Broken corn isn't sufficient food for ducks, not even chickens. They all still need a food like this every single day:No that is they true age, they eat only broken corn
http://poultry.purinamills.com/OURPRODUCTS/Products/FlockRaiser/default.aspx
Cracked corn should be fed in winter months in small amounts and ONLY in addition to their regular food (like the Flock Raiser). Cracked corn on it's own has nearly no protein, or essential vitamins and minerals to support a growing duckling let alone a full grown duck.
http://poultry.purinamills.com/OURPRODUCTS/Products/ScratchGrains/default.aspx
This is not a feed. Think of it like you would popcorn. Would you eat only popcorn everyday and never fruits of veggies or a bit of cheese or chicken here and there for protein? Duckies need need more than 'popcorn'. Can you get some other feed for your ducks? Once you get them on a good feed- they will pack on duckie pounds and grow some beautiful feathers for you. <3