It is interesting, I did some reading on nutritional vs brewers' yeasts. Brewers' yeast is actually naturally healthier since it feeds on grains which have lots of vitamins and minerals in them whereas nutritional yeast is grown on molasses which has none. Both are then deactivated through heat and are dried out (why you don't want to use bread yeast - it is still active). Nutritional yeast is often fortified though with additional vitamins after the fact (mostly B12). For the ducks, niacin is the important nutrient, and it is inherent in the yeast, so it is probably the same either way. I tried it both ways, and I prefer to add niacin (not time release or no flush) to their water with their probiotics and electrolytes and then feed them the crumble straight. You pretty much have to go to a health food store to get the niacin now since most of what is in the vitamin aisle of supermarkets / grocery stores is either time release or no-flush (make sure to get Nicotinic acid, the no-flush kind is also called Nicotinamide and does not provide the benefit to the ducks). To me the yeast is messier and they get it in their nares and that is just one extra thing to deal with.
Funny, i will do some more research but when i researched last spring with my ducklings everything pointed to nutritional yeast being better and having more niacin .
But i prefer the yeast over the niacin capsules.
I tried capsules last year and had a buff duckling develop leg problems. He was big.. bigger than the pekin. Anyway i switched to the nutritional yeast and a week or so later he was better.
I think they go thru wasting so much water on the ground it was harder for me to know how much they actually were getting.