This should help you http://www.cattlenetwork.com/cattle-news/Feeding-Hay--Do-you-have-enough-183792561.html
We feed 35 head of cow plus 35 calves and one bull 180 round 4x5 bales over 90 days so you will need a crap load.
I'm probably going to have to buy some then because I don't have enough. My heard is 130 and I only have a couple hundred bales