I'm confused. I have dealt a great deal with niacin deficiency in ducklings, and I had originally thought that they're pretty much safe once they reach the point where they're basically adults. But one of my male muscovies that had suffered from niacin deficiency when he was younger seems to be afflicted again. He's a little over 5 months old. Is this possible? What is the age in which possible niacin deficiency isn't an issue anymore?