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Distribution GEOLOGICAL DISTRIBUTION Selenium is widely distributed in minute amounts in virtually all materials of the earth's crust, having an average abundance of about 0.09 ppm (Lakin, 1972~. Its occurrence has been determined in a wide variety of rocks, min- erals, lunar and volcanic materials, fossil fuels, soils, plant materials, and waters. Selenium is rarely found in the native state. It has been found as a major constituent of 40 minerals and a minor component of 37 others, chiefly sulfides (Cooper et al., 1970~. The minerals are finely dispersed without forming a selenium ore. Selenium is located in mineral deposits and some soil formations where a high concentration of sulfur is found (Painter, 1941). The greatest abundance of selenium is in igneous rocks, where it occurs as selenite minerals; in sulfides, isomorphous with sulfur; in hydrothermal deposits, commonly associated epithermally with antimony, silver, gold, and mercury; and in massive sulfide and porphyry copper deposits, where it occurs in small concentrations but large quantities (Elkin and Margrave, 1968~. Selenium is richest in chalcopyrite, bornite, and pyrite minerals (Cooper et al., 19701. High concentrations of selenium are found in sedi- mentary rocks such as shales, sandstones, limestones, and phosphorite rocks. Considerable variation has been found in the selenium content of sulfide minerals (Lakin and Davidson, 1967), with values ranging from 0 to 2,100 10
Distribution 11 ppm. In a study of Canadian ores in which the selenium content was deter- mined in pyrite, pyrrhotite, pentlandite, and chalcopyrite minerals, the highest concentrations of the element (500 to 1,000 ppm) were found in Precambrian nonnickeliferous copper sulfide ores (Hawley and Nichol, 19591. The Canadian ores are considerably richer in selenium than those of Australia but less rich than some of the sedimentary deposits of the west- ern United States (Anderson et al., 19611. Selenium is obtained commer- cially by treatment of anode slimes produced during the electrolytic refin- ing of copper. The principal sources of selenium are the sulfidic copper ores in Canada, the United States, and the Soviet Union (Cooper et al., 19701. Sedimentary rocks cover more than three-quarters of the land surface of the earth and are therefore the principal parent materials of agricultural soils (Lakin and Davidson, 1967~. It has been estimated that 58 percent of all sedimentary rocks are shales, which in turn commonly contain the high- est concentrations of selenium (Anderson et al., 1961~. The average con- centration of selenium in shales has ranged from 0.24 ppm for Paleozoic shales of Japan to 277 ppm for black shales of Permian age from Wyoming (Lakin and Davidson, 1967~. Approximately 2 ppm selenium has been esti- mated to be present in Cretaceous Pierre Shale, the parent material for much of the seleniferous soil in the United States (Lakin and Davidson, 1967~. However, selenium concentrations found in members of the Pierre formation that have actually weathered to seleniferous soil are much higher (Moxon et al., 1939~. Shales are also the principal sources of selenium-toxic soils in Ireland, Australia, and several other countries of the world (Johnson, 1975~. It has been difficult to reach a realistic estimate of the selenium content of sandstones. Lakin and Davidson (1967) obtained values ranging from 0 to 112 ppm. Garde (1966) has reported selenium concentrations between 2 and 130 ppm. Apparently selenium is often concentrated in organic debris in sandstones (Johnson, 1975~. The selenium content of limestones is generally low, although some have contained relatively high levels (Lakin and Davidson, 1967~. The element has been found in seleniferous pyrite and in organic debris. The relatively high concentration of selenium in some phosphate rocks may be significant in agriculture because of the wide use of phosphate fertil- izers made from these deposits. It has been suggested that normal super- phosphate can be expected to contain about 60 percent and concentrated superphosphate about 40 percent as much selenium as the phosphate rock from which it is made (Robbing and Carter, 1970~. Samples from the west- ern U. S. phosphate field, extending over parts of Wyoming, Utah, Nevada, Idaho, and Montana, contained from 1.4 to 178 ppm selenium (Robbing and Carter, 1970~. Earlier analyses of phosphate rocks from Florida, South
12 SELENIUM IN NUTRITION Carolina, and Tennessee were lower, ranging between 0.8 and 9 ppm sele- nium (Racer and Hill, 1935~. Seleniferous sulfur is of agricultural interest as a source of selenium in phosphatic fertilizers and sulfur-containing inorganic salts included in livestock diets. The selenium content of Japanese and Hawaiian volcanic sulfur ranged from 67 to 206 ppm and 1,026 to 2,000 ppm, respectively (Lakin and Davidson, 1967~. However, not all volcanic sulfur was found highly seleniferous. Twenty-eight samples from various localities around the world contained between 2 and 15 ppm of the element (Lakin and Davidson, 1967~. Selenium has been found to occur in fossil fuels. In samples obtained in the United States, coal contained 1 to 5 ppm selenium and crude oil (Texas) 0.06 to 0.35 ppm (Cooper et al., 1970~. In a coal sample taken from a seleniferous region in the People's Republic of China, approxi- mately 90,000 ppm selenium were found (Levander, 19821. Fly ash ob- tained from electrostatic precipitators in stacks at coal-powered electricity generating plants in the United States has been shown to contain 1.2 to 16.5 ppm selenium (Gutenmann et al., 19761. Volunteer white sweet clover growing on a landfill containing fly ash showed up to 200 ppm (dry basis). Sheep (Furr et al., 1978) and swine (Mandisodza et al., 1979) fed such sweet clover exhibited large increases in tissue selenium. Swine fed fly ash directly also exhibited such an effect. COMMERCIAL SOURCES Known deposits of selenium are insufficient to permit their mining for the element alone. Virtually all new production of selenium is via its extraction from copper refinery slimes along with the recovery of precious metals (Na- tional Research Council, 1976b). Decopperization is the first procedure, after which selenium may be recovered either by volatilization during roasting or furnacing or by leaching of roasted calcine or furnace slag. In 1973, total free world production of selenium was 1.1 million kg, with Ja- pan, the United States, and Canada the leading producers in that order. The principal commercial selenium compounds are selenides of alumi- num, arsenic, bismuth, cadmium, calcium, copper, and indium; ammo- nium selenite and sodium selenite; selenates of copper, potassium, and sodium; selenium dioxide; selenium disulfide; selenium hexafluoride; and selenium monosulfide. These compounds are used mainly in the manufac- ture of glass; in xerography; in conductors, rectifiers, electron emitters, and insulators; as reagents; in remedies for eczemas and fungus infections in pets; in antidandruff agents for humans; and in veterinary therapeutic agents. In agriculture, early uses for selenium compounds were for control of
Distribution 13 mites and insects; these compounds are no longer used for this purpose. So- dium selenite and sodium selenate are presently used in agriculture as inject- ables and feed additives to control selenium-related deficiency disorders. SELENIUM IN SOILS The selenium content of most soils lies between 0.1 and 2 ppm (Swaine, 19551. The maximum quantity of selenium found in several thousand soil samples in the United States did not exceed 100 ppm, and the majority of the seleniferous soils analyzed contained on the average less than 2 ppm (Rosenfeld and Beath, 1964~. Soils developed from Cretaceous shale of South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colorado, and New Mexico tend to be high in selenium, ranging from 2 to 10 ppm (Jackson, 1964~. A portion of the selenium in soils is available to the vegetation they sup- port. Soils that supply sufficient selenium to produce toxic plants are com- monly referred to as toxic seleniferous soils. Nontoxic seleniferous soils, although their selenium content may be high, yield insufficient available selenium for plants to become toxic. The total selenium content of many toxic seleniferous soils is appreciably lower than that of some nontoxic soils. Because of the high levels of selenium in sedimentary rocks and the im- portance of such rocks as soil-forming materials, the processes contribut- ing to high selenium concentrations are of interest. The selenium content of sedimentary rocks varies considerably throughout a geological profile (Moxon and Olson, 1970~. This indicates that during their formation the selenium was provided from a primary source at a different rate than that at which sediments were deposited. In the United States, virtually all sele- niferous soils have weathered from sedimentary rocks of the Cretaceous period. Only a few such formations contain sufficient selenium that they become parts of soils that produce toxic vegetation. Lakin (1961) has sug- gested that selenium is concentrated in sedimentary rocks by the following processes: (1) precipitation by rain of selenium from volcanic emanations; (2) deposition of erosional products from volcanic sulfur, seleniferous tufts, and sulfide deposits; and (3) precipitation of selenium from streams or groundwater carrying unusual quantities of selenium from older selenif- erous sediments. Strock (1935) has suggested that selenium was removed from the erosion cycle and held in sedimentary deposits by its adsorption on freshly precipitated ferric hydroxide. Subsequent elevation and erosion would release selenium from sedimentary deposits and start it on a new cycle. The frequent association of high concentrations of selenium with limonite concentrations composed of ferric oxide and hydroxide (Rosen
