Vitamins are a group of organic compounds that poultry require in small quantities.

Despite the low requirement levels, vitamins are essential for normal body functions, growth, and reproduction. A deficiency of one or more vitamins can lead to a number of diseases or syndromes..
Vitamins are divided into two categories: fat-soluble and water-soluble. The fat-soluble vitamins are A, D, E, and K. Vitamin A is required for normal growth and development of epithelial tissue (skin and the linings of the digestive, reproductive, and respiratory tracts) and reproduction. Vitamin D3 is required for normal growth, bone development, and eggshell formation. Vitamin K is essential for blood clot formation.
The water-soluble vitamins include vitamin C and the B vitamins. The B vitamins include vitamin B12, biotin, folacin, niacin, pantothenic acid, pyridoxine, riboflavin, and thiamin. The B vitamins are involved in many metabolic functions, including energy metabolism. Poultry can make vitamin C, so there is no dietary requirement established for this vitamin. Vitamin C supplementation, however, has been shown to be useful when birds are stressed.
Some vitamins are produced by microorganisms in the digestive tract. Vitamin D can be produced when sunlight hits the bird’s skin. Other vitamins must be supplied because they are not formed by the birds. Many essential vitamins are partially supplied by feed ingredients such as alfalfa meal and distillers’ dried solubles. A vitamin premix is typically used to compensate for the fluctuating levels of vitamins found naturally in food and to assure adequate levels of all vitamins.
Vitamins and minerals are very important components of a chickens diet and unless a formulated ration is feed, it is likely that deficiencies will occur.
Poultry requires all known vitamins except C. Some vitamins are soluble in fats, while others are soluble in water. Some of the symptoms of a vitamin deficiency are as follows:
Fat Soluble Vitamins
Vitamin A

Decreased egg production, weakness and lack of growth

Vitamin D

Thin shelled eggs, reduced egg production, retarded growth, rickets

Vitamin E

Enlarged hocks, encephalomalacia (crazy chick disease)

Vitamin K

Prolonged blood clotting, intramuscular bleeding

Water Soluble Vitamins
Thiamine (B1)

Loss of appetite and death

Riboflavin (B2)

Curly-toe paralysis, poor growth and poor egg production

Pantothenic Acid

Dermatitis and lesions on mouth and feet

Niacin

Bowed legs, inflammation of tongue and mouth cavity

Choline

Poor growth, fatty liver, decreased egg production

Vitamin B12

Anaemia, poor growth, embryonic mortality

Folic Acid

Poor growth, anaemia, poor feathering and egg production

Biotin

Dermatitis on feet and around eyes and beak