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 D3 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 like Trisetum flavescens, which contain an active form of vitamin D-calcitriol or its glycosides, that are activated by digestion in the rumen.

中文翻译：

---

维生素 D 综述及其对奶牛健康的重要性

这篇研究反思简短回顾将讨论维生素 D 代谢、它在营养、疾病预防和奶牛福利中的作用，以及它的毒性。维生素D是一种重要的脂溶性维生素。然而，由于其在生物体中的功能，一些研究人员将其视为一种激素。它的作用不仅限于钙稳态和骨代谢，还与免疫有关。在奶牛中，它以预防牛奶热而闻名。奶牛可以通过多种方式获得维生素 D，例如通过饲料、肠胃外注射或通过来自太阳的 UVB 照射或人工照明。饲料中的维生素 D 可以是基于植物/真菌的麦角钙化醇或基于动物的胆钙化醇。目前欧盟只有一种注册的牛饲料维生素 D 补充剂，它是胆钙化醇。当皮肤中的 7-二氢胆固醇被 UVB 光照射时，动物也可以合成自己的维生素 D，从而产生胆钙化醇。尽管维生素 D 很重要，但由于补充不足或阳光照射不足，许多牛缺乏维生素 D。在斯洛文尼亚兽医学院进行的一项研究中，对商业奶牛场的 12 头高产荷斯坦弗里西亚奶牛进行了连续三个月的维生素 D 状态血液测试，除一头外，所有测试中的维生素 D 均不足。奶牛没有暴露在阳光直射和维生素 D 的含量 由于补充不足或阳光照射不足，许多牛缺乏维生素 D。在斯洛文尼亚兽医学院进行的一项研究中，对商业奶牛场的 12 头高产荷斯坦弗里西亚奶牛进行了连续三个月的维生素 D 状态血液测试，除一头外，所有测试中的维生素 D 均不足。奶牛没有暴露在阳光直射和维生素 D 的含量 由于补充不足或阳光照射不足，许多牛缺乏维生素 D。在斯洛文尼亚兽医学院进行的一项研究中，对商业奶牛场的 12 头高产荷斯坦弗里西亚奶牛进行了连续三个月的维生素 D 状态血液测试，除一头外，所有测试中的维生素 D 均不足。奶牛没有暴露在阳光直射和维生素 D 的含量3饲料中的含量低于 400 IU/kg 干物质，不到 NRC (2001) 建议的一半。由于影响胃肠道的疾病，例如副结核病，也会导致缺乏症，这会降低肠道的吸收能力。如果奶牛过度补充或食用大量植物，如苦参，其中含有一种活性形式的维生素 D-骨化三醇或其糖苷，它们在瘤胃中被消化激活。