14 S ELENIUM IN NUTRITION feld and Beath, 1964) and with pyrite and marcasite (Rosenfeld and Beath, 1964; Elkin and Margrave, 1968) in sediments lends support to Strock's explanation. TOXIC SELENIFEROUS SOILS Toxic seleniferous soils are usually alkaline in reaction and contain free calcium carbonate (Lakin, 1961; Rosenfeld and Beath, 1964~. They occur in regions of low rainfall, usually less than 8 cm total annual precipitation. The presence of water-soluble selenium is an important characteristic of toxic soils (Lakin, 1961~. Beath et al. (1946) concluded that selenate is the dominant water-soluble form of selenium in toxic soils. There are extensive areas of seleniferous soils in South Dakota, Wyo- ming, Montana, North Dakota, Nebraska, Kansas, Colorado, Utah, Ar- izona, and New Mexico that produce vegetation toxic to livestock (Rosen- feld and Beath, 1964~. The occurrence of toxic vegetation and indicator plants is most widespread in Wyoming and South Dakota (Rosenfeld and Beath, 19641. The average selenium content of 500 samples of soil from seleniferous areas in the western United States was 4.5 ppm, with a maxi- mum of 80 ppm (Trelease, 19454. Seleniferous soils supporting toxic vegetation in Canada are associated with Cretaceous rocks in large areas of Alberta, Saskatchewan, and Mani- toba (Rosenfeld and Beath, 19641. The range in total selenium content of 80 soil samples, taken where indicator plants were present, was 0.1 to 6 ppm, with 30 percent of the samples containing 1 ppm or more. Contamination of soils by seleniferous mine wastes caused a toxicity problem in a river valley in Mexico (Rosenfeld and Beath, 1964~. The mine wastes contained an average of 4.6 ppm selenium, while the contaminated surface soils contained between 0.3 and 20 ppm. Several seleniferous areas are found under humid conditions in Colom- bia (Rosenfeld and Beath, 1964~. Surface soils collected in Boyaca State contained from 1 to 14 ppm, and soil in the region located between the Negro and Negrito rivers averaged from 2 to 7 ppm selenium. Selenium occurs in toxic amounts under humid conditions in certain parts of Limerick, Tipperary, and Meath counties of Ireland (Rosenfeld and Beath, 19641. The seleniferous soils lie in a poorly drained valley, and leaching of topographically higher rocks and soils has led to selenium en- richment of these soils. In 1957, alkali disease was reported in cattle herds in the Huleh Valley of Israel (Rosenfeld and Beath, 1964) where soils had over 6 ppm sele- nium. In a seleniferous area in the Naot-Mordechai region the soils con- tained from traces to 6.0 ppm.
Distribution 15 In South Africa, the areas located on the Karoo sedimentary rock pro- duce chronic selenosis in livestock (Rosenfeld and Beath, 19641. NONTOXIC SELENIFEROUS SOILS Selenate has been identified as the main water-soluble form of selenium in soil that is translocated into vegetation containing toxic quantities of the element (Lakin, 1972~. Many soils of the world contain high levels of sele- nium but low levels of water-soluble selenium and consequently do not pro- duce vegetation that has a toxic selenium level for animals. Hawaiian soils containing 6 to 15 ppm and Puerto Rican soils containing 1 to 10 ppm selenium do not produce seleniferous vegetation, whereas soils of Israel and South Dakota with lower total selenium contents produce toxic plants (Lakin, 1972~. The nontoxic seleniferous soils of Hawaii and Puerto Rico have an acid pH range (4.5 to 6.5) which, in the presence of ferric hydrox- ide, renders the selenium unavailable to plants. The soils are characterized by a zone of accumulated iron and aluminum compounds and are devel- oped under humid conditions (Lakin, 1961~. LOW- SELENIUM S O IL S Recent volcanic deposits, which are low in selenium, and materials trans- ported from them are the principal soil-forming materials in the very low selenium region of the Pacific Northwest. Soils in the very low selenium region of the South Atlantic seaboard are formed from coastal deposits washed from a highly weathered land mass. The soil parent materials of the low-selenium areas in Montana are mostly derived from granites and very old metamorphic rocks. Low total selenium concentrations in the ter- tiary volcanic rocks of Arizona and New Mexico are suspected to be the cause of the low selenium levels in crops in this part of the United States. The soil-forming materials of the northeastern United States are derived primarily from sedimentary rocks that predate the major Cretaceous pe- riod of selenization of the North American continent. Most of the soils from low-selenium areas of the United States contain less than 0.5 ppm selenium (Cary et al., 1967~. Low-selenium soils of eastern Canada contain less than 0.2 ppm selenium (Levesque, 1974~. The low-selenium soils of Canada occur in the interior of British Columbia, in west-central Alberta, in northern Ontario, in the eastern townships and lower St. Lawrence re- gions of Quebec, and in the Atlantic provinces (Levesque, 1974~. Most New Zealand soils contain between 0.1 and 2 ppm selenium (Watkinson, 1962~. Low-selenium soils appear to be responsible for selenium deficiency disorders in livestock raised in certain regions of Australia, New Zealand,
16 SELENIUM IN NUTRITION Scotland, Finland, Sweden, Austria, Germany, France, Western Russia, Turkey, Greece, Canada, and the United States (Underwood, 1962, 1966; Oksanen, 1967; Allaway, 1968~. FORMS OF SELENIUM IN SOILS AND FACTORS AFFECTING AVAILABILITY TO PLANTS Separation and identification of the chemical forms of selenium in soils is difficult because of the small amounts of the element present (Trelease and Beath, 1949; Allaway et al., 1967) and the complexities of soil systems (Ro- senfeld and Beath, 1964~. The forms of selenium generally considered to be present in soil are selenides, elemental selenium, selenites, selenates, and organic selenium compounds. The chemical forms of selenium in soils and sediments are apparently closely related to oxidation reduction poten- tial, pH, and solubility (Lakin, 1961; Allaway et al., 1967; Cary et al., 1967; Allaway, 1968; Geering et al., 1968~. The principal chemical reac- tions of selenium in soils and weathering sediments, as summarized by Allaway (1968), are shown in Figure 1. There is evidence that insoluble selenides associated with sulfides may occur in some soils (Williams and Byers, 1936; Trelease and Beath, 1949; SOILS ACID-POOR LY AE RATED Heavy metal ~ Elemental Se selenides Se= ~ s Sea WELL AERATED-ALKALINE Selenites s, Selenates SeO3= ' s seo4= acid pH ~ Ik \ ~. ~. (insoluble) (insoluble) Fe(OH)SeO3 \ l complexes \ | (insoluble) ~ ~ PLANTS 1 oss loss 1 loss s: slow react) on loss: Process leading to loss of "biologically active" Se FIGURE 1 Generalized chemistry of selenium in soils. From Allaway, 1973. leach i ng 1 oss
Distribution 17 Allaway et al., 1967~. The low solubility of metal selenides, especially copper selenide, may lead to their persistence in agricultural soils (Allaway et al., 1967~. Although redox potentials indicate that selenides would be oxidized to selenite in most soils, the rate of oxidation is probably sufficiently slow to effectively stabilize this form of selenium under some soil conditions (Cary et al., 1967~. Elemental selenium is apparently present in small amounts in some soils. (Beath et al., 1937; Byers et al., 1938; Trelease and Beath, 1949; Olson, 1967~. It may be either an important intermediate in the oxidation of the element to a soluble form (Olson, 1967) or a transitory constituent of neutral and acid soils during the reduction of selenites under acid conditions (Allaway et al., 19671. There are indications that signifi- cant amounts of elemental selenium may be oxidized by microorganisms in neutral and alkaline soils (Geering et al., 1968~. The fate of elemental sele- nium in acid soils is uncertain. Watkinson (1962) and Allaway et al. (1967) have suggested that when elemental selenium is added to acid and neutral soils, it may be oxidized to selenites, which in turn react with hydrous ox- ides to form complexes of low solubility and availability to plants. A large fraction of the selenium in acid soils may occur as stable complexes of sele- nites with hydrous iron oxides (Williams and Byers, 1936; Trelease and Beath, 1949; Swaine, 