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I am feeding them an organic duck starter crumble but the ingredients didn’t say anything about B3 or niacin so I ordered some brewers yeast before the ducklings came but from this Coronavirus orders are backed up a bit.Welcome to BYC! Definitely try posting about your baby on the duck forum!
What are you feeding them? They actually make a duck pellet food that includes the much needed niacin ducklings need.
Regular chick starter works, but you will have to supplement it with brewer's yeast for the niacin. Another good choice is NutriDrench, and B complex.