Plain. I mix it with their starter crumbles so they don't make such a mess. I'm raising my second clutch of chicks and this has worked well for me. If your giving them sugar water that could be the cause of the pasty butt. I only give mine sugar water for the first day. Good Luck
I give mine all natural plain yogurt , most yogurts have active cultures, but you can check the ingredients list to make sure.