1955; Lakin, 1961; Allaway et al. 1967~. Geering et al. (1968) showed that the thermodynamically stable selenium compound in acid-to-neutral soils may be a ferric selenite-ferric hydroxide adsorption complex. As the pH rises above 8, decomposition of the ferric hydroxide- selenite complexes begins, and the equilibrium solubility of selenite in- creases rapidly. The rate of transformation of selenite to selenate proceeds rather slowly. Selenates have been reported to be present in water extracts of soil by several workers (Williams and Byers, 1936; Byers et al., 1938; Olson et al., 1942; Beath et al., 1946; Trelease arid Beath, 1949~. Accord- ing to Lakin (1961), selenates are stable in an alkaline, oxidizing environ- ment such as that found in many well-aerated, semiarid seleniferous soils. Selenates do not appear to be present in appreciable quantities in acid and neutral soils. Marked increases in selenium uptake by plants have resulted from application of soluble selenates to soils (Hurd-Karrer, 1935; Grant, 1965; Bisbjerg and Gissel-Nielsen, 1969; Gissel-Nielsen and Bisbjerg, 1970~. Very little is known about the nature of organic forms of selenium in soils. Beath et al. (1935) suggested that soluble organic selenium com- pounds are liberated through the decay of seleniferous plants. Williams and Byers (1936) found that soil organic matter contained water-soluble and easily recoverable organic selenium compounds. The availability of selenium in seleniferous soils was found by Olson and Moxon (1939) to be correlated with or dependent upon the selenium in the organic or humus
18 SELENIUM IN NUTRITION fraction. According to Cary et al. (1967), organic forms of selenium are probably more soluble under alkaline than under acidic soil conditions. The principal factors affecting the availability of soil selenium to plants have been summarized as follows (NRC, 1971~: In alkaline, well-aerated soils, selenium tends to form selenates. The selenates in these soils are very available to plants, and they may lead to toxic concentrations in plants. In acid soils, a ferric iron-selenite complex is formed that is only slightly avail- able to plants. This is the reason acid soils rarely produce plants that con- tain toxic concentrations of selenium. Elemental selenium appears to be stable in soils and, except for microbial action, is not readily oxidized to forms that are easily taken up by plants (Watkinson and Davies, 1967; Cary and Allaway, 1969~. There is evidence that there are some organic selenium compounds in soils that are water-soluble and available to plants (Moxon et al., 1939~. The uptake of soil selenium by plants is dependent on plant species; this will be discussed later. The overall relationships among the concentrations of selenium in rocks, soils, and plants have been summarized as follows (NRC, 19711: · Where rocks with a high content of selenium decompose to form well- drained soils in subhumid areas (less than 8 cm of annual rainfall), the selenides and other insoluble forms of selenium will be converted to sele- nates and organic selenium compounds. These compounds will be avail- able to plants, and vegetation containing potentially toxic levels of sele- nium will probably be produced on these soils. · Where rocks with a high content of selenium weather to form soils in humid areas, slightly soluble complexes of ferric oxide or hydroxide and selenite ions will be formed. These soils will also be slightly to strongly acid, and the plants produced on them will not contain toxic concentra- tions of selenium, but they may contain sufficient selenium to protect live- stock consuming them from selenium deficiency. · Where rocks with a high content of selenium weather to form poorly drained soils or where selenium from higher lying areas is deposited in poorly drained areas by alluvial action, and the soils are alkaline, plants containing toxic levels of selenium are likely to be produced. This will be especially probable if the aeration of these soils is improved by artificial drainage. The more acid the soils in an area, the less the likelihood of vege- tation containing toxic levels of selenium. · Where rocks with a low content of selenium decompose to form soils under either humid or dry conditions, the plants produced are likely to contain insufficient selenium to protect animals from selenium deficiency. The more humid the area and the more acid the soil, the greater the likeli- hood of extremely low selenium concentrations in the plants.
Distribution SOIL MANAGEMENT PRACTICES AND SELENIUM IN PLANTS 19 The value of management practices in control of selenium toxicity in various areas of the United States has been reviewed by Anderson et al. (1961), Rosenfeld and Beath (1964), and Olson (1969b). Subsequent to the map- ping of seleniferous areas, the U.S. government withdrew large areas from wheat production and converted the areas to controlled grazing (Anderson et al., 1961~. Muth (1955) and Schubert et al. (1961) have observed aggravated sele- nium deficiency following application of gypsum to soils. However, Johnson (1975) found that application of gypsum to seleniferous soils was ineffective in reducing selenium absorption by plants. Likely the sulfate content of the soils was already high, or the sulfate did not penetrate to the deep-rooted native plants. On the other hand, the concentration of selenium in some seleniferous soils has been markedly reduced both by leaching during the soil development process (Moxon et al., 1939) and by irrigation water (Lakin, 1961~. Kubota et al. (1967) found that forage growing on the bot- tomlands along the Missouri and Mississippi rivers contained more sele- nium than did forage growing on the upland soils, indicating that the rivers are transporting selenium from their upper watersheds. It appears, however, that selenium is being removed from the surface layers of the seleniferous areas of the United States and not from the lower profiles where deep-rooted plants can still accumulate toxic amounts of selenium (Johnson, 19751. In areas where soils are low in selenium, certain agricultural practices may have some effect in increasing the level available. Applying manure to low-selenium soils from animals fed imported selenium-adequate feeds in- creases the soil selenium content slightly. Superphosphate fertilizers con- tain selenium, but the extent of their contribution to soil selenium is not known. Cary et al. (1967) have shown that liming some soils deficient in selenium results in only a very small increase in selenium uptake by plants. SELENIUM IN PLANTS EFFECT OF SPECIES Factors influencing the selenium content of plants have been reviewed by Johnson et al. (1967~. One of the most important of these is the kind of plant. Rosenfeld and Beath (1964) have divided plants into three groups on the basis of their ability to accumulate selenium when grown on high-sele- nium soils. The first two groups of plants are referred to as selenium accu- mulator or indicator plants. These grow well on soil containing high levels of selenium and thereby assist in the location of seleniferous soils. Plants in
20 SELENIUM IN NUTRITION group 1 are called primary indicators and include many species of Astra- galus, Machaeranthera, Haplopappus, and Stanleya. They normally accu- mulate selenium at very high levels, often several thousand parts per mil- lion. Plants in group 2 are referred to as secondary selenium absorbers. They belong to a number of genera, including Aster, Atriplex, Castelleja, Grindelia, Gutierrezia, Machaeranthera, and Mentzelia. They rarely con- centrate more than a few hundred parts per million of selenium. Plants in group 3 include the grains, grasses, and many weeds, that do not normally accumulate selenium in excess of 50 ppm when grown on seleniferous soil. Some plants growing on seleniferous soils accumulate surprisingly low levels of selenium. White clover (Trifolium repens), buffalo grass (Hilaria belangeri), and gramma grass (Bouteloua spp.) are poor accumulators of the element (Beeson and Matrone, 1972~. On the other hand, high sulfur- containing plants such as the Crucifera (mustard, cabbage, broccoli, cau- liflower) are relatively strong concentrators of selenium. The accumulator plants in groups 1 and 2 have been found to grow in 140 counties in 16 states of the United States (NRC, 1971~. However, these plants probably add very little to the selenium content of feeds because they normally grow in dry nonagricultural areas. Hamilton and Beath (1963, 1964) have reported on the accumulation of selenium by field crops and vegetables grown in high-selenium soils, and Williams et al. (1941) have published data on the selenium contents of wheat and feed grains produced in the high-selenium areas of the United States. All of the vegetable and crop species grown in soils containing high levels of available selenium concentrated the element to potentially toxic levels ~ > 5 ppm). However, Williams et al. (1941) found that less than 10 percent of the wheat and feed grain samples grown in the seleniferous areas of the United States had selenium levels in excess of 5 ppm. Differences in the accumulation of selenium by plants growing in soils low in selenium have been reported by Davies and Watkinson (1966) and Ehlig et al. (19681. After the addition of selenite to a soil, brown top (Agrostis tennis) took up two to seven times as much selenium as white clover (Trifolium repens). Allaway (NRC, 1971) has found that for soils having moderately low selenium levels, alfalfa accumulates more selenium than red clover, timothy, or brome grass. No reliable differences were noted among species grown on very low levels of available selenium. Crops growing on neutral or acid soils absorb very little selenium, and any at- tempt to increase crop selenium uptake by shifting to some other species is not likely to be successful (Ehlig et al., 19681. SELENTUM AS A PLANT MICRONUTRIENT Early work by Trelease and Trelease (1938, 1939) indicated that the accu- mulator species Astragalus racemosus and A. beathii required selenium
