The vitamin D3 requirements listed in NRC (1994) are 200 IU per kg (90.9 IU per lb) of diet for broilers, geese and leghorn classes 0 to 18 weeks. Note: ICU (International Chick Units) and IU (International Units) are considered equal for Vitamin D3, but not equal for vitamin D2. Higher levels of D3 (IU per kg; IU per lb) are required for leghorns that are in prelay (18 weeks to first egg), laying (assuming 100 g feed intake per day) and breeding (300; 1,36.4) and for turkeys (1,100; 500), ducks (400; 181.8) and Japanese quail (750; 340.9). Edwards (1999) suggested for broilers different quantitative requirements for vitamin D3 to maximize responses to different criteria. The vitamin D3 requirement was 275 ICU per kg (125 ICU per lb) for growth, 503 ICU per kg (228.6 ICU per lb) for bone ash, 552 ICU per kg (250.9 ICU per lb) for blood plasma calcium and 904 ICU (410.9 ICU per lb) for rickets prevention.
It has generally been assumed that for all but a few species, vitamin D2 and vitamin D3 are equally potent. For poultry and other birds and a few of the rarer mammals that have been studied, including some New World monkeys, vitamin D3 is many times more potent than vitamin D2 on a weight basis. Vitamin D3 may be 30 to 40 times more effective than the D2 form for poultry, therefore plant sources (vitamin D2) of the vitamin should not be relied on to provide sufficient vitamin D for these species.
Poultry breed and age influence the requirements of vitamin D. Different broiler strains (Elliot and Edwards, 1994) and turkey breeds (Dudley-Cash, 1999) have responded differently to leg problems and vitamin D supplementation. Soares et al. (1976) concluded that, with age, there appears to be a progressive deterioration in the ability of the hen's liver to hydroxylate vitamin D3 to 25-hydroxyvitamin D3. Stevens and Blair (1987) stated that reduced hydroxylation of vitamin D3 in the liver or kidney could result in inadequate production of 1,25-(OH)2D3 for maximum absorption of calcium and phosphorus for bone formation. The laying hen is able to obtain 1,25-(OH)2D3 through metabolism of dietary vitamin D3 sources when fed sufficient amounts to maintain normal production and eggshell quality parameters. There is evidence, however, that the hormone is not produced from vitamin D3 at levels high enough to support or maintain tibia weight and perhaps tibia strength in the aged hen.
In addition to sunlight, other factors influencing dietary vitamin D requirements include (1) amount and ratio of dietary calcium and phosphorus, (2) availability of phosphorus and calcium, (3) species breed and age, and (4) physiological factors.
Vitamin D becomes a nutritionally important factor in the absence of sufficient sunlight. Sunlight that comes through ordinary window glass is ineffective in producing vitamin D in skin since glass does not allow penetration of ultraviolet (UV) rays, and its effectiveness is dependent on length and intensity of UV rays that reach the body. Poultry housed indoors for much or all of the year must depend on their feed for the vitamin D they need; in a modern agricultural economy this applies particularly to intensive poultry production.
Amounts of dietary calcium and phosphorus, and the physical and chemical forms in which they are presented, must be considered when determining requirements for vitamin D. Species differences can be illustrated by the fact that adequate intakes of calcium and phosphorus in a diet that contains only enough vitamin D to produce normal bone in the rat or pig will quickly cause the development of rickets in chicks. High dietary calcium concentrations can precipitate phosphates as insoluble calcium phosphate. Soluble calcium salts are more readily absorbed and oxalates tend to interfere with absorption, but dietary vitamin D or irradiation can overcome some of this interference. Phosphorus absorption is mostly independent of vitamin D intake, with the inefficient absorption in rickets being secondary to failure of calcium absorption and the improvement upon vitamin administration being a result of improving calcium absorption.
The need for vitamin D depends to a large extent on the ratio of calcium to phosphorus. The vitamin D needs of poultry are increased several fold by inadequate levels of calcium and (or) phosphorus or by improper ratios of these minerals in the diet. As this ratio becomes either wider or narrower than the optimum, the requirement for vitamin D increases, but no amount will compensate for severe deficiencies of either calcium or phosphorus. For poultry, the optimum dietary ratio of calcium:inorganic phosphorus is approximately 2:1. The consensus opinion of NRC (1994) was that 5 µg per kg (2.3 µg per lb) was a suitable D3 requirement for chicks between hatching and 21 days of age when diets contained adequate levels of calcium and available phosphorus. However, Baker et al. (1998) reported that chicks fed on diets that are severely deficient in available phosphorus continue to respond to D3 in excess of 37.5 µg per kg (17.0 µg per lb).
