A review of vitamin D and its importance to the health of dairy cattle

This Research Reflection short review will discuss vitamin D metabolism, its role in nutrition, disease prevention, and welfare of dairy cattle, as well as its toxicity. Vitamin D is an important fat-soluble vitamin. However, some researchers regard it as a hormone due to its function in the organism. Its role is not limited just to Ca homoeostasis and bone metabolism but is also associated with immunity. In dairy cattle it is known for preventing milk fever. Cows can acquire vitamin D in many ways for example through feed, parenteral injections or through UVB irradiation from the sun or artificial lighting. The vitamin D in feed can either be plant-/ fungi- based ergocalciferol or animal-based cholecalciferol. There is currently only one registered feed vitamin D supplement for cattle in the European Union and it is cholecalciferol. Animals can also synthesize their own vitamin D when 7-dihydrocholesterol in the skin is irradiated with UVB light resulting in cholecalciferol production. Despite its importance, many cattle are deficient in vitamin D due to inadequate supplementation or insufficient sun exposure. In a study performed at the Veterinary Faculty in Slovenia 12 high producing Holstein Friesian cows at a commercial dairy farm were blood tested for vitamin D status for three succeeding months and all but one were vitamin D insufficient in all testings. The cows were not exposed to direct sunlight and the content of vitamin D(3)in feed was <400 IU/kg dry matter, which is less than half of the NRC (2001) recommendation. Deficiency can also occur due to diseases affecting the gastrointestinal tract, such as paratuberculosis, which lower the absorptive capacity of the gut. Vitamin D can be toxic if cows are over-supplemented or consume large quantities of plants likeTrisetum flavescens, which contain an active form of vitamin D-calcitriol or its glycosides, that are activated by digestion in the rumen.