Colostrum is the calf's first meal and the foundation of its health. Because the bovine placenta does not transfer antibodies, all of the immune protection a calf has at birth must come from the dam's first milking. Careful harvesting and feeding protocols only work if the cow produces enough colostrum of sufficient quality in the first place, and many herds face periods when she does not. Nutrition of the close-up cow is therefore a decisive lever. Among prepartum strategies, 25-hydroxyvitamin D3 (calcidiol, Hy-D®) has accumulated a consistent body of evidence demonstrating benefits to both the volume and quality of colostrum, with effects extending to the newborn calf.
Why Colostrum Matters
Colostrum delivers immunoglobulins, cytokines, growth factors, and concentrated nutrients that the calf cannot obtain any other way. Calves that fail to absorb sufficient IgG face morbidity and mortality risks up to five to six times higher than herd-mates with successful passive transfer, while adequate transfer has been associated with better average daily gain in heifers and higher milk production in the resulting cows (Gavin et al., 2018).
Supply, however, is not guaranteed. A prospective study in a 2,500-cow Jersey herd documented a dramatic seasonal pattern: average colostrum yield fell from 6.6 kg per cow in June to 2.5 kg in December, and more than 35% of multiparous cows produced no colostrum at all in December, compared with about 1% of first-lactation animals (Gavin et al., 2018).
Photoperiod was the factor most strongly correlated with yield, alongside dry period length, age at calving, calf sex, and sire line. Such swings can force dairies into months of colostrum replacer use or heavy reliance on banked colostrum, so any prepartum strategy that supports colostrogenesis, particularly in older cows, carries real economic value.
Why 25-Hydroxyvitamin D3?
Conventional vitamin D3 (cholecalciferol) must undergo two hydroxylations before it becomes active. The first takes place in the liver, producing 25-hydroxyvitamin D3 (calcidiol), the main circulating form of the vitamin. The second occurs in the kidney and yields calcitriol, the hormone that regulates calcium and phosphorus absorption.
The hepatic step is a bottleneck: plasma 25-OH-D3 rises only modestly even when cholecalciferol is fed at several times the recommended level (Jensen et al., 2026). Feeding calcidiol directly bypasses this limitation. In Florida, cows receiving 3 mg/d of calcidiol reached prepartum plasma 25-OH-D3 concentrations of 237 ng/mL, compared with 60 ng/mL in cows fed the same amount of cholecalciferol (Martinez et al., 2018). Because calcidiol has a half-life of roughly two weeks, this elevated status persists into early lactation after supplementation has stopped.
Improved status matters at calving. Cows supplemented with Hy-D showed higher arterial ionized calcium on calving day than cows fed cholecalciferol (Jensen et al., 2026), and calcium is central to colostrum secretion.
There is also a more direct route. Mammary epithelial cells express both the vitamin D receptor and the enzyme needed to convert 25-OH-D3 into calcitriol locally, and calcitriol stimulates calcium and glucose uptake by mammary cells (Martinez et al., 2018). A larger supply of substrate may therefore support the gland as it prepares for lactation.
Evidence for Colostrum Yield
The most-cited trial was conducted at the University of Florida with 79 Holstein cows, arranged in a factorial design that combined positive or negative DCAD with 3 mg/d of either cholecalciferol or calcidiol during the last 21 days of gestation. Cows fed calcidiol tended to produce more colostrum, 7.8 versus 6.0 kg, and the response was observed under both DCAD levels, indicating that the benefit did not depend on an acidogenic diet. Calories secreted as colostrum, along with yields of protein, solids-not-fat, and total solids, increased significantly with calcidiol (Martinez et al., 2018).
These results were confirmed under very different conditions in Brazil, where 40 multiparous Holstein cows received an acidogenic diet with either 0.625 mg/d of vitamin D3, the conventional recommendation, or 3 mg/d of calcidiol for 30 days before calving. Calcidiol increased colostrum yield from 5.44 to 7.50 kg (P = 0.02), a gain of more than 2 kg per cow (Silva et al., 2022).
The supplemented cows also had higher plasma glucose on the day of calving and a tendency toward greater prepartum dry matter intake, which the authors linked to the extra colostrum, since glucose is the main substrate for lactose synthesis. That this response appeared in a tropical environment, where cows have abundant sunlight for endogenous vitamin D synthesis, underscores how limiting the hepatic bottleneck can be.
The largest trial to date enrolled 173 Holstein cows and compared calcidiol with cholecalciferol at 1 or 3 mg/d from 250 days of gestation. Calcidiol tended to increase colostrum yield (5.5 versus 4.6 kg) and significantly increased yields of fat and total solids, raising the net energy secreted as colostrum from 7.0 to 9.0 Mcal (Poindexter et al., 2023). In each study, the direction of the effect was the same: more colostrum and more nutrients within it.
Beyond Volume: Quality and Calf Status
More colostrum is only useful if quality is preserved, and here the evidence is encouraging. In the Florida study, calcidiol increased colostrum IgG concentration from roughly 45–51 g/L with cholecalciferol to 58–60 g/L, together with true protein yield (Martinez et al., 2018). One proposed explanation is that vitamin D metabolites stimulate IL-10 production by B cells, favoring immunoglobulin secretion in the weeks before calving.
Not every trial has reproduced the IgG effect; Poindexter et al. (2023) found no difference in concentration, although the greater colostrum volume still delivered more total protein and solids to the calf.
Calcidiol also reaches the calf directly. In an Ohio study, cows fed 6 mg/d of 25-OH-D3 with a negative DCAD diet produced colostrum with roughly four times more 25-OH-D3 than cows fed cholecalciferol, and their calves were born with nearly double the serum 25-OH-D3 (80 versus 42–46 ng/mL), remaining higher after their first feedings (Weiss et al., 2015). Since vitamin D status is associated with innate immune function in cattle, this may give newborns an additional layer of resilience during their most vulnerable days.
Conclusion
Hy-D addresses a physiological bottleneck in vitamin D metabolism and supplies a metabolite the transition cow can use immediately. Across independent trials in temperate and tropical systems, prepartum calcidiol has consistently increased colostrum yield by approximately 1-2 kg per cow, improved its solids and IgG content, and enhanced the vitamin D status of the newborn calf. For herds facing seasonal colostrum shortages, or simply aiming to secure a strong start for every calf, Hy-D is a science-based tool that begins working before birth.
