Letting ducks out for the first time

In an effort to keep things cleaner and save some money on food, I'd like to let my ducks out. In the past, we've kept chickens, and had no trouble letting them out a few hours every day -- they'd coop themselves up at night, and we'd just have to close the door to the coop to keep them safe overnight. If the chickens were unwilling to be put away, tossing a bit of food into the coop never failed.

My ducks don't seem as interested in food as the chickens -- they don't come running the moment they hear food.

The ducks are smart enough to put themselves into the little house I've got for them inside their pen (the house is a converted calf hutch, and by "converted" I mean "filled with straw").

These are ducks from a late hatch last year -- hatched in June, so I just kept them penned up until now, as they weren't mature before the days started getting colder and shorter. If I let them out, should I be worried about getting them back in at the end of the day? And, if they aren't so willing to do so themselves, what can I do to coerce them?

Thanks!