My fear at switching from a heated barn with crates/pens to outdoor farrowing was unfounded. The first full year of outdoor farrowing averaged 7.8 piglets weaned per litter. The heated farrowing barn typically weaned 8.5 to 9 piglets per litter. So it appears I lost about a piglet per litter, but that’s not the full story. I’ll list the farrowing groups below.
Month Ave. Weaned
January 5.9
March 8.3
May 8.3
July 9.3
September 8.2
November 8.0
Look at how most of the months were in that 8 range. If I take out the two litters which froze in January, the average weaned for the year is bumped up to 8.1. If I take out all the January litters, the average weaned moves to 8.4. Now we’re getting close to my indoor numbers.
And I think I can do better this year. The first thing I’m doing different is I’m not farrowing during the worst winter months. My first group will farrow the beginning of March. The other thing is I experimented with sows making their own farrowing sites. Some chose the farrowing huts I provided, some chose their own spots in the woods or pasture. All of the sows did ok, and I don’t have data, but I’m pretty sure the sows in the huts weaned an extra piglet or so compared to the independent-minded sows. I’m going to encourage more hut farrowing this year, but will try to keep data on each.
All in all, it has been an interesting and enjoyable transition to outdoor farrowing. I would have a difficult time going back to indoor farrowing as the work environment is much nicer for me and the pigs seem happier.
Curiousfarmer 
I find that our sows have to be forced to stay in a hut during warm months (May-October). In the winter they do like the shelter of the greenhouses, open sheds and huts we’ve made. But in the warm seasons they prefer building their own nests out in the shade of trees or brush along the margins of the pastures. They also like to move their nest a few days post farrowing. This leaves behind the scent of farrowing and probably naturally helps with disease and predators with pigs in the wild.