I've done a ton of research for my babies. I'm posting a couple of different articles regarding niacin for ducks. Everything I've read state that it is very important to supplement with niacin in the beginning weeks for ducks to help strong leg and get development since ducks are much heavier than chickens. I get my niacin from the tractor supply or southern states farm supply. There are instructions for dosage based on age right on the label.
Article 1
Niacin is a critical vitamin required for the correct development of ducklings and goslings. In fact, waterfowl have a higher requirement for
niacin (also called nicotinic acid) than chickens.
Niacin is formed from the amino acid tryptophan and waterfowl do not synthesize
niacin well from tryptophan.Mar 13, 2011
Article 2
You can feed ducklings regular chick starter feed but since they require more niacin than chicks do, to help with bone growth especially in their legs, you will need to add brewers yeast to the feed (at a ratio of 5% or roughly 1/2 lb (3 cups) brewers yeast per 10 pounds of feed). I use
Thomas Labs Brewer's Yeast & Garlic Powderand just sprinkle a bit over the top of their feed a few times a day. This mix gives them the niacin they need plus garlic which has immune system and other benefits to growing ducklings.
The ducklings should start on chick feed (20% protein) for the first 2 weeks, then can switch to a waterfowl starter or starter/grower (15-16% protein) until they are 18 weeks old or so and then switch right over to a regular (16% protein) layer feed - or waterfowl feed if you can find it in your area.