Thanks for all the replies everyone! It's interesting to see the different brands of commercial feeds in different areas. There are two stores close to us that carry poultry food and they don't really have anything specific for ducks. My husband usually buys the food from a feed store close to his work and he has always gotten whatever the store owner recommends based on their age. Our ducks are about 4 months old now and they're on a pelleted feed, but they don't seem to like it so I bought some scratch grain to add to it and they like that. I'm thinking scratch grain alone isn't enough as far as nutrition, but they do forage all day, too.
You've all given me some ideas of other types of natural foods to give them, too. One thing I like to give them as a treat once in a while is prepared plain oatmeal, but I haven't seen that mentioned. Is that okay for them to have?