He could be called “Dr. Vitamin D,” based on the amount of time and effort Dr. Corwin Nelson has spent researching the fat-soluble vitamin in dairy cattle. Nelson is an Associate Professor of Physiology at the University of Florida.
On a recent episode of The Dairy Podcast Show, Nelson shared how insights about Vitamin D have evolved over the course of history and his own career as a dairy researcher. He said the importance of Vitamin D in dairy animals now spans far beyond its first identified function of preventing rickets (soft bones) in mammals.
Nelson said Vitamin D is very critical for maintenance of calcium homeostasis that not only affect proper skeletal development, but also conditions like hypocalcemia (milk fever) in fresh cows. “A lot of what we know about Vitamin D was really discovered in relation to control and prevention of milk fever in dairy cows,” he stated.
More recent knowledge points to even broader functions of Vitamin D, including tissue development, immune system support, gut physiology, and reproductive physiology. He said the value – and subsequent optimal levels – of Vitamin D span beyond those needed for calcium regulation.
Nelson noted that the way Vitamin D supplementation is delivered to lactating dairy cows also has evolved. Rather than feeding straight-up Vitamin D (cholecalciferol), nutritionists can “skip” a metabolic step, delivering Vitamin D in a form closer to its active form in the cow’s body. This is accomplished by feeding 25-hydroxyvitamin D3 (calcidiol) prepartum.
His team’s research has focused almost exclusively on 25-hydroxyvitamin D3, because it is more available to the cow and serves as a substrate to the enzymes that make the active form of Vitamin D, which is 1,25-dihydroxyvitamin D3 (calcitriol).
In terms of milk fever, severe clinical milk fever that cause downer cows has been nearly eliminated by DCAD diets for prepartum cows. But as researchers have drilled deeper into subclinical hypocalcemia – defined as low blood calcium without clinical signs – 25-hydroxyvitamin D3 has been found to have a helpful role.
His team’s research has shown that supplemental 25-hydroxyvitamin D3 does not have much effect on “transient” subclinical hypocalcemia, which he described as a short-term drop in blood calcium immediately after calving. Cows in the transient state tend to quickly rebound and launch into a productive lactation. But prepartum supplementation has been shown to improve the chronic/delayed cases, which are described as persistently low calcium 4-5 days into lactation and beyond.
Those cows tend to be plagued with increased cases of metritis, lower milk production, reproductive challenges, and ketosis.
While the mechanism by which supplemental 25-hydroxyvitamin D3 prepartum protects cows from longer-term subclinical hypocalcemia is not completely understood, Nelson has a couple of theories. He said it could be due to the increased intestinal calcium absorption it promotes, creating a greater flush of calcium prepartum, especially when combined with a low DCAD diet. “It may help those cows’ ability to recover from transient subclinical hypocalcemia and climb out of that hole even faster,” he explained.
Or, he said it could be that the presence of more of the active components of Vitamin D3 in the cow’s system helps mitigate the effects of inflammation, which can cause a temporary drop in systemic calcium because it is shuttled to the tissues of the liver, lungs, and peritoneal cavity during bouts of inflammation. Nelson said the presence of more Vitamin D3 provides checks and balances to contain the inflammatory process during postpartum challenges like metritis.
The upshot of prepartum 25-hydroxyvitamin D3 supplementation: more milk. Nelson believes that is due largely to less severe cases of metritis, meaning inflammation is kept in check and cows go off feed less. “A consistent outcome that we’ve seen is increased milk production – up to 8 pounds per day for the first several weeks of lactation,” he stated.
