Have we seen your ducks? Sure would like to! Even sometimes when on duck specific feed there can still be one or more come up with a deficiency best to catch it quick.
What breed? Pekins could probably benefit their entire lives something like Nutritional yeast sprinkled over their feed 2-3 times a week just for support of their legs. I have Runners Buffs and Muscovy and mine get Nutritional yeast 1X a week now that they are all adults.