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We keep our boars and sows together right through farrowing in the warm seasons. In the winter we provide the sows with stalls they can go into and defend so other pigs can not enter - this mimics their seeking out private farrowing spaces on the margins of the pastures during the warm seasons. The boars do not hurt the piglets.
Realize we are not penning at all. Our pigs our out on pasture. We have about 400 pigs on about 70 acres of pasture doing managed rotational grazing. They're divided into herds. At any one time there are 50 to 200 pigs on three to ten acre paddocks. For example, the north field which is about eight to ten acres has about sixty sows and a breeder boar right now. There are many piglets in there in addition to the adults. This extensive system where there is plenty of room for everyone works.
In an intensive penned situation I would expect it to be different. If you pen the pigs then I would suggest separating the farrowing sow from other pigs so the piglets have a chance to get up and able. Normally on pasture the sows don't introduce the piglets to the herds until about four to ten days after birth. That natural timing gives you an idea of what they need.
Your mileage may also vary with the boar, and sows. We specifically select for pigs that have good temperaments and do well in these situations. Mothering instincts and temperament are very inheritable. Eat mean people.
If the boar is in with the sow he will re-breed her possibly sooner than you want, before she has a chance to regain weight after nursing, which can be demanding on her. Also if you want to control when the sows deliver, either so they can all raise the babies together or one can take a few from another's too-large litter then you might want to pen him until you want them bred.
My boar was very careful and protective of the piglets. He'd lay across the entrance to their shelter at night, keeping the piglets within safe. With coyotes around it was a prudent move. On the other hand, I had to separate my sow and her piglets from the herd because the larger pigs were nursing on her, and the piglets weren't getting enough milk! The other pigs exerted their dominance over the piglets by biting them, which made them really holler but it didn't look like it actually injured them. From being chased and bitten by the other pigs, the piglets got very timid, even of us. The piglets can go through the fence, tho and spend most of their time with the larger herd, away from their mother. So I don't think they're very scared of the rest of the herd!