The biggest hurdle on the “best” feed I’ve been trying to sort out is that many are labeled as “layer”, and the instructions specify to not feed it to male birds...
I have not resolved that yet.
Especially because I am allergic to soy and corn, and try to avoid feeding it to my birds that we raise for meat and eggs.
The feed we are currently using is 18% protein. I add (quick) oats to it to cut the protein levels for the ducks.
How much to add depends on the feed, and how many ducks you are managing.
I just buy Quaker quick oats at Costco and estimate how much to add/ watch the “kiddos” and adjust as needed.
I also add brewers yeast tablets to the feed, ground up w a mortar and pestal, that I get at my local feed store. It is labeled for dogs, but the math of the dosage and the approximate weight of the total number of birds you are feeding, is pretty straightforward. The oats and niacin are added to every batch of feed for the ducks.
It’s kind of an art, and kind of a science at the same time