Compound feeds for horses most often have added vitamin D3. The amounts added may vary, but are commonly between 500 and 2000 IU per kg.
Grains contain very little or no vitamin D. Fresh grasses, haylage and hay contain vitamin D2 in significant, but varying amounts. In general, vitamin D levels in grass increase in the later stages of maturity, meaning that late cut grass will have more vitamin D than early cut grass. When hay or haylage are dried in the sun, more vitamin D2 will be formed, depending on the duration of exposure to sunlight.
Vitamin D is normally not analyzed in forage samples due to cost. In the planning of rations, horse owners therefore have to rely upon mean values given in the literature. In PC-Horse we use values for vitamin D in grass, haylage and hay according to Ballet et al., 2000.