Wet wrapped bales generally run approx 25% moisture. If the cows you are inquiring about have no other feed source (pasture, ect) then the would eat 15-20 lb dry matter or a little more. If these are 1000 pound bales, then one bale would feed 37 cows a day. If the bale has been innoculted, it will generally last a little longer. Other wise, in real hot and humid weather, it wnt last muh more than a couple days. It would be similar in bunk life as haylage.