For ruminants in confinement in climates where the sunlight is not adequate for vitamin D production, the vitamin D content of the diet becomes important. In modern dairy operations, cattle are often completely or nearly completely confined without significant direct exposure to sunlight. These conditions, combined with higher levels of production and stress, warrant that supplemental vitamin D be provided. Cattle grazing during spring and summer months will obtain significant vitamin D from sun exposure. During fall and winter, however, sun exposure is reduced to levels that can result in marginal vitamin D status. This is a concern with both dairy and beef cows pastured during the last 60 to 90 days of pregnancy. In northern latitudes, it is unsafe to rely on exposure to sunlight to provide adequate vitamin D, at least during the six months of shortest day length (September 21 – March 21 in the Northern Hemisphere).
Milk is not an especially rich source of vitamin D, but the calf, kid or lamb can obtain adequate amounts by skin irradiation if exposed to sunlight for 12 hours per day. Sun-cured forage is the best natural source of vitamin D, although the vitamin D activity of forage is variable. Due to these uncertainties and the value of replacement animals, it is prudent to provide supplemental vitamin D to young calves, lambs or kids in milk replacer and starter diets.
The cost of vitamin D supplementation in livestock diets is nominal (Rowland, 1982). In contrast, the cost of inadequate vitamin D supplementation is very high. For optimum animal performance, vitamin D levels in feeds and mineral supplements should be adjusted to provide a margin of safety needed to offset the factors influencing the vitamin D needs of the animal and effects of storage. Factors that increase the amount of vitamin D required to optimize animal productivity are often not reflected in the NRC minimum nutrient requirements. Consequently, many nutritionists use greater than the minimum levels of vitamin D in feeds.
Vitamin D deficiencies may result from (1) errors in addition of the vitamin to diets; (2) inadequate mixing and distribution in feed; (3) separation of vitamin D particles after mixing; (4) instability of the vitamin D content of the supplement; or (5) excessive length of storage of feeds under environmental conditions causing loss of vitamin D loss (Hirsch, 1982).
Supplementation considerations are also dependent on other dietary ingredients. The vitamin D requirements are increased several fold by inadequate levels of calcium and (or) phosphorus or by improper calcium:phosphorus ratios in the diet. Several reports have indicated that molds in feeds interfere with vitamin D metabolism (Cunha, 1987). For example, a toxin produced by the mold Fusarium roseum interferes with vitamin D3 absorption from the intestinal tract of the chick. Similar deleterious effects of mycotoxins on vitamin D metabolism may occur in other species, including ruminants.
Other factors that influence vitamin D status are diseases of the endocrine system, intestinal, liver or kidney disorders and certain drugs. Liver or kidney disease or degeneration can limit production of the active form of vitamin D [1,25-(OH)2D]. Enteric disease or heavy infestation by internal parasites can reduce absorption of vitamin D and the other fat-soluble vitamins. Unknown factors in feeds, possibly other mycotoxins, may increase vitamin D requirements. For example, there is evidence of a factor in rye grain and soybeans that interferes with vitamin D absorption from the intestine (MacAuliffe and McGinnis, 1976).
Optimum levels of vitamin D recommended for ruminants are shown in Table 1. Although 15,000 IU vitamin D per day appeared adequate in lactating cows (Reeve et al., 1982), improvements in milk yield (Hibbs and Conrad, 1983) and reproduction (Ward et al., 1971) have been recorded when higher levels up to 50,000 IU per day were fed to lactating cows (Weiss, 1998). Given these data and the uncertainties of sun exposure and rumen metabolism of vitamin D, the levels indicated are believed to represent a safe optimum.
