The key is keeping the pigs dry in the cold. You're south of us so I think you should be fine. If anything your issue is likely to be that you're warmer yet cold while being wetter than it is here. The deep cold of winter is easier for us than the mud season on either end, especially spring mud when the mountain drains off the blanket of snow.
As you note, we raise pigs all year round and our sows (44 now) farrow right through the winter here in Vermont. We are in the northern part of the state in the mountains. We get deep snows and down to -20°F regularly with down to -45°F sometimes. No barns. Just simple sheds to block the wind and lots of hay.
Big thing is to keep their beds dry. Let me repeat that, keep the bedding dry. If you get rain in the winter then you want a shed roof. Ditch, swale and slope the ground to make water stay out of the shed. Plan the pitch of the roof to take the water away from the animals and bedding. Raise the bedding area up above the surrounding area. Start bedding earlier than you think so you build a deep pack. Use more hay in the wet time. Allow enough hay outside the shed for the pigs to clean off. They go into the mud, come back and wipe themselves on the bedding bringing in mud and moisture. Add hay regularly. I like the round bales but if I didn't have a tractor I would use square bales to be able to move them more easily. In the fall stack the hay close to where you'll use it and under cover unless it is wrapped round bales.
The pigs do eat the hay as well as bedding down in it. This is why we use hay instead of wood chips or shavings which have no food value and pack densely. The hay they fluff up, make nests and crawl under during cold nights. It's a quilt.
We don't use heat lamps, crating, stalls or anything fancy. They do great.
All that said, the warm season is easier - I just wouldn't miss out on the winter. Almost half the year around here and I can't garden then.
Cheers
-Walter
Sugar Mountain Farm
in the mountains of Vermont
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