Solgar put's out Brewer's Yeast, which is a great product to add in powder form to your duckling feed if you are looking to supplement niacin. You can feed your duckling non-medicated chick feed. Manna pro makes a nice product, but you must add a little sprinkle of the brewer's yeast for the niacin. Solgar products are sold at your local vitamin Shoppe. I had a hard time finding definite answers on when to change over the food to a gamebird (higher protein) feed. Some sources say 2 weeks, some 4. In the event that you have chicks in the same area, the rule of thumb is to keep the non-medicated chick starter available for all.
Hope this helps.