It takes a very nice hut and a lot of bedding to raise pigs without supplemental heat. They will survive in the coldest weather if they have a dry,draft free hut with enough straw they can get completely covered up. Once a year would be an excellent addition to a crop farm. My pigs that are farrowed in late April make about the right size to hog down corn. If you were going to farrow only once a year you could move that back to late May. Farrow on pasture.
Bare dirt is either mud or dust.
Dust and mud don't make very healthy environments for animals. Self feed the sows for lower labor requirements especially if they have big litters. A good legume pasture will save a lot on the feed bill.