There's big differences per farm. According to the CBS about 75% of farms let their cows graze outside. The dairy industry has published an intention to get this number up to 80% of cows being outside on the field for at least 6 hours.
I haven't been on a farm in 20 years, I have no idea if farmers rotate so they can have 200 cows on a 100 cow field each day. I imagine they wouldn't bother with making cows graze at night.
