Technical Abstract: Maintenance of production suitable to today's agricultural industry may require supplementation of mineral nutrients to ruminant livestock. Minerals may be supplied to livestock in several forms and knowledge of mineral levels and bioavailability in feedstuffs are important to economically optimize mineral supplementation with animal performance. Changes in forage maturity affect both forage mineral level and bioavailability. Both absorptive efficiency and the ability of an animal to store and use an element may vary with the animal's age, nutritional state, and reproductive status. Sodium and phosphorus are the macrominerals most consistently deficient in western rangeland forages. The need for magnesium is greatest in the early spring, when the potential for grass tetany exist. There is limited information on the economic value of potassium supplementation for mature grasses, but this nutrient is often provided in needed levels through supplementation of other limiting nutrients such as protein. Sulfur supplemetation should be considered when livestock are being provided a non-protein nitrogen source. Decisions about timing of supplementation can be difficult due to variations in macromineral profiles in time and space. The economic cost of supplementation must be determined over a time scale of several years due to variations in mineral profiles among years and the animal's ability to store some nutrients. Improper balancing of minerals in a supplement can negatively impact the supplementation program.