I use dry mown grass, the smell pretty well harmonizes with the ducklings poo, sweetens and smooths it up a little.
Anyway, I do not find their poo stinky, which is interesting cause chickpoo makes me keck.. Lucky me
There are two area of bedding in our duckhouse: one is a few handful of sundried mown grass&weeds near their waterplate - they step on it after swim, and they poo and splash water there, so it has a wet bottom layer and whenever I change their water in the day I spread a new dry layer on top. It is not just grass but all kinds of weed (without poisonous ones of course, like jimsonweed - those go to the burnbarrel immediately if found). This wet patch of bedding is changed completely every day, and the room is cleared with highpressure water, oxidizing spray and water again...
Also, my ducks have a few inch high tray, filled with silky mown grass, and grass only - this is where they rarely poo, they use it as a night nest, for rest and sleeping. If I find some poo there it goes very dry in minutes, so changing that area and filling it up with a handful of fresh dry grass is easy. The full "bedclothes" is changed every week, but checked and repaired daily.
They really accustomed to this "dual area bedding" thing, and keep their bed in relatively clean (makes me very proud).
But next week I will buy a few bales of straw and hay, I will chop it up and it will be their bedding for the autumn and winter when we run out of mowngrass.
A brand new wood storage house is being built just now, so their "silk blankets" we collect from the frontyard grass area will stay dry regardless of weather.
And they can sleep in hay, we are living in an agricultural town so it is an easily affordable luxury for a few ducks.