They occur in feedstuffs in varying quantities and in different combinations. Some vitamins are produced by micro-organisms of the intestinal tract. Vitamin D can be produced by sunlight on the bird's skin. Other vitamins must be supplied in the ration, because they are not formed by the birds. Vitamin A is necessary for the health and proper functioning of the skin and lining of the digestive, reproductive and respiratory tracts.
Vitamin D plays an important role in bone formation and the metabolism of calcium and phosphorus.
The B vitamins are involved in energy metabolism and in many other metabolic functions. Many necessary vitamins are partially supplied by feed ingredients, such as alfalfa meal and distillers' dried solubles. Feeding the Right Ration Commercial poultry feeds contain numerous similar feed ingredients. There are, however, several different types of rations available. These are designed to meet the specific needs of different type birds.