Wow, my cousin works in dairy management (travels from farm to farm) and even she'd need more information than number of cows and winter. Like, what else is are the girls getting, any corn or other feed? What type of hay are you planning to feed? Less nutritive types of hay mean you need to feed more.
How many are currently milking? Pregnant? Dry? For those milking, how many pounds of milk are they giving on average? That all goes into the feed calculations to determine the amounts that should be fed, as milking and pregnancy take additional energy.
Another question, is why are you asking here? I mean, yea we have a subforum for livestock here, but this is your first post, which is rather odd. BYC has a sistersite, Backyardherds. That would be a better place to ask a dairy management question.