Yes, you have accurately measured the dry matter removed per bale.  NOW.... if you get a feed test done on those bales, and use the nutrient analysis from that test, you can easily compute the pounds of P,K, S. and Ca removed and replace it with fertilizer.  The S will leach, and should probably, not be put on this fall , as it will be gone next year for the most part.  It along with boron, should be put on early in the growing season, imo.