The requirement for vitamin A is listed in NRC (1994) as 1,500 IU per kg of diet (681 IU per lb) for broilers, growing geese and growing layer replacement birds. The requirement for laying and breeding chickens, assuming a feed intake of 100 g per day, is higher at 3,000 IU per kg (1,364 IU per lb) of diet. The requirement for all classes of turkeys is 5,000 IU per kg (2,273 IU per lb). The requirement for ducks is listed as 2,500 IU per kg (1.136 IU per lb) of diet for the growing phases and 4,000 IU per kg (1,181 IU per lb) of diet for breeders. The requirement for starting and growing Japanese quail is 1,650 IU per kg (750 IU per lb) of diet and 3,300 IU per kg (1,500 IU per lb) for breeders. Shrivastav and Panda (1999) reviewed the quail nutrition research in India. Under tropical and subtropical conditions, Shim and Vohra (1984) suggested a dietary vitamin A requirement of 4,000 IU per kg (1,181 IU per lb) for growing and laying quail. For most animals, vitamin A has been considered to be required at 100 to 200 IU per kg (45 to 90 IU per lb) of body weight per day (Olson, 1984).
Different species of animals convert beta-carotene to vitamin A with varying degrees of efficiency. Some factors that influence the rate at which carotenoids are converted to vitamin A are type of carotenoid, class and production intake of animal, individual genetic differences in animals, and level of carotene intake. Compared to other species, poultry are very efficient in conversion of beta-carotene to vitamin A. Efficiency of vitamin A conversion from beta-carotene is decreased with higher levels of intake (Van Vliet et al., 1996). For the chicken, as beta-carotene intake level is increased, conversion efficiency drops from a ratio of 2:1 to 5:1.
Stress conditions, such as extremely hot weather, viral infections, and altered thyroid function, have also been suggested as causes for reduced carotene to vitamin A conversion. Vitamin A requirements are higher under stressful conditions such as abnormal temperatures or exposure to disease conditions. As an example with poultry, coccidiosis not only causes destruction of vitamin A in the gut but also injures the microvilli of the intestinal wall, thereby decreasing absorption of vitamin A and at the same time causing anorexia for several days (Scott et al., 1982).
Vitamin A requirements should be sufficient to provide for satisfactory growth, reproduction and hatchability, egg production and prevention of deficiency signs under laboratory conditions. Present requirements should be designed to be adequate for these purposes under practical conditions of feeding and management as well as allow for a certain amount of storage. The decision as to the minimum vitamin A requirements of young poults and chicks depends upon whether the criterion of optimum requirement is based upon growth and feed utilization or also considers liver storage. Storage of vitamin A in the liver certainly appears to be desirable since under conditions of little or no liver storage, stresses and diseases may precipitate vitamin A deficiency.
The amount of vitamin A added to poultry diets is usually in excess of the requirements contained in the NRC publication because no safety factors are built into the NRC figures (NRC, 1994). Additional vitamin A is added to allow for loss of activity due to oxidative destruction of the vitamin A ester during feed processing and storage, variability of carotenes in feedstuffs, changes in feed consumption, genetic differences in animals and stress due to disease and other environmental factors. The minimum vitamin A requirement for normal growth may be lower than those required for higher rates of gain, resistance to various diseases, and normal bone development. Sklan et al. (1995) suggest that maximal immune response in the poult may be achieved at dietary intakes of vitamin A at or higher than those recommended by NRC. In chicks with no added dietary vitamin A, antibody production and proliferative response were depressed in comparison with chicks receiving vitamin A. Supplementation of small amounts of vitamin A enhanced the responses; both antibody production and proliferative responses increased with dietary vitamin A until the diet contained 6,660 IU per kg (3,027 IU per lb), above which the responses decreased (Sklan et al., 1994). This suggests that maximal immune responses in the chick may be achieved at dietary intakes of vitamin A considerably higher than NRC recommendations. High stress affects the immune system and, therefore, increases nutrient requirements including vitamin A (Friedman and Sklan, 1989; Panda and Combs, 1963). Friedman and Sklan (1997) report that optimum immune responses in growing chicks and turkeys were obtained with vitamin A intakes that were three- to tenfold higher than NRC recommended levels.