Niacin is so good for their legs it helps them become strong. Chickens don't need as much so when we haven't a choice of what to feed then we have to supplement. I have a mixed flock so I can't feed just feed for waterfowl anyway. My birds are all adults except my chicks and they still get NY in their feed a couple of times a week just because it's so good for them and us to actually.
this will help explain it much better than I.
https://www.backyardchickens.com/threads/niacin-deficiency-in-waterfowl.1367557/
Just be sure it's Nutritional yeast and not baker's yeast.