Distribution 21 for growth. Broyer et al. (1972a,b) were not able to repeat the findings and concluded that in the Trelease work phosphate toxicity had occurred in the plants, with the condition being alleviated somewhat by the addition of selenium. Broyer et al. (1966) found no beneficial effect of adding selenite to cultures of alfalfa (Medicago saliva L.) and subterranean clover (Trifo- lium subterraneum) up to 2 ppm, at which a depressing effect was ob- served. Bisbjerg and Gissel-Nielsen (1969) observed a growth depression in some plants when 0.5 to 2.5 ppm selenate selenium was added to the soil. TOXICITY TO PLANTS In some of the nonaccumulator species, soluble selenium compounds have been shown to interfere with seed germination (Levine, 1925) and growth (Levine, 1925; Hurd-Karrer, 1934, 1937; Martin, 1936~. In some cereal crops, selenate toxicity produced a characteristic snow-white chlorosis (Hurd-Karrer, 1933~. The rate of crossing over in barley was found to be reduced by selenium (Walker and Ting, 1967), apparently by causing a relaxation of the meiotic chromatin. The accumulator plants on the other hand are able to absorb high levels of selenium without any adverse effect. Apparently there are no published accounts of naturally occurring sele- nium causing damage to crops (Cooper et al., 19701. It has been found by Rosenfeld and Beath (1964) that crop plants are not injured until they ac- cumulate more than 300 ppm selenium, a concentration never found in even the most seleniferous areas of the United States. CHEMICAL 170RMS In early studies on the alkali disease syndrome it was found that high levels of selenium were associated with the protein in grains (Franke and Painter, 1936; Horn et al., 1936~. This was confirmed in more recent studies in which most of the selenium in nonaccumulating species was found in the form of protein-bound selenomethionine (Butler and Peterson, 1967~. Early investigations also showed that the selenium in indicator plants was mostly water-soluble and was not associated with the protein (Beath et al., 1934~. Horn and Jones (1940) were the first to isolate an organic selenium compound from plant material. They isolated a mixture from water extracts of Astragalus pectinatus that they considered to be the isomorphic compounds cystathionine and Se-cystathionine. Later, Trelease et al. (1960) reported the isolation of Se-methylselenocysteine from Astragalus bisulcatus. Since then many other selenium compounds have been isolated from plants, including Se-methylselenomethionine, the glutamyl peptide of selenocystathionine, sel- enohomocystine, selenocystine and its oxides, selenomethionine selenoxide, selenoglutathione, selenite, selenate, selenocysteic acid, selenocysteine seleninic acid, dimethyl selenide, and dimethyl diselenide (Moxon and Olson, 1970;
22 SELENIUM IN NUTRITION NRC, 1976b). It has been pointed out by Walter et al. (1969) that diselenide- sulfhydryl and diselenide-selenol interchange reactions might occur during isolation of selenium metabolites and might lead to the identification of arti- facts. Shrift (1973) has pointed out that there appear to be some biochemical distinctions between selenium-accumulator and selenium-nonaccumulator Astragalus species. Se-methylselenocysteine has been consistently found in much higher amounts in the accumulator than in the nonaccumulator species (Martin et al., 19711. Both species methylate selenomethionine to give Se- methyl-selenomethionine, but only the accumulators convert it to selenoho- mocystine and Se-methylselenocysteine (Virupaksha et al., 1966~. Another distinction is the large amount of selenocystathionine in the accumulator species of Astragalus; only trace amounts occur in the nonaccumulators (Mar- tin et al., 1971~. METAB OLISM IN PLANTS Shrift (1973) has summarized present knowledge of the metabolism of se- lenium by plants. Although the selenium metabolites identified in plants are analogs of sulfur compounds, the metabolism of selenium in plants cannot be identified from known mechanisms involving sulfur metabo- lism. For example, selenocystathionine has been found in plants without cystathionine (Martin et al., 1971), and glutathione has been found in the absence of selenoglutathione (Shrift and Virupaksha, 1965~. Nissen and Benson (1964) found that the roots of several crop plants formed choline sulfate but not the selenate derivative. They failed to detect 3'-phos- phoadenosine-5'-phosphoselenate in plants provided selenate and con- cluded that this was due to the conversion of SeO4 2 to adenosine 5-phos- phoselenate by sulfate adenyltransferase and subsequent reduction to SeO3 2. A similar metabolic reaction, however, was observed by Lewis et al. (1971) for sulfur and selenium in Brassica oleracea. They obtained an en- zyme preparation that cleaved Se-methylmethionine into dimethyl sulfide and homoserine, and S-methylselenomethionine into homoserine and di- methyl selenide. The latter compound was found earlier in Astragalus bi- sulcatus by Froom (1963~. Our understanding of the metabolic pathways for selenium in plants re- mains very limited. SELENIUM IN ANIMAL FEEDSTUFFS The selenium content of feedstuffs varies with plant species and geographi- cal area of production. Concentration of selenium in plants is governed
Distribution 23 primarily by the presence and availability of the element in the soil. In some areas of the United States, forages contain sufficiently high selenium concentrations to cause selenium toxicity in livestock; in other regions the levels of selenium in crops and forages are too low to meet animal require- ments. At the same time, there are extensive areas where virtually all the crops and forages contain sufficient selenium to meet livestock require- ments. In the seleniferous areas, accumulator plants frequently contain sele- nium at levels that are toxic to farm animals. However, the impact of these plants on the livestock industry in the seleniferous areas is small because of the widespread adoption of practical techniques for controlling the prob- lem (Olson, 1969b). There are relatively small differences among species of forage and feed plants in the accumulation of selenium when they are grown in the seleniferous areas. Hamilton and Beath (1963, 1964) have described the accumulation of selenium by field crops growing in soil con- taining a high level of available selenium. Lakin and Byers (1941) and Wil- liams et al. (1941) have published an extensive compilation of the selenium content of wheat and feed grains produced in the high-selenium areas of the western United States. Lakin and Byers (1941) found 82.5 percent of their wheat samples contained 1 ppm selenium or less, and 7.5 percent contained in excess of 4 ppm. Similar concentrations were found in bran, shorts, and middlings. In Canada, Thorvaldson and Johnson (1940), ana- lyzing 230 composites made up from 2,230 samples of wheat from wide- spread areas of Saskatchewan and Alberta, found an average value of 0.44 ppm selenium, with a maximum of 1.5 ppm. Robinson (1936) found con- centrations between 0.1 and 1.9 ppm selenium in samples of market wheat obtained in various parts of the world. Davies and Watkinson (1966) and Ehlig et al. (1968) studied the differ- ences among plant species in accumulating selenium from soils having low concentrations of the element. In New Zealand, browntop (Agrostis tennis) contained more selenium than white clover (Trifolium repens) when grown on low-selenium soil. In the United States, alfalfa accumulated more sele- nium than red clover, timothy, or bromegrass from soils containing mod- erately low selenium concentrations, but differences among forage species have not been consistent when the soils contained very low levels of avail- able selenium. In Canada, Miltimore et al. (1975) found that British Columbia wheat grain had a considerably higher mean selenium concentration than barley and oats. Significantly higher levels of selenium also occurred in wheat than in grasses and legumes. The percentages of samples below 0.1 ppm selenium were wheat, 12 percent; barley and oats, 32 percent; legumes, 22 percent; grasses, 21 percent; and corn silage, 76 percent.
