticed your new to Ag talk--
Welcome Aboard


edit to clarify supplement feed, sure I'm still clear as mud :)