Roughage can be a significant source of vitamin D in herbivore diets. The vitamin D content of forages can vary considerably and depends, among others, on plant species, plant maturity and climate fluctuations. Ultraviolet B (UVB) exposure causes conversion of provitamin D to vitamin D and thus represents the most important influence. To characterise this effect, two batches of a young-cut roughage of high hygienic and nutritional quality were exposed to constant artificial UVB-irradiation up to 192 h and were sampled at regular intervals. In previous studies, both ergocalciferol (vitamin D