Phosphorus of inorganic orthophosphate tends to be well absorbed, other factors being favorable, while that of phytic acid, which is the predominant phosphorus compound of unprocessed cereal grains and oilseeds, is poorly available to poultry. Supplemental dietary microbial phytase has been shown to increase the availability of phytate phosphorus for poultry and pigs fed a commercial corn-soybean meal diet. The phosphorus equivalency of microbial phytase for 1 g of nonphytate phosphorus is reported to be 650 to 750 units of phytase in broilers (Schoner et al., 1991; Kornegay et al., 1996; Yi et al., 1996), and 520 to 700 U of phytase in turkey poults (Qian et al., 1996, 1997; Ravindran et al., 1995). Kornegay et al. (1996) suggested that phytase, D3, and the calcium:phosphorus ratio are important factors in degrading phytate and improving phytate phosphorus and calcium utilization in broilers. Results show that supplemental phytase improved body weight gain, feed intake, toe ash content, and calcium and phosphorus retention of broilers fed a corn-soybean based diet; these improvements were negatively influenced by wide calcium:phosphorus ratios, and positively influenced by higher levels of D3. High levels of D3 added to the diets resulted in an increase in the retention of phosphorus and calcium, which seemed independent of supplemental phytase but synergetic with it. Maximum responses to supplemental phytase were achieved when broiler chicks were fed diets with 600 to 900 U of phytase per kg (272.75 to 409.1 U per lb) of diet with dietary calcium:phosphorus ratios of 1.1:1 to 1.4:1, and a D3 level of 660 µg per kg (300 µg per lb) of diet.
Apparently, there is a competitive interaction between vitamins A and D and vitamins E and D. Poults that were fed a diet containing the NRC recommended level of vitamin D and a high level of vitamin A developed hypocalcemia and rickets (Muirhead, 1987). An extremely high level of vitamin A (45,000 IU per kg or 20,454 IU per lb) when fed to broiler chicks receiving diets ranging in supplemental vitamin D3 levels from 0 to 3,200 IU per kg (0 to 1,454.5 IU per lb) decreased body weight, bone ash, and plasma calcium levels while increasing the incidence and severity of rickets (Aburto et al., 1998). Supplementation with high levels of either 25-(OH)D3 or 1,25-(OH)2D3 overcame the toxic effects of the excess vitamin A. Similarly, supplementing chicks and turkey poults with extra vitamin D3 partially overcame the effects of vitamin A toxicosis as measured by growth and skeletal abnormalities (Veltmann et al., 1987). The mechanism of vitamin A toxicosis, suggested in work with rats, is that excess vitamin A decreases bioactive serum parathyroid hormone (PTH) and 1,25-(OH)2D3 (Frankel et al., 1986). Supplementing a moderate level of vitamin E (150 IU per kg or 68.2 IU per lb) did not aggravate a mild cholecalciferol deficiency induced by feeding 75 IU of vitamin D3 per kg (34.1 IU per lb) (Bartov, 1997). However, exceptionally high levels of either vitamin A (>80,000 IU per kg or 36,364 IU per lb) or vitamin E (10,000 IU per kg or 4,545 IU per lb) will limit utilization of low supplemental levels of vitamin D3 if chicks have no exposure to UV light (Aburto and Britton, 1998a).
It is not clear that the feeding of any vitamin D3 metabolite will alter eggshell quality or egg specific gravity. While a few reports indicate a benefit of feeding 1,25-(OH)2D3 (Tsang et al., 1990; Tsang and Grunder, 1993), other workers were unable to observe any difference between vitamin D forms (Frost et al., 1990). Keshavarz (1996) observed an increase in the number of cracked eggs and (or) poor shell quality when supplemental vitamin D3 levels were below 2,000 IU per kg (909.1 IU per lb). Thus, it appeared levels of vitamin D equal to or above this level were needed to optimize shell quality in a large-egg type hen.