ABSTRACT
In the 1930s, selenium was discovered to be accumulated and concentrated from soils by some plants (Franke, 1934). Within the Great Plains states of the United States, plants, principally of the genus Astragalus (Beath and Lehnert, 1917), Stanleya, Xylorrhiza, and Oonopsis, when growing on seleniferious shales, concentrated the more toxic inorganic selenium (Rosenfeld and Beath, 1946) into the less toxic organic selenium compounds (Shrift and Virupaksha, 1965). Such concentrations of selenium by the “selenium accumulator plants” could exceed 1 g Se/kg of dry matter, and selenium became known to be toxic among grazing livestock, principally cattle and sheep, but also horses (Figure 5.2). Manifestation of selenium toxicity affected the nervous system, growth of hair, and other keratotic tissues. The sloughing off of horses’ hooves observed in cases of selenosis in the Great Plains states was apparently also noted in horses by Marco Polo while traveling the Silk Road in the thirteenth century as attributed by his fellow prisoner and biographer, Rustichello da Pisa (In French, 1298; translated by William Marsden, 1818, https:// archive.org /stream/travelsofmarcopo92polo#page/60/mode/2up, and again in the translation by Thomas Wright, London, 1892, from the 1818 translation by Marsden).
