LAA gave you a very good answer.
Where bread works is if you can get out dated milk for next to nothing or you have left over whey from making cheese with family cow milk. The bread becomes a filler for the milk you are other wise throwing away.
This combined with pasturing and left over scraps from the garden becomes a way for a family pig to be fed at a lower cost. The pig will grow slower but with lower feed cost per lb of gain some use this as a alternative feed source.
You should also mix a mineral supplement with the milk/bread.