Here in the Netherlands, cows only go into the fields half of the year, simply because during winter time it is too cold for them to be outside with no protection against wind and rain.
It might no longer be true for the cows that have been breed for producing lots of milk. I guess most of them might still be able to survive outside during the winter, but I suspect that their milk production will be strongly affected.
It depends on the breed for sure, but as a rule of thumb when you're talking about the 40-55 latitude band, they're more comfortable/gain weight easier in the winter when it's cold outside than in the summer when it's hot outside.
