To start out, they should eat only starter. If it is chick starter instead of duck starter, it will need to be fortified with niacin. This can be either from brewer's yeast in their food or a vitamin supplement in their water. Also adding probiotics and electrolytes to their water is a good idea. Once they start eating things other than "feed", they will need access to grit. Their feed provides enough that they can eat it by itself, but when they are a little older you might give them romaine lettuce or peas as treats or when they start going outside where they can eat bugs and grass, they will need grit available to them; Of course grit is nothing more than small rocks and sand so depending on where you live, they might always have access to it anyway
.
After 8 weeks, you should switch them to their primary feed. In the chicken world that is grower feed. I feed mine all-flock since I don't have duck/waterfowl feed available, I am guessing it is better for waterfowl than straight chicken food since ducks and geese are taken into account in its formulation.
Once they are laying, they will need a supply of calcium. The only difference between grower food and layer food is the calcium (and I believe a trace of manganese). The label shows it has a lower protein content, but typically it is only 2% lower and the feed has more than 2% calcium. So the rest of the feed is really just as high in protein as the grower food just some of it has been replaced with oyster shell. To me, the problem with layer food (at least for my flock) is:
1). Some ducks aren't laying yet.
2). I have 2 Drakes.
Ducks that aren't laying and get too much calcium can have other problems.
3). My ducks can free range so they eat grass and bugs and I really have no idea on an individual basis how much feed each duck eats. If one of my laying ducks eats mostly bugs and grass and very little feed, then it doesn't matter if the feed has calcium, the duck isn't eating it.
So for myself, I feed them all all-flock all the time. But they have a constant supply of oyster shell available to them. I see mostly the laying girls eat the oyster shell so I think it is auto regulating that way. (although I have one drake that ate a bunch of it through weeks 10-12 but I don't see that happening much anymore - maybe he needed it.)
Finally I am NO expert. I put this out as the best info I've got and it is hopefully better than nothing and may attract the attention of an expert if I have any flaws in my logic.
Hens go broody when you don’t want them to… and won’t go broody when you do. 
