Storey’s Guide recommends 150 mg niacin per gallon of drinking water if duckling shows signs of deficiency, if I recall correctly.
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I think it was Metzers who published an article that each duck should get 100mg/day of niacin if they are having deficiency related issues. I can’t remember if wry neck if from a certain vitamin deficiency or not. If the vitamins you are giving him are water soluble, then the duck will pee any extra vitamins it doesn’t need. But if they are fat soluble, you can over due it.
If you have nutri drench, I would give it straight to the duckling with a dropper. Dribble on the bill and he will swallow it. Then give access to lots of water. Anytime you add vitamins to the water, they will spill and waste quite a bit. So keep that in mind.
If the neck problem is a vitamin deficiency the treatment is thiamine.
http://www.duckdvm.com/condition/thiamin-deficiency
I had a chicken chick with this earlier this year and about an hour after giving her a shot she was 100%.
Edited to add:
I gave her a small amount of the b complex for cattle into her breast muscle, but you could probably give it orally too.