Minerals are required by all dairy cows, and the amounts found in most feeds normally are inadequate for high milk production or growth. If these requirements are not met, deficiency symptoms can occur. Pastures in the USA are no exception and are often deficient in several minerals compared to the NRC requirements. Even with grain supplementation of pasture-based diets, most rations need to be supplemented with concentrated sources of certain minerals.
Usually minerals are categorized as “macrominerals” and “microminerals." The macrominerals are generally expressed as percent of the dry matter of a feed. Minerals needed in lesser amounts, often called trace minerals, are usually expressed as parts per million (PPM) or milligrams per kilogram.
Source: Penn State University