With adequate direct exposure to sunlight, ruminants do not have an absolute dietary requirement for vitamin D, due to its production in the skin through the action of UV radiation on 7-dehydrocholesterol. Other factors influencing dietary vitamin D requirements include (1) amounts and ratio of dietary calcium and phosphorus, (2) bioavailability of calcium and phosphorus, (3) species and (4) physiological state of the animal.
Vitamin D becomes nutritionally important in the absence of sufficient sunlight. Sunlight passing through ordinary window glass is ineffective in producing vitamin D in skin because glass blocks penetration of UV radiation. Both time of exposure and intensity of UV radiation affect vitamin D production in skin. Additionally, coat color, coat thickness and skin pigmentation are important determinants of vitamin D production by irradiation. Radiation is more effective on bare skin than through a heavy coat of hair or wool. Irradiation is less effective on dark-pigmented skin. Ruminants housed in confinement depend on their diet for their vitamin D requirements. In the current agricultural economy, confinement livestock production is common. A significant proportion of dairy cattle and calves are housed under conditions with little or no exposure to sunlight. Cattle that are not confined are subject to seasonal variation in sunlight exposure.
The vitamin D requirements of ruminants are listed in NRC publications in units of IU per kg (lb) of body weight per day, IU per kg (lb) of diet, or IU per head per day. One IU of vitamin D is defined as 0.025 µg of cholecalciferol (D3) or its equivalent, and therefore 1 mg of cholecalciferol possesses 40,000 IU of vitamin D activity. Typical requirements of cattle, sheep and goats are shown in Table 1. The vitamin D requirement of beef cattle (NRC, 1996) is 275 IU per kg (125 IU per lb) of diet on a dry basis. Vitamin D requirements for dairy cattle are 1,000 IU per kg (455 IU per lb) of diet for lactating cows, 1,200 IU per kg (545 per lb) of diet for dry, pregnant cows and 300 IU per kg (136 IU per lb) of diet for all age groups of calves, heifers and mature bulls (NRC, 1989). No direct allowance is made for varying levels of milk production, which is likely justified because the vitamin D content of milk does not increase markedly even when massive doses of vitamin D are fed (McDermott et al., 1985). The vitamin D requirement of all classes of sheep, except early-weaned lambs, is 555 IU per 100 kg (252 IU per 100 lb) of body weight per day. For early-weaned lambs, it is 666 IU per kg (303 IU per 100 lb) of body weight per day. Dietary vitamin D requirements of ruminants exceeds that of nonruminants in part due to ruminal degradation (Parakkasi et al., 1970; Sommerfeldt et al., 1979). Vitamin D3 (cholecalciferol) is more biologically active than vitamin D2 (ergocalciferol) in dairy calves (Sommerfeldt et al., 1983).
