My ducks are on 50:50 whole wheat and cut maize, ad lib. Can imagine it's much the same for geese. Allen and Page do a goose and duck grower/finisher which is good but our ducks finished on that last year and needed a bit more fat covering hence trying straight grains this year. Restrict their grazing space if you can, or they will just eat grass which at this time of year is starting to lack in goodness. My ducks are restricted to a patch in our garden now.