How much to feed ducks

Does anyone have any recommendations on how much to feed ducklings as they are growing and as adults? I would like them to free range for some of their food, but my ducks are lazy and they won't forage if I feed them as much as they can eat in a sitting. I've been restricting their food some so they do. I have six ducks fenced into around a 1/16 of an acre with plenty of grass and greens (bermuda grass, nut grass, plantains, lettuce, poppies, pigweed, kale, and collards).

We raised lots of ducks when I was a kid. They were provided with a shed and pond, but no food from us during the warm months. A few breeders were fed through the winter.