The biggest enemy to outdoor cattle is wet and wind together. They can be wet, and they can be cold, and they'll be fine. But if they're soaked and it's windy, then they can't keep their heat because the wind sucks it away. So you need to either keep them dry or out of the wind, or both, with some sort of shelter or lean to or windbreak.
The digestion/fermentation of dry matter in a ruminent keeps the metabolism up in the animal, and that creates heat. That's why you feed more hay when it's really cold. I'm in Ohio and mine get an extra flake apiece when it gets frigid or it's wet/windy.
Watch your cow's weight. If she's getting thin, then she needs more food because she's burning all her calories staying warm and making milk. Her milk production will also go down if she's burning hay to stay warm. Just adjust as needed.
~red
