Brewers yeast questions
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Unless you are feeding a duck starter food that has enough niacin then yes you need to add niacin. The recommended levels is 55 mg (per kg). Brewer's yeast is a byproduct of the beer brewing process. I got mine from a local health food store. DO NOT use bread yeast, backing yeast, or wine making yeast.
You'll want to add one tablespoon per cup or food or 2-3 cups per 10 pounds of food. You'll want to give it for about the first 10 weeks.