Supplemental UV-B lighting to naturally enhance the vitamin D content of milk of housed dairy cows

Abstract

Publication history: Accepted - 19 October 2025; Published - 27 January 2026.The increasing incidence of vitamin D deficiency in Europe has generated interest in naturally enhancing vitamin D levels in commonly consumed foods, such as milk, by biofortification (defined here as increasing the concentration of a specific nutrient in a food by changing the diet or management of an animal). Endogenous, solar-driven biosynthesis of vitamin D is a potentially modifiable option for boosting vitamin D concentrations in milk. The objective of this study was to quantify milk and plasma vitamin D responses in housed cows exposed to artificial UV-B light over 12 wk. Two 2 × 2 factorial experiments (experiment 1 and 2) were conducted, each using 32 late-lactation Holstein dairy cows (separate cows in each experiment). Factors examined were level of supplementary UV-B lighting (no UV-B vs. UV-B) and the inclusion of a dietary additive, L-cysteine (Control vs. Additive). Cows were exposed to supplemental UV-B light for 30 and 60 min/d (experiment 1 and 2, respectively). Exposing cows to UV-B increased mean milk concentrations of vitamin D3 by 44.2% (experiment 1) and by 112% (experiment 2), compared with no-UVB. Milk concentrations of 25-(OH)-D3 also increased by 24.2% (experiment 1) and 20.9% (experiment 2). Mean milk vitamin D2 concentration was increased by 33% by UV-B treatment in experiment 2. Milk 25-(OH)-D2 levels were unaffected by UVB exposure. Milk concentrations of vitamin D3 changed with time, as did milk concentrations of vitamin D2. Milk concentrations of 25-(OH)-D2 changed with time in experiment 1, there was a treatment × time interaction for milk vitamin D3 concentrations in experiment 1, and a trend for an interaction in experiment 2. Mean plasma 25-(OH)-D3 concentration increased with UV-B treatment in experiment 1 only. Mean plasma 25-(OH)-D3 concentration decreased over time in experiment 1 but was unaffected in experiment 2. Dietary additive had no effect on any aspect of cow performance, and there were no interactions between UV-B and additive. The UV-B treatment did not affect milk yield or milk fat, protein, or lactose concentrations. Milk yield and milk fat, protein, and lactose concentrations changed with time, and there was a treatment × time interaction for milk yield (experiment 2 only). This study demonstrates the potential for increasing the vitamin D3 content of milk produced by housed cows by short exposure (30–60 min/d) to supplementary UV-B light as a successful biofortification strategy.The study was cofunded by the Agri-Food Quest Competence Centre (Belfast, UK; AFQCC) and was conducted in partnership with Devenish Nutrition Ltd. (Belfast, UK), Agri-Food and Biosciences Institute (Belfast, UK; AFBI), John Thompsons and Sons Ltd. (Belfast, UK), Dale Farm Limited (Belfast, UK), Karro Food Group (Malton, UK), College of Agriculture Food and Rural Enterprise (Antrim, UK; CAFRE), and Ulster University (Coleraine, UK; Nutrition Innovation Centre for Food and Health: NICHE)