Niacin

What is the "Niacin " you may ask? And How to cure Niacin Deficiency.
Well, Niacin is Vitamin B3. Ducks receiving low-niacin diets show (severely) bowed legs and ultimately become so crippled and weak, Niacin Deficiency (Low Niacin Levels) make it almost impossible for the ducks (Also Geese and Turkeys) to walk. 

How to cure Niacin Deficiency in ducks, geese, and turkeys

 Brewer's and/Or Nutritional yeast is a good source of niacin. If you don't have Yeast, you could also try Greens like Peas (Whole Peas and pods) or other greens. EDIT: You can go to your local pharmacy or drug store and buy Niacin Capsules. They are water soluble, but open up the capsule and dump the contents into either the water or food. Mix well. You may need to use up to 5 capsules per cup for a day and you will see how much so little effort and cost will help.

Credits:
 https://www.google.com/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjsk9vuxvvWAhWH5YMKHZEIBhAQjRwIBw&url=https%3A%2F%2Fwww.backyardchickens.com%2Fthreads%2F4-week-old-pigeon-toed-pekin-loosing-weight-help-please.668584%2F&psig=AOvVaw3N9hoVWOCqt5Bi3xOpnqcx&ust=1508463568981244