The short answer is, "anything they can get", they're pigs after all.
My pigs' two staples are "hay" and yogurt whey. The hay is wrapped in white plastic and ferments into so called "balage", which improves its digestibility. The whey is a byproduct of greek yogurt manufacturing. My farm is close to one of the big greek yogurt makers and they deliver whey by the truck load. This is the first winter I've had pigs, and have not had time to improve the genetics of the swine I'm raising. Since 99.99% of hogs in the USA subsist largely on a corn/bean ration and have been selected to perform on that type of feed and not the ration I supply them with, I'm also giving my pigs a little wiff of soybeans/barley/oats during the cold weather. They don't have much in the way of hair and they burn a lot of calories staying warm when the ambient temps fall into the teens and below. The hay/whey ration I'm feeding is not very calorie dense and the littlest piggies struggle to eat enough to stay healthy - hence the bit of grains. The "wiff" is only a very small fraction of a full ration, 1/10 or less, but even that small amount of easy to digest carbs is enough to keep them looking happier. I also feed them stale bread from the local healthfood store, and this amounts to an even smaller "wiff" than the hog ration they get. Finally, I also feed them my kitchen scraps, which is a treat, but I don't even know how to count it in the grand scheme of calories since it amounts to very little when spread over 21 pigs.
I was not much of a bread eater before the start of this year even though I generally like the taste of it. Bread was usually a take-it-or-leave-it sort of thing for me. Now when I crack the stale loaves out of their bags to feed to the pigs I think they smell incredibly good, even frozen and stale... I've been tempted to take a bite... but haven't yet. I find I focus on anything in the house that is starchy and carby and it takes will power to avert my gaze. I suspect this is because my total calorie intake per day has diminished on the homegrown challenge. It is just plain difficult to eat a lot of calories as roots because they have so much water in them. I guess that is how my pigs feel about their whey - too much water in it.