I just feed chicken layer pellets to my adults. It is true that access to fresh forage will tend to even out any deficiencies. Even Holderread says that ducklings with access to fresh greens and insects will rarely show signs of niacin deficiency. I also know that free ranged chickens & ducks produce eggs with a firmer, more vividly colored yolk, which to me means, "HEALTHY!"
Enjoy!
Enjoy!