Hypomagnesemia is a metabolic disorder of ruminants not receiving adequate utilizable magnesium, either because of low Mg concentration in the forage or because of complicating factors affecting Mg availability. The disorder is more likely to affect cattle than sheep or goats and generally affects older, lactating animals. Magnesium deficiency may result in decreased milk or beef production (Grunes & Mayland, 1975; Grunes et al., 1970). An intensified hypomagnesemia, known as grass tetany, has caused a large number of cattle deaths in the temperate regions of the United States and other countries, resulting in losses of up to 3% in dairy or beef herds in a bad year (Baker & Gould, 1976; Grunes et al., 1970; Mansfield et al., 1975; Molloy, 1971). In some individual herds in some years losses have been even higher. Reid et al. (1978a) estimated annual losses in the United States at 1 to 2% of mature grazing animals. The severity of the problem is quite sporadic. Many factors affect Mg uptake by plants and its subsequent availability to animals. It is our objective to discuss the soil, climate, and plant factors that may ultimately affect Mg intake and availability to ruminants.