Not an expert by any means, but this is what I was able to sum up after some research (Critiques appreciated since research can get very complicated):
You can feed poultry crumbles and it will work, just make sure that you're reaching the right level of B3.
Niacin B3
Bare minimum is 10mg/duck/day, recommended is 55mg/2.2lbs feed/day.
Usually poultry feed has 30mg/kg, double check label.
Ducks will need high niacin until 10 weeks; past that point they are generally okay with levels in poultry feed.
Can use 1 tbsp. Nutritional Yeast/1 cup feed
Foods to supplement niacin with are peas, sweet potato raw or cooked, feeder fish, sardines/tuna packed in water, salmon, and pumpkin
If using human grade niacin: 500mg/8gal water if little spilling, or 500mg/4 gal water if half of it gets spilled daily.
Human grade brewer's yeast: 5mg/15g (1tbsp) = 2 tbsp
Livestock grade brewer's yeast:1.5mg/15g (1tbsp) = 6 tbsp
Brewer's yeast also contains high protein levels, which may interfere with birds' development depending on the stage they're in.
Protein:
20% ducklings
15% juveniles
16-17% adults