You do not mention what sort of duck, but any laying female will need extra rations to non-laying females. We have a female pekin that has not taken a break from laying since she first started in September last year. She is always hungry. We measure out 1 cup of feed each per duck morning and again in the evening. Overall that suffices but I think the one laying pekin eats more than the two females [one small crested pekin and one very small muscovy] that took a break from laying.
We use organic layer feed but as we currently cannot get organic duck layer feed, the ladies are on chicken layer feed and get extra niacin every day on their mealworm treats.
If you are using all flock pellets/crumbles, your females will need extra niacin as well as calcium on the side. But it is the feed -- pellets/crumbles that have to be given in enough quantity to fill you duck!