The order Anseriformes includes ducks, geese, and swans. Waterfowl vary from strictly herbivorous (swans, most geese, most ducks) to highly piscivorous (marine ducks, mergansers). The following are commonly accepted recommendations, originally derived from the Nutrient Requirements of Poultry published by the NRC:
Reared and growing chicks up to 8 weeks old should be fed a starter diet (pellet diameter ≤ 2 mm) that contains 25–28% protein, 5–8% fat, 0.85–1.2% calcium, and 0.3–0.6% phosphorus.
From 8–12 weeks, waterfowl should be fed a mixture of a starter diet and maintenance diet.
After 12 weeks, waterfowl should be kept on a maintenance diet, such as commercial duck or game-bird pellets, which should contain 14–17% protein and 3–6% fat with an adequate vitamin/mineral supplement. During breeding season, the amount of protein can be increased to 16–21%.
In winter in colder areas, to provide extra calories for thermoregulation, more fat can be added by adding corn as 20% of the total diet. Piscivorous waterfowl generally consume a fish-based pellet containing 25–40% protein and 5–10% fat. Some breeders prefer to feed a “sinking” or “floating” waterfowl pellet in the water. However, pellets fed in water should not be allowed to dissolve before consumption, or the bird may not consume all the nutrients contained within the pellet. Feeding waterfowl a starter-grower, adult maintenance, or layer chicken pellet is not advised, because in general these pellets contain lower amounts of protein and vitamin and mineral supplements than waterfowl require.
Swans and geese are fed the same diet as herbivorous ducks. Although waterfowl can live on only nutrient-adequate pellets, usually some lettuce is also fed for psychological stimulation and to help mimic their natural foraging behavior.
Diets of only bread, lettuce, and corn, which are unfortunately often provided, lead to deficiencies of protein and multiple vitamins and should be prevented. These deficiencies often manifest as poor plumage, swollen joints, and pododermatitis.
As is true for most bird species, young waterfowl should be fed starter pellets with higher fat (5–8%) and protein (25–28%) content. However, excessively rapid growth of large waterfowl must be avoided to prevent bone and joint deformities, such as angel wing (in which the flight feathers so grow rapidly that the last wing joint twists and the feathers point out laterally, rather than lying against the body) and perosis (also called "slipped tendon," in which the gastrocnemius tendon slips out of its normal position at the hock joint, causing lameness).
