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SELENIUM IN SELENIFEROUS ENVIRONMENTS 29 of Se tolerance. Selenium absorbed by the nonaccumulator plants is con- verted into Se metabolites, which act as analogues of essential S compounds and interfere with cellular biochemical reactions. SELENIUM IN ANIMALS Selenium toxicosis in aquatic and terrestrial animals has been well described in several reviews (NAS-NRC, 1983; Sorensen, 1986; Chapter 8 of this publication). Selenite, selenate, and organic Se sources produce simi- lar toxicological effects (Moxon &Rhian, 1943; unpublished data by L.F. James et al., 1986). Absorption rates vary between chemical forms. Mini- mum lethal doses maybe 1.5 to 3.0 mg Se/kg body weight as selenate (Moxon & Rhian, 1943), although tolerance levels differ between and within animal species and nutritional conditions (Volcani et al., 1957). Chronic toxicity studies have indicated that diets providing 4 to 5 mg Se/kg body weight or more of Se daily result in chronic toxicity in laborato- ry animals (NAS-NRC, 1983). The Food and Nutrition Board of the Na- tional Academy of Sciences (1980) has accepted 5 mg Se/kg diet as the critical level between toxic and nontoxic feeds. In general, Se as selenomethionine is more readily absorbed when in- gested by animals than is Selenite, selenate, or selenocystine (NAS-NRC, 1983). Nearly one-half of the Se in wheat grain is present as selenomethio- nine (Olson et al., 1970), so the Se in wheat is more readily available to animals than is the Se in selenocystine, Astragalus, or fish meal. Generally, Se from plant forms is more available to animals than Se from animal forms. As in plants, Se in animals interacts with other trace elements. Frank et al. (1986) found 20% lower liver Cu concentrations in cattle (Bos taurus) supplemented with 0.58 mg Se/kg diet compared with 0.18 mg Se/kg in the control diet over a 7- to 8-month period. Koh and Judson (1987) evaluated the efficacies of oxidized Cu particles and Se bullets on the production responses in young cattle marginally deficient in both elements. The Se bullets exerted an antagonistic effect on the availability of Cu from the Cu0 parti- cles, as indicated by reduced Cu concentrations in the whole blood, plasma, blood cells, and liver. Animal weight gains were significantly increased when both Cu and Se were supplemented, but not when provided individually. This has practical applications because Cu deficiency has been reported in areas where cattle and sheep (Ovis arees) are often supplemented with Se (H.F. Mayland, 1986, personal observation). Sulfate S is another anion that interferes with Se absorption and metabolism. Halverson et al. (1962) reported that the addition of sodium sulfate to rat (Rattus rattus) diets reduced the toxicity of 10 mg Se as selenate per kilogram diet. However, adding the sulfate salt to diets containing Selenite or seleniferous wheat was ineffective in reducing the toxicity. Thus, increas- ing the sulfate-S levels in diets that are marginally adequate in Se could be expected to increase the clinical incidence of white muscle disease (Hintz & Hogue, 1964). Pope et al. (1979) also found that blood-Se levels in sheep were inversely related to dietary S levels, but this effect could be countered by increasing the dietary intake of Se.