Knowledge about vitamins has evolved greatly since they were discovered < 100 yr ago, but the quantitative determination of the vitamin requirements of dairy cows has been difficult. Current requirements are based on data that are at least 20 yr old and, in some cases, > 50 years old. The response variables in those studies were the prevalence of overt signs of deficiency, level of milk production, reproductive performance, and vitamin concentrations in blood and tissues. Since those studies were conducted, milk production per cow has increased substantially, and the feeding and management of cows have changed (less pasture, less forage, and more total confinement). More refined measures of vitamin adequacy have been developed. Immunocompetence may be a useful tool for determining the requirements of some vitamins. Requirements that are based on measures of immune function are usually higher than those that are based on production or reproduction. The current requirements for vitamin D appear to be adequate, but the requirement for vitamin A should be increased by approximately 50% because of differences in bioavailability between beta-carotene and retinyl esters. Based on health data, the vitamin E requirement should be increased at least 500% and perhaps as much as 700% for dry cows and lactating cows.
