Vitamin A is necessary for support of growth, health and life of major animal species. In the absence of vitamin A, animals will cease to grow and eventually die.
Vitamin A itself does not occur in plant products, but its precursors, carotenes, do occur in several forms. These compounds (carotenoids) are commonly referred to as provitamin A because the body can transform them into active vitamin A.
The combined potency in a feed, represented by its vitamin A and carotene content, is referred to as its vitamin A value.
Concentrations of vitamin A in feedstuffs are highly variable. The naturally occurring sources with the highest vitamin A activity are fish oils and liver. Among the common foods of animal origin, milk fat, egg yolk and liver are sources of high vitamin A activity, provided the animal from which they were obtained received adequate vitamin A for an extended period.
