How long should I be adding niacin to my ducks’ feed? I’ve used just about 1 pound of brewer’s yeast for 3 ducks the first 8 weeks of their lives. Do I give it to them the rest of their lives? Until they are on layer crumble? They are currently almost 9 weeks old and on poultry starter crumble with the brewer’s yeast scooped into it and sprinkled onto it.