Because it's dog food. Fowl have different nutrition requirements. Just because the label has niacin in it doesn't mean it's an appropriate quantity or source.
I feel very badly for your duck. I'm also questioning her quality of life at this point. All alone, can't walk, and covered in maggots and needs a bath daily, and an inadequate diet.
I feel bad for the duck too but what do you suggest I do? I'm here for suggestions.
She always has been alone even at the pond because the geese and rouens aren't exactly her friends. I can't take on more of her species because I already have too many birds for the size of my pond.
She can walk and even climb the banks of the pond, but it's just not easy for her because she steps on her feet.
She's not covered in maggots. She gets an infestation only because I wasn't paying attention because I thought the problem was resolved. It won't happen again because I will always be vigilant now that I know the problem is perpetual.
And I'm not yet convinced her diet is inadequate. I'll concede she could benefit from B vitamins but I have no evidence that the B vitamins would have prevented such a popular problem with that species. Without controlled studies the niacin is only a theory. None of my other birds (4 rouens + 4 geese) have that trouble so there is something specific to the genetics of the pekin.
Maybe the duck is inbred. I've read on other threads that people won't buy ducks from
TSC for whatever reason. Here is one
https://www.backyardchickens.com/threads/duck-with-turned-in-feet.645617/
Who knows what the problem is. Everything is speculation at this point.
I'm skeptical you would be able to get her back on the pond safely with the geese since she's the only duck. Like you found out, the geese can do a lot of damage quickly.
Truly you must love her if you have been willing to do all this extra work. I'm very practical when it comes to my homestead animals and I know I would have put her down a while ago.
I don't feel like her life is so bad that it warrants putting her down.
Normally when a duck is alone I suggest getting new friends for her, but unless you radically change how you are feeding her and we figure out why she's getting maggots late, I don't think that would be wise.
As another poster said, are you positive that they are maggots and not perhaps some other bug? It's very odd that without a wound they would be laying on her continually.
You said she sits all day. Does she preen at all? Using that oil gland above their tail, they spread oil around their feathers and helps make them waterproof.
I don't know if they are maggots but they are worms that look like maggots and they correlate with flies.
Yes she preens. She is waterproof. Peroxide rolls off like water on a waxed car.
By the way, it isn't easy, but it is worth it.
