I thought that you needed to add it once a week for adult birds?
Shaky legs? Runner duck. - Page 2
I know what you mean about reading a bunch of information. Then trying to keep it all straight. Seems like there are a dozen opinions on each subject. Everything I remember reading about giving niacin said to give until 10-12 weeks old.
Anyway, I have not given my ducks additional niacin since they were about 11 weeks old (because I was using up the last of the mixed food) and I haven't noticed any issues as they got older. I did have one with a niacin deficiency but it was corrected (she was about 4 weeks old at the time). I will say that I give mine Purina Flock Raiser as opposed to a chicken feed. That may be a factor.
Yeah, right now we are using something meant for ducklings and chickens sold at tractor supply, but usually it's just chicken feed. It has seeds, and pellets, and other things in it. I might continue it for awhile just incase, and then see what happens if I wean them off. It would be nice, to not have that expense. They free-range too though, so hopefully they are eating lots of bugs.