I just let mine out in the yard - they eat all day long and are especially happy when it rains....a lot! Mine eat around the house and hit any low area - I have chickens who free range too but the ducks seem to do well with virtually no additional food! Of course, that's during warm weather when the ground is not frozen. I do give my duckies treats - mostly to help me get them into their shelter area at night - I like for them to come when I call and for them to be trusting enough to be around me - so I give them chicken scratch, cracked corn and sometimes layer pellets - but nothing much. I have and LOVE my Muscovy's - they are pretty robust! I would also suggest to not clip the wings - give them a chance to get away if something tries to eat them. As long as they are used to your area - know you will give them treats and shelter - they will fly back -or come back, they like being around their crowd! I only know about Muscovy's as the Pekins were eaten very early - big, fat, white and they cannot fly - wasn't good - and they were VERY dirty where Muscovy's are clean.
I hope this helps - let'em free range - they'll eat the bugs and such in your yard!
Connie