Pig food (pigfood), you can buy it from the dealership in pallets or big bags, which you can load into regular tractor trailer so they are easy to transport into pig barn. At pig barn you can dump pig food from the trailer into the unloading point which is designated by the "dumping trailer" icon. Please note; (giants claim) buying pig food pallets from the vehicle shop is very expensive, grow your own crops to feed your pigs. Pig food consists of (note, this came from savegame placeables.XML file I believe): WHEAT, SOYBEAN, MAIZE (corn to humans), POTATO, so when you dump "pigfood" into the placeable these "fill levels" are increased in game internal data variables, there is no "pigfood" in pig barn placeable, just these individual crop names. Pig food consists of, base food: corn or sorghum, grains: wheat or barley, protein: canola, soybeans or sunflowers and root crops: potatoes or sugar beets (source: in-game giants documentation).
Pigs eat sugar beets and potatoes. See Root crops storage shelter mod.
Grimme sugar beet harvester needs a tractor with narrow tires, otherwise tractor destroys crop before the harvester gets to it.