See if your feed store stocks or can special order duck and goose maintenance formula (my local brand here, Armada Mills, calls it "Maintainer"). Its a little lower in protein than the grower formula. If they don't have that, you could probably mix some regular chicken layer pellets in with the duck grower, about a 1 to 3 ration, and add a few drops of some kind of avian vitamin with niacin to the feed to compensate for the need for a little extra niacin for the ducks.