24 REGIONAL DISTRIBUTION IN CROPS SELENIUM IN NUTRITION The regional distribution of forages and grain, containing low, variable, or adequate levels of selenium in various areas of the United States and Can- ada is shown in Figure 2. Under most circumstances the selenium require- ments of ruminants and nonruminants will be met by a dietary level of 0.1 to 0.2 ppm (NRC, 1973 iswinei, 1975 isheepi, 1976a [beef cattle), 1977 [poultry], 1978 [dairy cattle]~. If feed containing less than 0.10 ppm sele- nium is fed to livestock, selenium/vitamin E-responsive disorders may de- velop in varying degrees, with a much higher incidence occurring when the selenium concentration drops below 0.05 ppm. Figure 2 indicates where selenium deficiency might occur if farm animals are fed locally grown crops. Feed supplements that have been grown elsewhere and are fed lo I.. . ~ 1 ~-~ 1~ _~ ) ~2 -¢ LOW-APPROXIMATELY 80% OF ALL FORAGE AND GRAIN CONTAIN<0.10 PPM SELENIUM VARIABLE-APPROXIMATELY 50% CONTAIN>0.10 PPM SELENIUM (INCLUDES ALASKA) ADEQUATE-80% OF ALL FORAGES AND GRAIN CONTAIN>0.10 PPM SELENIUM (INCLUDES HAWAII) FIGURE 2 Regional distribution of forages and grain containing low, variable, or adequate levels of selenium in the United States and Canada.
Distribution' 25 cally may contribute sufficient selenium to the diet to prevent the develop- ment of selenium deficiency disorders. Figure 2 shows that there are broad areas of Canada and the United States where plants contain low levels of selenium. In the United States the most selenium-deficient areas are the Northwest, Northeast, the Atlantic coastal area, Florida, and regions surrounding the Great Lakes. In Can- ada, in almost all areas east and north of the Great Lakes, in the northern areas of the prairie provinces, and in parts of the Rocky Mountains, sele- nium deficiency disorders have been most prevalent. Crops grown in the central and west-central regions of Canada and the United States, and in the southern states, usually contain adequate ~ > 0.10 ppm) levels of sele- nium for livestock. Presently, it is legally possible to provide farm animals in the United States and Canada with specific dietary selenium supple- ments to prevent the deficiency disorders. The preparation of countrywide and even international maps illustrat- ing the relative concentrations of selenium in forages and grains, as in Fig- ure 2, has become possible by integrating the results of numerous surveys conducted on specific areas of the United States and Canada, taken in conjunction with data obtained on a national basis. Some of these include: United States Low-selenium areas: Kubota et al. (1967) and Scott and Thompson (1971), many different regions; Carter et al. (1968), Northwest; Pa- trias and Olson (1969) and Ullrey (1974), Midwest Selenium-adequate areas: Kubota et al. (1967) and Scott and Thomp- son (1971), many different regions; Ullrey (1974), Midwest Seleniferous areas: Byers and Lakin (1939) and Lakin and Byers (1941), Central Plains; Williams et al. (1941), Western States; Bee Canada son (1961) and Rosenfeld and Beath (1964), Wyoming. Low-selenium areas: Lessard et al. (1968), Beauchamp et al. (1969), and Young et al. (1977), Ontario; Arthur (1971), most regions; Walker (1971), Martin et al. (1973), and Macdonald et al. (1976), Alberta; Winter et al. (1973), Gupta and Winter (1975), and Win- ter and Gupta (1979), Atlantic Provinces; Miltimore et al. (1975), British Columbia Selenium-adequate areas: Arthur (1971), most regions; Walker (1971), Martin et al. (1973), and Macdonald et al. (1976), Alberta; Miltimore et al. (1975), British Columbia; Owen et al. (1977a), Sas- katchewan Seleniferous areas: Byers and Lakin (1939), Thorvaldson and Johnson (1940~.
26 VARIABILITY IN CONCENTRATION SELENIUM IN NUTRITION The concentration of selenium in feed ingredients varies widely depending on the area in which the feedstuff is produced. This can best be seen from analyses described by Williams et al. (1941), Thompson and Scott (1968), Patrias and Olson (1969), Scott and Thompson (1971), NRC (1971), and Wauchope (1978) for the United States; and Arthur (1971) and Miltimore et al. (1975) for Canada. Table 3, in which data have been taken from the above published studies, indicates the variable selenium content of animal feed ingredients. BIOLOGICAL AVAILABILITY There have been several published studies illustrating differences in the biological availability of selenium occurring in various feedstuffs. Mathias et al. (1965) found that selenium in alfalfa was comparable to sodium sele- nite for the prevention of liver necrosis in the rat. The effectiveness of the alfalfa selenium in preventing exudative diathesis in the chick was markedly reduced by a high level of sulfur. Mathias et al. (1967) observed that the sele- nium present in milk had higher potency than that in sodium selenite for prevention of exudative diathesis, although the two sources of selenium had equal effectiveness in preventing liver necrosis in the rat. Miller et al. (1972) reported a study in which selenium retention by chicks was compared when the element was derived from fish meal, fish solubles, selenite, or selenomethionine. Selenium from selenomethionine was re- tained better than that from selenite; and compared to selenomethionine, the fishery products were only 31 percent as effective. Cantor et al. (1975a) found that selenomethionine was much more potent than either selenite or selenocystine for preventing pancreatic fibrosis in the chick. The selenium in seleniferous wheat was highly effective for prevent- ing both pancreatic fibrosis and exudative diathesis, possibly due to the high percentage of selenomethionine (Olson et al., 1970~. In another study, Can- tor et al. (1975b) determined the biological availability of selenium in vari- ous feedstuffs for the prevention of exudative diathesis. Selenium potency in most of the feedstuffs of plant origin, in comparison to selenite selenium, was highly available, ranging from 60 to 90 percent, but was less than 25 per- cent available in animal products. Scott (1973) has reported the biological availability of selenium in a number of natural feedstuffs for protection against exudative diathesis as follows: alfalfa meal, 140 percent (vs. selenite selenium); brewers' grains, 89 percent; brewers' yeast, 81 percent; wheat, 110 percent; corn, 83 percent; soybean meal, 64 percent; cottonseed meal, 78 percent; menhaden fish meal, 35 percent; meat and bone meal, 36 per
Distribution 27 TABLE 3 Variation of Selenium Concentrations in Various Feed Ingredients (as-fed basis) Selenium (ppm) Ingredient United States Canada Alfalfa meal 0.01-2.00 0.02-0.27 Barley 0.05-0.50 0.02-0.99 Bentonite 1.00-20.00 Blood meal - 0.50- 1.20 Brewers' grains 0.15-1.00 0.29-1.10 Corn 0.01-1.00 0.01-0.33 Dicalcium phosphate 0.15- 1.00 0.39- 1.00 Feather meal - 0.60-1.20 Fish meals 1.00-5.00 1.30-3.40 Gluten feed 0.15-0.50 0.12-0.25 Gluten meal 0.10-1.50 0.20-0.57 Linseed meal 0.50-1.20 0.70-1.50 Meat meal 0.08-0.50 0.20-0.81 Oats 0.01-1.00 0.01-1.10 Poultry by-product 0.50-1.00 Rapeseed meal - 0.46- 1.90 Soybean meal ~0.06-1.00 0.04-0.78 Whole soybeans 0.07-0.90 Wheat 0.01 -3.00 0.02- 1.50 Wheat middlings 0.15-1.00 0.41-0.89 Wheat bran 0.10-3.00 0.24-1.30 SOURCES: Williams et al. (1941), Thompson and Scott (1968), Pa- trias and Olson (1969), Scott and Thompson (1971), NRC (1971), and Wauchope (1978) for the United States; and Arthur (1971) and Milti- more et al. (1975) for Canada. cent; poultry by-product meal, 33 percent; tuna meal, 33 percent; and rock phosphate (Curacao), 50 percent. However, Gabrielsen and Opstvedt (1980) suggested that the assay conditions used by Scott and his associates were not valid and published quite different relationships for the bioavaila- bility of selenium in plant and animal products. Relative to selenium in sodium selenite (100 percent), the effectiveness of selenium in fish meals was 32 to 60 percent, in soybean meal was 18 percent, and in corn gluten meal was 26 percent for restoration of serum gluathione peroxidase activity in selenium-depleted chicks. Very few studies have been carried out to determine the bioavailability to humans of selenium from various foods (Levander, 1983~. Alexander (1982) and Douglass et al. (1981) used animal models and found that the
