At best, corn is okay for extra calories in winter, but lacks many nutrients the ducks need for good health. Yes, they often can survive on it, but I am told they can be more prone to illness. Variety certainly is helpful when it comes to healthy diet for animals.
I have used Blue Seal and Nutrena grower/maintenance rations as well as some poultry layer mash. They need a balanced ration and some variety.
There is an online article about raising your own poultry feed that's worth a look. It covers some good material about how to feed fowl a healthy diet. ("Grow Your Own Poultry Feed")
Many of us also provide some vitamins and electrolytes from time to time for a boost.
Carol Deppe's book, The Resilient Gardener, has information on what she feeds her ducks, and I found that helpful, also.