Seasonal variation in vitamin D status of beef cattle reared in the central United States.

The objective was to retrospectively measure seasonal sunlight-associated variation in serum concentrations of 25-hydroxyvitamin D (25OHD) in beef cattle. The concentration of 25OHD was measured in crossbred animals born from March to May in 2011 and 2012. Vitamin D status 2 to 3 mo after birth (period 1) was only available for 2012 calves and was measured in June 2012. Period 1 animals had serum 25OHD concentrations of 26.3 ± 1.5 ng/mL. The 25OHD concentrations for late summer (period 2) were 46.6 ± 1.4 and 51.0 ± 1.5 ng/mL for 2011 and 2012, respectively. Serum concentration of 25OHD in early fall (period 3) were 63.8 ± 1.4 and 55.2 ± 1.5 ng/mL for calves in 2011 and 2012, respectively. Values observed for both late summer and early fall indicated vitamin D sufficiency (P < 0.001) compared with period 1. With diminishing exposure to ultraviolet B and consuming ∼800 IU or 1800 IU (2011 and 2012, respectively) of supplemental vitamin D, the calves' midwinter (period 4) 25OHD concentrations fell to 15.2 ± 1.6 and 16.7 ± 1.5 ng/mL for 2011 and 2012, respectively, after 4 to 5 mo on a finishing diet (P < 0.0001). This is considered vitamin D insufficiency in most species. Results indicate that calves are marginally sufficient to insufficient for vitamin D based on serum 25OHD concentrations soon after birth and during winter. Some individual animals would be classified vitamin D deficient. In the absence of sufficient UVB exposure, the dietary vitamin D requirements for rapidly growing beef cattle may need to be increased.