I poo pick the horses fields, and poo pick where the llama is (only because he poos in one specific place, and its easier to pick up than goat and sheep droppings!). If you can pick the pig poo up easy enough, they I would, because it reduces smells and flies, and helps keep the field/pen in good order.
My two pigs poo in one corner of their area, making it easier to pick up ....although, with the recent hot weather, when I went to collect it, it make me feel rather sick!!!!