I, too, would love information about this. There's a wonderful, long
thread about perennial/self-seeding
chicken feeds (
https://permies.com/t/997/chickens/perennial-chicken-feed), for
chickens to do the harvesting themselves. But,
chickens aren't ducks, and I would assume their nutritional needs are different, just as their eating habits are different (ducks can't peck and claw their way through food like chickens can). I do recall reading in Story's Guide to Raising Ducks that Halderread was able to reduce his feed bill by half--and eliminate the corn feed entirely--by feeding his ducks cooked potatoes (pages 234-235). Potatoes are very vitamin rich, and so are sweet potatoes. My ducks (especially ducklings) LOVE sweet potatoes, and those do not need to be cooked before feeding--I just chop them up. So, you probably could substitute a lot of your duck feed with potatoes/sweet potatoes.
But, I'd love to plant things that the ducks can feed themselves on, that complete their nutritional needs. I just don't know what those plants would be....
Some plants that might be good would be duck weed, dandelion, and mulberry. These are all things the duck can forage for by itself. My ducks also forage under salmonberry bramble, and I hope they'll enjoy the salmonberries as they drop. They sadly do not eat the leaves or the shoots. I know they like huckleberries (the woman I bought them from had a huckleberry bush and a rooster, and the ducks would all congregate under the bush when the rooster came near it, in hopes the rooster would hop up and shake down the berries). My ducks also enjoy creeping buttercup, clover, and grass. Anyone have other ideas of perennial/self-seeding plants for ducks to feed themselves on?
Another thing to consider is that ducks eat a lot of bugs and slugs. So, maybe try encouraging the bug population. I've found that a
compost heap that's all "greens" without the carbon-rich leaves/wood, tends to bring a LOT of black flies. Free duck food! I like to throw out old foodwaste to the ducks in a pile in their enclosure. They can eat the food, and the flies that breed on it as rots.
If you want more slugs, you could make a nice moist area full of deep decomposing leaves. Slugs bred like CRAZY on a
hugel mound that put leaf and forest litter on. You could even sew some seeds or put succulent plants or carrot tops in the leaf mulm. The slugs will devour them--at least, mine sure did! Nothing grew in there other than the grass I was trying to kill, because the slugs ate it all before it could get big
enough.