Pigs can’t be “grassfed” the way cattle can. Cattle are ruminants; they have four stomachs that allow them to break down the fiber in grass and get the most out of growing plants. Pigs have a digestive system that is very similar to a human digestive system. Pigs can digest green vegetables, and get a lot of nutrition from them. But pigs can’t grow fast and get fat on pasture alone, any more than we can get fat eating only salad.
My pigs do live outside, and they do have access to green stuff year-round. Depending on the pig’s age and stage of life, forage can be anything from just a garnish (for little ones just starting out) to the vast majority of the diet (for adult pigs who aren’t working very hard). My preferred diet to fatten pigs for the freezer is about turnip pasture and corn. I think the pigs are eating about 50/50 turnip greens and corn. It’s a little slower that confinement feeding, but it’s good for the pigs and it makes great pork.