It took me a while to realize that if I feed piggies 3 growable food items that I could recoup seeds from their, erm, plops.
Unfortunately, trying to select just 3 of a type of produce to feed them is very difficult if you have a large stack in your inventory. For "growable" types of food, I think a quantity of three should _always_ be a option, rather than its current default to 10, 20, 30, and so on when you have a large stack.