I had ducks for over 20 years and never had a problem giving Ducks and geese scratch feed. Once I had 28 babies from 3 females and 2 males. Also had given them lay mash and or pellets/crumbles. I also supplement scratch with different salad mixes. First I have to ask you if you are raising your ducks for meat or for pets? You might want to ask someone else about their nutrition if you are raising for meat.
Mine are pets, and I eat the eggs. I supplement their feed with kale mix from Costco, and Power greens, and iceberg salad mix (iceberg lettuce isn't very nutritious but has carrot slivers and red cabbage in the salad mix-from Costco). Unfortunately for me Costco doesn't carry the scratch feed or I'd get it there! You do want to give them some fresh greens also...
My Ducks are gone now, I had a few Pekins, a Muscovy that flew into my yard and never left because I fed and watered her, she liked it =) also had a plethora of Mallards. Sadly my dogs just attacked them one day after getting along well with them for years =(
Now I have the geese in pens, they don't get to eat the weeds like when they were "free range" on my 1/2 acre (every day was like an Easter egg hunt, haha...)