It doesn't matter HOW {explicative} smart the {explicative} cow is if its penned into a pasture of all one grass, courtesy us humans. Same for your captured duck. and that its dietary needs differ from a rouen or a goose shouldn't surprise either - the dietary needs of gallus gallus domesticus (the chicken) differs between fast growth meat birds like the CX, high production white egg layers (like the leghorn) and hybrid brown egg layers (i.e. red sex links). Similar differences exist between distinct duck lines, and between ducks and geese.That makes sense. Live and learn. No one told me this before.
My duck is 7.5 years and in good health other than the leg. She should make it to 10 without a doubt unless a predator interferes. Therefore the feed must be more than adequate since I've accomplished the impossible.
Perhaps living too long is what is wrong.
As I said, for the first few years the legs were straight, then they gradually start turning inward year after year until they break.
And it's only with the pekins, not the rouens or geese.
The rouens and geese walk and run much more than the pekins that usually just sit, so maybe it's the sitting that causes the deformities (ie lack of stimulation of the bones).
I wouldn't describe it as dogfood either.
Lemons?
How long could you eat jellybeans before you refuse to eat anymore and start craving something more substantial?
Every fall I crave salmon because it's the only food with significant vitamin D. My body knows what it needs.
William Albrecht devoted his life to the study of this https://en.wikipedia.org/wiki/William_Albrecht
"I might say the cow is smart enough to be a better chemist than we are, even with
our finest of laboratories. She was a chemical assaying agent measuring the quality of feeds long before we were. " https://www.verdeterreprod.fr/wp-content/uploads/2020/01/Lets_Live_1952.pdf
The foremost authority on the relation of soil fertility to human health with four degrees from the University of Illinois and emeritus professor of soils at the University of Missouri said a cow is smarter than he is.
Modern ducks have substantial and well studied dietary needs (thank you China, particularly with regard the Pekin, which I raise), which differ significantly from the wild duck, or even the French meat duck, due to selective breeding choices we humans have made over centuries to emphasize some traits (early size, particularly) over others. Trace mineral imbalances have a progressive effect on ducks (chickens, too, of course) which normally results in deformities of increasing severity over time.
If you weren't aware of the need for additional niacin in ducks, a nutritional tidbit of infomation that even Purina knows enough to mention, I'm not certain its worth my time to get into the weeds on this. If you do your own research, and you should, look for Huaiyong Zhang, who has done small studies on the effects of D3 supplimentation, low nutrient density (LND) diets to slow body mass growth (allowing for greater bone density early on and superior bone structure), Ca supplimentation in LND diets, and a host of related studies looking at duck leg bones. Nor is Zhang H the only one - China is really interested in this stuff, and funds research for their own food production needs. The biggest limitiation of these works is that there is no emphasis on old ducks, because old ducks make no economic sense.
This should be a free overview/meta study for you to read.
Here's another free overview with links to individual studies.
those should get you started.
My time is frankly too valuable to spend more of it here, when there are other posters more eager to learn, less enamoured of ignorance.