28 SELENIUM IN NUTRITION availability of selenium from tuna to previously depleted rats was only about half as great as that for selenium from selenite for elevation of hepatic glu- tathione peroxidase activity. Selenium from beef kidney or various products made from seleniferous wheat were roughly equivalent to selenite. A recent bioavailability trial conducted on Finnish men of low-selenium status who were supplemented with different forms of selenium indicated that a com- prehensive assessment of selenium bioavailability requires the determina- tion of several parameters. These include a short-term platelet glutathione peroxidase activity measurement to determine immediate availability, a medium-term plasma selenium measurement to estimate retention, and a long-term platelet glutathione peroxidase measurement after discontinua- tion of supplements to determine the convertibility of tissue selenium stores to biologically active selenium (Levander et al., 1983~. SELENIUM IN WATER DRINKING WATER, SPRINGS, AND WELLS Various aspects of selenium in water have been recently reviewed (NRC, 1980b). Selenium occurs as a minor constituent in drinking water in a con- centration range of 0.1 to 100 ,ug/liter (Davis and De Wiest, 1966~. The U.S. Department of Health, Education and Welfare (1962) has set the up- per limit for selenium in drinking water to be 10 ~g/liter. It would appear from published data that one rarely finds surface waters containing toxic concentrations of the element or even levels that would provide a signifi- cant fraction of the nutritional requirements of animals (NEC, 1971~. In surveys conducted by the U.S. Department of Health, Education, and Welfare (1959-1962) on the selenium content of waters from the major wa- tersheds, only two samples contained more than 10 ,ug selenium/liter. In analyzing 194 public water-supply sources, Taylor (1963) found that sele- nium was barely detectable in most samples; only a few samples averaged as high as 8 ,ug/liter. In a seleniferous area of South Dakota, Smith and Westfall (1937) could not detect any selenium in drinking water from 34 of 44 wells; the other 10 wells contained 50 to 300 ,ug selenium/liter. Beath has reported a few instances where appreciable amounts of selenium occur in springs and wells in seleniferous areas. In one instance an Indian family near Ignacio, Colorado, had well water containing 9,000 ,ug/liter (Beath, 19621. Cannon (1964) reported 5,800 ,ug/liter in spring water from a ura- nium-mineralized area in Utah. The selenium content of well water in sele- niferous areas is highly variable; however, the vast majority of samples have contained well below the limit of 10 ,ug/liter (Cooper et al., 19701.
Distribution 29 Byers (1935, 1936) has reported that in seleniferous areas, water in deep wells contains very little selenium. Hadjimarkos and Bonhorst (1961) ana- lyzed well water from farms located in three Oregon counties. They found that most samples contained between 2 ,ug and less than 1 ,ug/liter. RIVERS, LAKES, AND IRR:IGATION WATER In the extensive surveys conducted on the major watersheds in the United States, only two samples had selenium contents equal to or above 10 ,ug/ liter (U.S. Department of Health, Education, and Welfare, 1959-19624. These were a sample from the Animas River at Cedar Hill, New Mexico (10 ,ug/liter), and a sample from the Missouri River at St. Louis (14 ,ug/liter). Using a more sensitive analytical method, Scott and Voegeli (1961) found the selenium content of Animas River samples to contain 1 to 40 ,ug/liter, averaging close to 1,ug/liter. These authors observed that higher selenium levels in Colorado surface waters were correlated with higher water pH val- ues. There have been reports of high selenium values in river waters where irrigation drainage from seleniferous soils has contained as much as 2,680 ,ug/liter (Williams and Byers, 1935a; Byers et al., 1938~. Rivers at the point of entering the Colorado River have contained up to 400 ,ug/liter. Water in lakes, including those in seleniferous areas, has been found to contain very little selenium (Beath et al., 1935~. These low levels have been explained by the precipitation of selenite with oxides of such metals as iron and manganese (Goldschmidt and Strock, 1935; Byers et al., 19381. Sele- nium has been detected in a number of deep sea deposits (Goldschmidt and Strock, 1935; Williams and Byers, 1935b; Moxon et al., 1939; Edgington and Byers, 1942) suggesting further that the element can be removed from water by precipitation. OCEANS In extensive studies, Schutz and Turekian (1965) found an average of 0.090 ~g/liter for selenium in the major oceans. Others have found values of 6 ,ug/liter or less for ocean waters collected in various locations (Goldschmidt and Strock, 1935; Strock, 1935; Byers et al., 1938; Ishi- bashi, 19531. The low levels of selenium in most ocean waters have been attributed to its precipitation, under certain conditions, with metal oxides (Strock, 1935; Williams and Byers, 1936; Olson, 1939; Olson and Jensen, 1940~.
30 SELENIUM IN HUMAN FOODS DISTRIBUTION BY FOOD GROUPS SELENIUM IN NUTRITION The amount of selenium in a plant-derived food varies largely with its pro- tein content and with the area of the country in which it is grown (Levan- der, 1976a). The concentration of selenium in the milk, eggs, and meat of animals is influenced by the level of selenium in the plant material they consume (Allaway, 19781. In North American diets, cereals are the domi- nant food of plant origin for supplying selenium, with much of the cereal consumption in the form of bread. The United States and Canadian wheat crops are produced primarily in selenium-adequate regions, and this results in moderately high average concentrations of the element in wheat- related foods in both countries (NRC, 1976b). Meat and fish also are good sources of selenium for humans, whereas most fruits and vegetables pro- vide little selenium. Higgs et al. (1972) concluded that ordinary cooking techniques did not appear to result in major losses of selenium from most foods. Little or no loss of selenium occurred as a result of broiling meat, baking seafoods, frying eggs, or boiling cereals. Ganapathy et al. (1977) found that food preparation methods did not affect the selenium content of legumes and vegetables. Ferretti and Levander (1976) found that although some soybean meat extenders contain comparable or higher selenium lev- els than the beef or chicken they replace, others have much lower levels. Ferretti and Levander (1974, 1975) also found that small losses of selenium occur during the manufacturing of breakfast cereals, with the lost sele- nium appearing in the by-products destined for livestock feeding. CONCENTRATIONS IN SELENIFEROUS REGIONS Smith and Westfall (1937) reported the selenium content of some foods produced on seleniferous farms in South Dakota, finding 0.16 to 1.27 ppm selenium in milk, 0.25 to 1.0 ppm in bread made from local flour, 0.25 to 9.14 ppm in eggs, and 1.17 to 8.0 ppm (dry basis) in meat. Williams et al. (1941) found 0.1 to 0.5 ppm selenium in bread milled from wheat grown in seleniferous areas of the United States. On analyzing 951 samples of wheat from 8 states in the more seleniferous regions of the United States, Lakin and Byers (1941) found that only 7.5 percent of their samples contained over 4 ppm selenium. The other samples contained 1 ppm or less. Of the 66 samples of flour milled in the regions, only 5 contained more than 1 ppm selenium. Williams et al. (1941) found most samples of mustard seed and dry beans obtained from seleniferous areas contained less than 3 ppm and
