Synthesis of vitamin D in the skin of swine through exposure to UV irradiation is limited because of the production trend of complete confinement for swine in commercial production. In addition, feedstuffs supply negligible amounts of vitamin D activity in the diet. Consequently, all swine diets should be supplemented with vitamin D. Vitamin D3 is the primary source of supplemental vitamin D in foods, feeds and pharmaceuticals (Adams, 1978).
Some irradiated sterol resins used as the source of vitamin D3 in various vitamin D3 products for feed use may contain excessive amounts of biologically inactive impurities, such as tachysterol and isotachysterol. The vitamin D3 activity in low-purity vitamin D3 resins and supplements (Baker, 1978) has been overestimated by the United States Pharmacopeia (USP) Chemical Assay, which does not correct for these biologically inactive compounds. A commercially available vitamin product containing stabilized, high-purity vitamin D for feed or drinking water use should be used to ensure the vitamin D levels needed to prevent deficiency and allow optimum performance in swine.
Pure vitamin D3 crystals or vitamin D3 resin is very susceptible to degradation on exposure to heat or contact with mineral elements. In fact, the resin is stored under refrigeration with nitrogen gas. Dry, stabilized supplements retain potency much longer and can be used in high mineral supplements. It has been shown that vitamin D3 is much more stable than D2 in feeds containing minerals.
Stabilization of the vitamin can be achieved by (1) rapid compression of the mixed feed, for example into pellets so that air is excluded; (2) storing feed in cool, dry, dark conditions; (3) preventing close contact between the vitamin and potent metallic oxidation catalysts, for example, manganese; and (4) including natural or synthetic antioxidants in the mix. The vitamin can also be protected by enclosing it in durable gelatin beadlets.
Stability of dry vitamin D supplements is affected most by high temperature, high moisture content and contact with trace minerals, such as ferrous sulfate, manganese oxide and others. Hirsch (1982) reports the results of a "conventional," or nonstabilized, vitamin D3 product being mixed into a trace mineral premix or into animal feed and stored at ambient room temperature (20 to 25°C) for up to 12 weeks. The mash feed had lost 31% of its vitamin D activity after 12 weeks, and the trace mineral premix had lost 66% of its activity after only six weeks in storage.
In addition to providing supplemental vitamin D in feed and water, injectable sources are available. Parenteral vitamin D3 treatment of sows before parturition provided an effective means of supplementing piglets with vitamin D3 (via the sow's milk) and its dehydroxy metabolites by placental transport (Goff et al., 1984).
Cost of vitamin D supplementation to livestock diets is nominal (Rowland, 1982). In contrast, the cost of not adding enough vitamin D to prevent deficiencies is very high. Supplemental levels of vitamin D3 administered to pigs through the feed should be adjusted to provide the margin of safety needed to offset the factors influencing the vitamin D needs of swine. This is important to prevent deficiency and allow optimal performance. Factors that increase the amount of vitamin D needed to maximize productive and reproductive responses often may not be reflected in NRC minimum requirements. Consequently, nutritionists often use more than the minimum levels of vitamin D in feeds. Successful nutrition programs may greatly exceed the NRC minimum of vitamin D. However, no amount of vitamin D can make up for lack of enough calcium or phosphorus in the diet.
Besides inadequate quantities of dietary vitamin D, deficiencies may result from (1) errors in vitamin addition to diets, (2) inadequate mixing and distribution in feed, (3) separation of vitamin D particles after mixing, (4) instability of the vitamin content of the supplement, or (5) excessive length of storage of diets in environmental conditions causing vitamin D loss (Hirsch, 1982).
Supplementation considerations are dependent on other dietary ingredients. The requirements for vitamin D are increased several fold by inadequate levels of calcium and (or) phosphorus or by improper ratios of these two elements in the diet. A number of reports have indicated that molds in feeds interfere with vitamin D (Cunha, 1977), for example, when corn contains the mold Fusarium roseum, a metabolite of this mold prevents vitamin D3 in the intestinal tract from being absorbed by the chick. A similar deleterious effect on vitamin D metabolism would be expected in the pig.
Other factors that influence vitamin D status are diseases of the endocrine system, intestinal disorders, liver malfunction, kidney disorders and drugs. Liver malfunction limits production of the active forms of the vitamin, while intestinal disorders reduce absorption. It is possible that swine with certain diseases or heavy infestation of internal parasites may be unable to synthesize the metabolically active forms of vitamin D as a result of liver or kidney damage. Unknown factors in feeds may increase vitamin D requirements. For example, there is evidence of a factor in rye and in soybean fractions that can produce malabsorption of this vitamin in the intestine (MacAuliffe and McGinnis, 1976).
Hendricks et al. (1967) investigated whether antagonism occurs between the source of vitamin A activity and vitamin D. The authors reported that feeding fermentation beta-carotene at 8 mg per kg of diet did not increase the need for ergocalciferol above the level normally required by baby pigs fed a purified isolated-soy diet containing retinyl acetate.