Whether you need to supplement with B vitamins depends on the niacin content of the feed your using, and the breed is which your rasing. So, the bag you're feeding, just like most other feeds (unless your feeding Mazuri) is probably around 55mg of niacin per kilo of feed. That requirement will be sufficient for light-medium weight breeds, but with heavyweight breeds such as Pekins, you'll really want the niacin content around 70mg/ per kilo.
So to simplify what I said, if the ducks you have are lightweight/ or medium breeds like Indian Runners, Calls, or Buff's you should be ok without supplementation, but if you're raising Pekins, or Saxonys, it may be good to supplement.
Why supplement with the heavyweight breeds (?) the heavyweight breeds tend to be raised for meat, and have been bred to grow fast, and put on weight fast. This cause's an increase in their niacin requirement to keep up with their fast leg growth. Once they reach around eight-ten weeks their niacin requirement drops down to 55mg per kilo, so you can then stop supplementation.
In regards to which niacin supplement to use, there are several. I personally avoid Brewer's yeast because after doing some research I found several very common brands that contained little niacin. Nutritional Yeast on the other hand almost always has high niacin content, and tends to taste better than the BY too. The dosage for NY, is one/two tablespoons per cup of feed.
Another alternative, which might be better for larger flocks because NY supplementation can be costly, you can instead dissolve one 500mg plain niacin capsule in ten/twelve gallons of water, and offer that to them a few times a week, with another plain clean water source.
If you'd like more information pertaining to preventing niacin problems in waterfowl, what to feed, etc. You can read the link in my signature,
