Corn, soy, wheat....
It's basically just sugar.
In the wild, ducks and geese wouldn't have much access to grains until fall. In spring when they hit that vital growth spurt where angel wing occurs they are munching on weeds, aquatic plants, fish, tadpoles, and bugs. The carbohydrates they get are not so rapidly digestible and not in large quantities.
Since I also have quail I feed my ducks gamebird feed, which is 24% protein. I also give them greens daily. If high protein was the main cause of angel wing, I should have seen a case or two in my flock by now.
I would like to get away from commercial feed all together but can't due to my small space. I do, however, grow as many veggies as I can and then shred and freeze for "duck veggies". Excess zucchini is a cheap supplement. Put the frozen shredded zucchini in a bowl of hot water and let it set for a few minutes, then you have a nice soup that disappears in seconds. They love that on cold days.
I am also trying to get a good compost pile going so I can add red wiggler worms and give the ducks a natural snack.