I always thought that dried hay was considered to be 10% moisture. If so, it would be 1285 x .85 / .90 = 1213 lb/bale.  Multiply # of bales per acre x 1213 lb/bale and you have lbs of alfalfa per acre for that cutting.  Divide that number by 2000 lbs (= 1 ton), and you have tons/acre. 

So, how many cuttings will you have?  Do that for each cutting, total them up, and you will have tons/acre.  Yeah, it's that easy.