Swine do not have a nutritional requirement for vitamin D when sufficient sunlight is available, since vitamin D3 is produced in skin through action of UV irradiation on 7-dehydrocholesterol. In addition to sunlight, other factors influencing dietary vitamin D requirements include (1) amount and ratio of dietary calcium and phosphorus, (2) availability of calcium and phosphorus, (3) species, and (4) physiologic 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 UV rays, and its effectiveness is dependent on length and intensity of UV rays that reach the body. Animals housed in confinement must depend on their feed for the vitamin D they need; in a modern agricultural economy this applies particularly to intensive swine and poultry production.
The colors of the skin are important in determining response to irradiation. Irradiation is less effective on dark-pigmented skin. This has been shown to be true for white and black breeds of hogs. White pigs have been shown to resist vitamin D deficiency signs about twice as long as colored pigs; an average of 45 minutes of daily exposure to January sunshine for two weeks was sufficient to cure rickets in white pigs in Minnesota (Cunha, 1977).
The vitamin D requirements of swine as listed in NRC (1998) range from 150 to 220 IU per kg (68 to 100 IU per lb) of diet. The estimated requirement for breeding and lactating animals is 200 IU per kg (91 IU per lb) of diet. However, no studies of the vitamin D requirement of sows during gestation or lactation have been reported (NRC, 1998). Miller et al. (1964) reported that the vitamin D2 requirement of the baby pig fed a casein-glucose diet was 100 IU per kg (46 IU per lb) of diet. However, if isolated soy protein is fed, the requirement is higher (Miller et al., 1965b).
Vitamin D requirements of swine are suggested to be sufficiently high to produce normal growth, calcification, production and reproduction in the absence of sunlight provided that diets contain recommended levels of calcium and available phosphorus. Wahlstrom and Stolte (1958) found that there is no need for supplemental vitamin D in highly fortified rations for growing pigs confined in the absence of sunlight with regard to growth. However, Wahlstrom and Stolte (1958) suggested that they did not use an adequate depletion period to induce true vitamin deficiency, as endogenous stored vitamin D was apparently available. 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.
The need for vitamin D depends to a large extent on the ratio of calcium to phosphorus. 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. It is recommended (NRC, 1998) that the dietary dry matter of rapidly growing young pigs should have a calcium:phosphorus ratio in the range of about 1:1 to 1.5:1; for adult maintenance, wider calcium:phosphorus ratios are possible. In these situations vitamin D requirements seem to be at a minimum and risks of vitamin D deficiency are less. Combs et al. (1966b) indicated the need for vitamin D supplementation may be reduced or eliminated when accompanied by "near optimum" dietary calcium and phosphorus concentrations. Their findings appear to be supported by those of Wahlstrom and Stolte (1958). However, Bethke et al. (1946) concluded that swine have a fundamental requirement for vitamin D even when the feed supplies a satisfactory ration and adequate amounts of calcium and phosphorus. Differences in the length of vitamin D depletion before initiation of the experiments or the inclusion of B vitamins and antibiotics in the basal ration in some of the experiments but not others may partially explain the conflicting findings.
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. 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 some of this interference can be overcome by dietary vitamin D or irradiation. Correspondingly, while the phosphorus of inorganic orthophosphate tends to be well absorbed, other factors being favorable, that of phytic acid, which is the predominant phosphorus compound of unprocessed cereal grains and oilseeds, seem to be poorly available to swine. 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 D administration being a result of improving calcium absorption.
In light of the cost of phosphorus and environmental concerns over its abundance in pig manure, the interrelationships and possible additive benefits of adding phytase and 1-alpha-hydroxylated vitamin D3 compounds are being investigated. In poultry, both phytase and 1-alpha-hydroxy D3 have been demonstrated to be efficacious for releasing phosphorus and trace minerals from phytase complexes (Edwards, 1993; Biehl et al., 1995). However, Biehl and Baker (1996) reported that 1-alpha-hydroxycholecalciferol was either significantly less effective in pigs than in chicks or not at all effective in improving phytase-phosphorus utilization in young pigs. Baker and Biehl (1996) indicated that 1-alpha-(OH)D3 seems to be much less efficacious in pigs than in chickens. Cromwell et al. (1996) indicated that addition of 1,25-dihydroxycholecalciferol at the rate of 10 mg per kg diet did not improve utilization of phytate phosphorus in pigs. Similarly, Cromwell et al. (1997) determined that addition of 5 to 200 mg per kg of D3 to low-calcium, low-phosphorus diets did not improve phytate phosphorus utilization in growing pigs. The 200 mg per kg level of D3 depressed growth rate, bone strength and plasma P in trials conducted with pigs (14 to 21 kg initial weight). Adeola et al. (1998) determined that phytase or cholecalciferol supplementation of a low-calcium, low-phosphorus diet produces similar growth performance as a diet with adequate calcium and phosphorus when fed to 20 kg pigs. Li et al. (1998) found that the addition of 2,000 IU per kg Vitamin D3 to a diet containing phytase tended to increase pig performance and tended to further increase digestibility of dry matter, phosphorus and calcium. However, the addition of vitamin D3 did not significantly increase the effectiveness of phytase. Whether or not vitamin D will be found to improve the effectiveness of phytase, both phytase and 1-alpha-hydroxy D3, alone or in combination, have been suggested to offer potential for reducing feed costs while simultaneously reducing phosphorus in manure (Baker and Biehl, 1996).
It was generally 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 provided to these species. More recently, studies with swine indicate that D3 is likewise more potent for swine than D2, but the difference between forms is much less dramatic than for poultry. Horst et al. (1981) demonstrated that pigs discriminate in their metabolism of the two forms. Horst et al. (1982) indicated that the rat and pig as well as the chick discriminate between ergocalciferol and cholecalciferol when these vitamins are administered together orally. Pigs given oral doses of a mixture of vitamin D2 and D3 had significantly higher concentrations of plasma vitamin D3, 25-(OH)D3, 24,25-(OH)2D3, 25,26-(OH)2D3 and 1,25-(OH)2D3 than corresponding vitamin D2 counterparts.
