Greenhouse and field experiments were conducted at the High Plains Grasslands Research Station to determine what might cause tetany-prone forage. The soil was analyzed for ammonium acetateextractable cations, cation exchange capacity, and alkaline earth carbonates; and the successive harvests of forage plants were analyzed for magnesium (Mg), calcium (Ca), potassium (K), and nitrogen (N). Forages used in the study included legumes: 'Lutana' Cicer milkvetch (Astragalus cicer L.), 'Remont'

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... n (Onobrythis viciofoliu Stop.), 'Dawson ', 'Vernal', 'Team', and 'Fremont' alfalfas (Medicugo sativa L.); and grasses: 'Latar' orchardgrass (Ductyhs glomerutu L.), 'Fawn' tall fescue (Festuca urundinuceu Schreb), 'Regar' bromegrass Bromus biebersteinii Roem & Schult.), 'Manchar' smooth bromegrass (Bromus inermis Leyss), 'Garrison' creeping foxtail (Alopercurus prutensis L.), 'Luna' pubescent wheatgrass [Agropyron trichophorum (Link) Richt.], and 'Greenar' intermediate wheatgrass [Agropyron intermedium (Host) Beauv.]. Forage Mg level increased when average soil temperature increased from 16.6 to 22.7" C. Fertilization with 1,134 kg of Mg as MgSOd/ha did not increase forage Mg level. Latar orchardgrass and Fawn tall fescue consistently produced forage containing more than 0.20% Mg, whereas wheatgrass species produced forage with Mg levels as low as 0.11%. All legumes had Ca levels ranging from 1.0 to 2.5%. One field crop of Latar orchardgrass produced forage with a high K accumulation (K/Ca •t Mg ratio of 2.7). Predication of blood-serum Mg from forage nutrient content indicated values from 17 to 31 mg/l in lactating cows.