Cattle, like people, can get vitamin D from food and from exposure to the sun. Specifically, vitamin D2 is acquired from plants, while vitamin D3 is synthesized by cells in sun-exposed skin. Vitamin D availability within the body is often measured as the concentration of 25(OH)D in the blood (serum). 25(OH)D is a product of metabolism of both vitamins D2 and D3. With summer sun exposure and no exogenous vitamin D, cows will have 40-100 ng/mL serum 25(OH)D. For cows without the opportunity to graze out on the range all summer, alfalfa hay can provide pretty significant amounts of vitamin D2, and even corn silage contains some. Additionally, vitamin-mineral supplement mixes usually contain vitamin D3, which is metabolized more efficiently than D2.
In a study published last year (full citation below), the investigators compared various management practices with 25(OH)D levels across several herds. Levels of vitamin D in the cows and heifers looked pretty good: all herds had 25(OH)D averages above 30 ng/mL, which is considered the minimum for vitamin D sufficiency. However, the herd that was supplementing at only 20,000 IU/day (rather than the 30,000-50,000 IU/day of the others) did have 22% of their cows below that sufficiency threshold.
The situation for calves in this study wasn’t quite so rosy. Now, newborn calf levels of serum 25(OH)D are typically much lower than in older animals—in this study they averaged 15 ng/mL. In the six herds examined, 25% of newborn calves had serum 25(OH)D concentrations below 10 ng/mL. If left uncorrected, these calves in particular could suffer impaired health. If we look at the figure below, we see distinct differences across farms as the calves age. (We’re looking exclusively at the pre-alfalfa-eating stage here.) Spring/summer sun exposure or supplemental vitamin D in the diet would seem to make a significant difference in a calf’s vitamin D level.
Given that vitamin D is associated with growth, development, and immune function, this nutrient is required starting with a calf’s first days. Most milk replacers contain adequate vitamin D. If raising calves on milk, one should provide supplemental vitamin D3 at a rate of 6000-10,000 IU/kg of dry matter. Additionally, giving calves a 50,000-100,000 IU bolus of vitamin D3 at birth may also be helpful.
(Before initiating a new treatment or nutrition regimen, you should consult your veterinarian/nutritionist.)