If you already have chickens it should be a piece of cake.
I feed my ducks game bird feed. Others use flock raiser. It kind of depends on what is available in your area.
My ducks also get some kind of greens everyday...either lettuce from the garden, peas, cabbage, or whatever. On top of that they free range in my back yard every day and get lots of weeds and bugs to munch on. They also get cat food as a treat. A handful a day split between them. They act like it is crack!!!