How much hay is need to feed a 100 head of cows?

For how long a period are you feeding them? And what size are these cows, if they are really "cows" at all? Either way, I'll give you an estimate.

One bovine will eat 2.5% of it's body weight, no matter if it's a cow, a bull, a steer, a heifer or a calf. Thus, one 1000 lb (454 kg) cow, with or without a calf, will eat 25 lb per day. For 100 cows, that's 2500 lbs of hay per day.