Below is Porky grazing in the yard. There isn't much grass, but he is up and about a good part of the day finding weeds, roots, acorns depending on the time of year and I"m sure some bugs.
Grazing occupies about a third of the pig's day, providing great emotional satisfaction as well as nutrition. The experts say that pigs seldom root if there is adequate grazing area...there are a few here that root no matter what. A yard that is just dirt will get rooted as they search for roots underground. Pigs know what they need to be healthy. They will seek it out and won't eat dangerous toxic weeds or roots unless starving. On a warm spring day they will graze and then sprawl out to enjoy the sun.
Have you ever found chewed up clumps of grass in the yard? Presumably, the pigs are taking big mouthfuls of grass, chewing them to extract the soluble nutrients, and then spitting out the fibrous remains. Pretty interesting if you ask me, but does it tell us anything useful about swine nutrition?
Why are the pigs spitting out the grass instead of swallowing it? Obviously, we can't ask them, so here's a guess: The pigs have a finite amount of stomach and intestinal space. The only way they can extract nutrition from the fibrous parts of the grass is to let it get through their digestive tract and into their large intestine, where microorganisms can go to work on breaking down the cellulose, pectins, etc. They're not particularly efficient at extracting the energy from the fiber, and thus are better served by ingesting more digestible material. By grinding the plant matter in their mouths, they extract the easily digestible juices, then spit out the hard to digest parts. Of course, I don't think they make this a conscious calculation, but rather rely on instinct.
While our pigs will eat just about anything, but they still have preferences. They know what to eat in their foraging. It is a very hungry pig who will eat any wild food that is unsafe. Nor do pigs generally overeat to the point of it being a problem.