Ducklings are doing most of their growing from hatch till 10-12 weeks and need a good supply of nutrients for that growth. They can have all kinds of health issues if they are deprived. First one that comes to mind is leg deformity as it is we have to supplement extra niacin with our ducklings so they have good strong legs some even need the extra if fed ducks specific feed. They are like growing children they need a lot of good wholesome nutrients. Once ducks reach maturity and have acreage to forage then I can see cutting their feed but not many have the area to allow for good quality forage. So we keep feed out so our layers get what they need and our youngins are getting all they need for growth. My older ducks eat when they want too. It’s never been a problem here and to me just good husbandry.