Distribution 31 1 ppm, respectively. Anderson et al. (1961) have reported that because many of the most seleniferous areas have been retired from farming, it is likely that there are very few locations today having the high concentration of selenium in foods that was found by Smith and Westfall (1937) and others. CONCENTRATIONS IN NONSELENIFEROUS REGIONS There are relatively few reports on the concentration of selenium in foods representative of normal diets in the United States and elsewhere. Some analyses have shown lower levels of selenium in foods produced in low- selenium areas. Characteristically, there are large differences in selenium levels for the same food item among different investigators. One of the main factors contributing to these differences is undoubtedly the use of a small number of samples from a few localized areas. Problems in analytical preci- sion sometimes appear, particularly at low selenium levels (Schroeder et al., 1970). Nevertheless, despite the variability, the data show at least in a relative way which foods are likely to provide a lower or higher dietary intake of selenium. Information on the nutritional availability of selenium in various foods for humans is meager and is largely inferred from experiments with livestock, poultry, and laboratory animals. The data subsequently described on selenium levels in various foods are on a fresh-weight basis unless otherwise stated. DAIRY PRODUCTS AND EGGS Levels of selenium in milk reflect the level of naturally occurring selenium in the diet. Mathias et al. (1967) found dried skim milk from cows fed either low- or high-selenium diets to contain 0.06 and 0.28 ppm selenium, respectively. Allaway et al. (1968) reported that milk obtained from South Dakota, a relatively high-selenium area, contained 0.05 ppm, whereas a sample from Bend, Oregon, a low-selenium area, contained 0.02 ppm. Other analyses for whole milk sampled in the United States include average values of 0.012 ppm (Morris and Levander, 1970), 0.010 ppm (Schrauzer and White, 1978), and 0.021 ppm (Hadjimarkos, 1963~. Val- ues obtained in other countries for cows' milk averaged: Germany, 0.09 ppm (dry basis, Kiermeier and Wigand, 1969~; Denmark, 0.2 ppm (dry basis, Bisbjerg et al., 1970~; Japan, 0.030 ppm (Sakurai and Tsuchiya, 1975~; Canada, 0.015 ppm (Arthur, 1972~; USSR, 0.013 ppm (Suchkov, 1971~; Great Britain, 0.010 ppm (Thorn et al., 1978~; and New Zealand, 0.006 ppm (Miller and Sheppard, 1972~. When selenium supplements have been provided to lactating dairy cows
32 SELENIUM IN NUTRITION there has been very little increase in milk selenium level (Conrad and Moxon, 1979; Maus et al., 1980~. Of other dairy products analyzed, butter and cream (Morris and Levan- der, 1970; Arthur, 1972) had the lowest concentration of selenium (0.003 to 0.006 ppm) and cheese (Morris and Levander, 1970; Arthur, 1972) the most (0.010 to 0.123 ppm). Whey selenium concentrations generally re- flect regional differences in natural selenium levels in feeds consumed by dairy cows (Hitchcock et al., 1975~. In Oregon the selenium concentrations (dry basis) of whole egg samples averaged 0.317 ppm (Hadjimarkos and Bonhorst, 1961~. Other values reported for whole eggs have been 0.39 ppm (Arthur, 1972), 0.20 ppm (Schrauzer and White, 1978), and 0.52 ppm (Higgs et al., 1972~. CEREAL PRODUCTS Levander (1976a) has presented in tabular form much of the reported data on the selenium concentration of various grain products and breakfast ce- reals. Arthur (1972) reported that the selenium concentration of breakfast foods can vary widely with species of grain and geographical origin. Prod- ucts made from corn grown in Ontario, Canada, or in the midwestern United States had the lowest values, averaging 0.07 ppm, whereas those made from Canadian western wheat contained the highest amount of sele- nium, averaging 0.56 ppm. Puffed wheat products made from western Canada durum wheat had an average selenium concentration of 1.27 ppm. Breakfast cereals from rice grown in the southern United States varied in selenium concentration from 0.01 to 0.24 ppm. Analyses for the United States show a wide variation among various breakfast cereals: 0.024 to 0.451 ppm (Morris and Levander, 1970~; 0.032 to 0.51 ppm (Higgs et al., 19721; and 0.201 to 1.26 ppm (dry basis, Ganapathy et al., 1977~. Bread appears to be a relatively good source of selenium ranging from 0.28 to 0.68 ppm in various reports (Levander, 1976a), with whole wheat bread containing more selenium than white bread (Morris and Levander, 1970; Arthur, 1972; Schrauzer and White, 1978~. MEAT, POULTRY, AND FISH AND OTHER SEAFOODS Meats are a good source of selenium. It is apparent from the results of many studies that levels in animal tissues tend to be reflections of the con- centrations of available "natural" selenium in the diets (Hoffman et al., 1973; Jenkins et al., 1974), although when supplements of inorganic sele- nium are added to the diets, the tissue levels of selenium may not be in- creased appreciably (NRC, 1971~. Ku et al. (1972) reported that the sele
Distribution 33 nium concentrations of longissimus muscle (loin) of swine fed typical diets in various states ranged from 0.034 ppm (Virginia) to 0.521 ppm (South Dakota) and were linearly correlated (r > 0.9) with dietary selenium con- centrations. Reports from various countries on the selenium concentration of selected meats and fish products have been summarized by Levander (1976a). Morris and Levander (1970) found the average selenium value for steak, ground beef, chicken, pork chops, and lamb chops was 0.22 ppm, a value similar to that obtained by others for beef, pork, and poultry meats (Arthur, 1972; Schrauzer and White, 1978~. Kidneys were found by Mor- ris and Levander (1970) to contain the highest concentrations of selenium (1.4 to 3 ppm) in animal tissues, followed by liver (0.20 to 0.85 ppm). Fish and other seafoods are good sources of selenium; Arthur (1972) reported trout to contain 0.36 ppm and shrimp to contain 2 ppm. Other workers (Morris and Levander, 1970) found an average value of 0.63 ppm in cod and flounder fillets and 0.63 ppm for various shellfish. FRUITS AND VEGETABLES Fruits and vegetables are recognized as poor dietary sources of selenium (Levander, 1976a). Many have less than 0.01 ppm (Morris and Levander, 1970; Arthur, 1972; Ganapathy et al., 1977; Schrauzer and White, 1978; Thorn et al., 19784. Cucumbers, carrots, cabbages, onions, and radishes had slightly higher values, 0.015 to 0.140 ppm, and mushrooms and garlic between 0.060 and 0.249 ppm selenium. B A B Y F O O D S The average values for processed meats, cereals, fruits, and vegetables fol- lowed those previously described except that they were considerably lower than fresh, unprocessed samples (Morris and Levander, 1970; Arthur, 1972; Levander, 1976a; Thorn et al., 19781. DIETARY SELENIUM LEVELS IN VARIOUS COUNTRIES An indication of the selenium concentration of certain selected foods for various countries is presented in Table 4. It is evident that in each country there are wide differences in the sele- nium level in foods, depending on the kind of food and location where it was produced. In the United States, because diets have a varied nature and the ingredients a varied origin, it is unlikely except under unusual circum- stances that an excess or inadequacy of dietary selenium would arise. Indi- vidual intakes of selenium will vary with the amounts of selenium-rich
