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Seasonal Variation in Vitamin D Status of Beef Cattle Reared in the Midwest and Fed to NRC Recommendations
The objective was to measure seasonal variation in concentration of circulating 25-hydroxyvitamin D (25OHD) in beef cattle reared in the Midwest and fed to NRC recommendations. The concentration of 25OHD reflects adequacy of vitamin D intake and indicates vitamin D status. Vitamin D is an important modulator of calcium homeostasis and adequate vitamin D has many positive effects on the immune system. Concentration of 25OHD was measured in crossbred animals from the USMARC Germplasm Evaluation Project. Sixty five steers and 31 heifers were used in the study. Animals were born in March and April of 2012. Plasma samples were collected on June 1 (JUN), September 11 (SEP), and October 1 (OCT), of 2012, and in March 1 (MAR), of 2013. Significant differences (P < 0.0001) were observed throughout the year for 25OHD. The concentration of 25OHD in JUN was 26.1 ± 1.7 ng/mL, which was an intermediate value of 25OHD, and statistically significant than the other measurements (P < 0.0001). The SEP and OCT concentrations of 25OHD were similar (P > 0.05) between them, and the highest of the study (50.8 ± 1.7 ng/mL and 53.0 ± 1.7 ng/mL, respectively). The lowest concentration of 25OHD was observed in MAR (16.6 ± 1.7 ng/mL), which was statistically different (P < 0.0001) from all other concentrations. The 25OHD concentrations closely followed seasonal UV exposure for animals housed outdoors in the Midwest. Sex was not a significant source of variation. The concentration of 25OHD representing optimal vitamin D status in cattle for calcium homeostasis has been established at greater than 20 ng/mL, and greater than 30 ng/mL for optimal function of the immune system. Results from the present study indicate that calves soon after birth (JUN) and during winter (MAR), are deficient in 25OHD concentration, therefore, in vitamin D status. In the absence of sufficient UV exposure from sunshine, the dietary vitamin D requirements for beef cattle need to be reevaluated.
Keywords: Cattle, Seasonal variation, vitamin D