34 V, ·_4 5 - o o .~ o o o C) o o ·~ - a; ~_ V, ,_ C~ 3 - a, ~: Ct s~ o - Ct o V .= st C~ C~ e~ _3 V, V) ._ ._ s~ r3 Ct Ct V V, ._ o o C~ oo . . o o 1 1 CN ~ _ o o o 0 oo r~ oo o 0 r~ . o o o o 0 r~ ~ 0 0 ~ ~ ~ 0 0 o ~ o o o o o o o o o V V V V I~ ~1 Ir) ~ 1 1 O 0 1 ~0 1 1 _- ~S o o ~ _ ~r~ O O O r~ . o o ~_ o 1 o o . o - ~ o ~ ~ 0 oo r~ ~ ~ 0 o o o ~ o o . . . . . . o o o o o o U~ _ ~0 ~ (~) ~s ~_ ~ 0 ~0 ~ 0 O 0 1 0 1 1 1 ~1 ~ ro O ~0 0 0 . . . . . O 0 0 0 0 _ _ _ _ _ r~ oo 0 ~ ~ r~ r~ ~ oo ~ O oO ~ oO ~ O r~ ~ ~ ~ ~ 0 0 0 0 r~ 0 0 . . . . . . . . . . . . O ~ 0 0 0 0 0 0 0 0 0 0 CQ ~: ._ C) ~ _ ~0 _ S ~ ~ ,,= -., ~ m) ~ ~ ~ ~ U ~ ~ ._ ~ 0 _ ~V' _ ~ ~ ~0 ~ C ~ ~ C) oo - - Ct S~ ~ ~0 r~ 0 ~ 0 0 0 . S~ S~ _ . ~ ~: O ~ Ct ~.~ - ~ ~o V' cd ~U . ~i 5-, :> ~ C~ _ ~ Ct ._ 5- ~ ~ _f ~ . ~ . .~ C) ._ Ct ~ ~ s~, .~ C~ C: _ Ct _ ._ 0 ~ ~ ~ au g 5- ~ ~ ~ . 5- ~, 5- Ct ~ ~ ~ ~ 5- Cd . s~ ~ ~ ~ o ~ ~ ~ ~ o ~ o ~ ^] ;^ s~ ~ c\ · .= d . .- v) ;^
Distribution 35 foods eaten. Those consuming more of the high-selenium cereal products, seafoods, and animal organs such as kidneys and liver would have higher dietary selenium levels. However, the intake of most Americans would fall within the suggested safe and adequate range of approximately 50 to 200 fig selenium/day (NRC, 1980a). The average daily intakes of selenium by humans in various countries have been reported by several investigators (Table 5; Levander, 1976a). Watkinson (1974) calculated that the per capita dietary intake was 56 ,ug/ day in New Zealand; 132 ,ug/day in Maryland, USA; and 151 ,ug/day in Ontario, Canada. Thompson et al. (1975) found a range of 98 to 224 fig/ day in the diet of Canadians. Sakurai and Tsuchiya (1975) found that a typical Japanese diet provided about 88 ,ug/day. Mondragon and Jaffe (1971) estimated the average daily intake by Venezuelans as 326,ug. Other investigators have reported average daily selenium intakes for humans as: Great Britain, 60 fig (Thorn et al., 1978~; New Zealand, 28 to 32 ,ug (Thomson and Robinson, 1980~; Sweden, 23 to 210 fig (Bostrom and Wester, 1968; Wester, 1971, 1974~; northeastern United States, 60 to 150 ,~4g (Schroeder et al., 1970~; and Finland, 30 ,ug (Koivistoinen, 1980~. Welsh et al. (1981) determined by analysis the actual selenium intakes of Maryland, USA, residents consuming self-selected diets as 81 + 41 fig/ day. Analyses of market basket samples collected in four different regions of the United States have not revealed appreciable differences in selenium intake (U.S. Department of Health, Education, and Welfare, 1974~. How- ever, great extremes in the dietary intake of selenium have been reported in the People's Republic of China, ranging from 11,ug/day in Keshan disease areas to 5 mg/day in areas of endemic selenosis (G. Q. Yang, personal communication). C O NTRIB UT! O N S O F WATE R Present U.S. standards for drinking water suggest 10 ,ug selenium/liter as the acceptable upper limit (U.S. Department of Health, Education, and Welfare, 1962~. Drinking water rarely contains selenium at levels above a few micrograms per liter. Therefore, this source of selenium is unlikely to be significant from either a nutritional or a toxicological standpoint, a con- clusion that is in accord with a recently published report (NRC, 1980b). SELENIUM CYCLING IN NATURE Several proposals have been made for the cycling of selenium in nature. An early scheme developed by Moxon et al. (1939) described a geological cy
36 Cal - A: _) X Ct _` Ct - _' V2 so o V: Ct ._ a o 5- . ~2 o - . a . ~ .= V ~ rat ICES O A: O So O C<S O O So O C: ~ Ct Ct ~V, . Ct 3 By o o A:' - 1~ AS) oO .. . . . Go o Cal Do ~ Lr) ~ rat r~ ~ c~ .. . . . U~ oo oo o oo o C~ .. . . . U~ ~ o o C~ - ~ oo ~ ~ oo . . . . . a~ ~ ~ 0 CN ~D oO oo o ~ U~ . . . . . U~ o oo - o . . . . . U) C ~ ~ oo cr o . . . . . r~ ~ O C~ ~ ~ ~ ~u~ ~ o u~ ~r~ oo oo ~ ~ ~ ~ ~oo ~ ~ r~ u~ cd ~- - ~D c~ 'e p" c) s~ . =, - ~ ~ u .= ~ ~ - cd o s~ 'e 3 o .= o 5- U~ a~ Ct ~ ._ _ V ~ ~:5 g ~ C~ ._ Ct o o s~ o
Distribution 37 cling of selenium in which plants and animals had a role. In 1964 a biological selenium cycle was postulated, involving the oxidation and reduction of the element by plants, fungi, and bacteria (Shrift, 19641. Later, Lakin and Davidson (1967) summarized knowledge of the geochemical cycling of sele- nium, and the cycling of low and high levels of the element in soils, plants, and animals was reviewed by Allaway et al. (1967) and Olson (1967), respec- tively. Recent versions of the cycling of selenium in nature are shown in Figure 3 (NRC, 1976b) and Figure 4 (Frost, 1973~. // // IMPLANTS ~ SOI LS¢: //\ ANIMALS J1 \\ T (~ / / AQU ATI C \ Ll FE \ / / ~/ /OCEANS, \ ~ //:ES \\\ ~ ATMOSPH E R E \ \ \ MOLTEN \ ROCK VOLCAN I SM 1 EARTH'S CORE SEDIMENTS & \ SEDIMENTARY ROCKS RUNNING\\ / and | / GROUND / WATE RS /f~' / ~ / IGNEOUS ROCKS / FIGURE 3 Cycling of selenium in nature. From NRC, 1976b.
38 SELENIUM IN NUTRITION Organic Selenides in Soil and Water - Se 2 ~ ~( Selenite Animal ~ / Se+4 Microorganisms ~ ~ ~ / Fe(OH) IRON SELENITE \ insoluble in acid soils I` - \ ? \ Burning of Fossil Fuels SO2 + SeO2 \ / \ ~ ~ Set / Digestive tract ~ '' reductional i/ / ,' Only at pH greater// than 8 / Bloc ''§0+ Selenate ~~~ CAGE Se+6 l FIGURE 4 Some possibilities of biological cycling of selenium. From Frost, 1973. Quantitative information on each of the cycling processes, involving rocks, water, air, soils, plants, and animals, is meager. However, it is possi- ble to describe each of the cycle components in a general way. Selenium is transported from the core to the surface of the earth through igneous extru- sion and volcanic gases. Soils may obtain selenium from the rocks that form the parent material, from volcanism, from industrial airborne wastes, from irrigation water, and from fertilizers. Geological processes such as wind erosion, glaciation, water erosion, and leaching all affect the selenium con- tent of soils. Although selenium in sedimentary rocks is insoluble and un- available to plants, chemical weathering and plant and microbial action transform much of it to soluble and available forms. The soil pH can have a marked influence on the selenium content of the plants. Chemical oxidation in alkaline soils produces selenate, which is available to plants; in acid soils the forms of selenium are much less available. The concentration of sele- nium in feed and food plants is governed largely by the amount and avail- ability of the element in the soils. The levels of selenium in milk, eggs, and meats reflect the concentration of the element in the plant material fed to the
Distribution 39 livestock and poultry producing the food. Plant and animal wastes return selenium to the soil. Oceans, seas, and lakes obtain selenium from inflowing waters, with some of the element deposited in the sediments. Selenium is transported in running water to lowlands and poorly drained areas. The atmosphere is supplied with selenium via soil dust, volcanoes, burn- ing of fossil fuels, industrial emissions, and volatile products produced by plants and animals. Some of the airborne element returns to the land and water as solid particles or in water precipitation. In the biological cycling of selenium, selenite and selenate are taken up by the plant roots with much selenium subsequently reduced within the plant to the-2 oxidation state. Monogastric animals consume food con- taining selenium compounds in the-2 oxidation state and appear to re- duce dietary selenite and selenate. The trimethyl selenonium ion is ex- creted in the urine (Palmer et al., 1970), and primarily elemental selenium and metal selenides are excreted in the feces (Peterson and Spedding, 19631. Thus, animal feces return the element to the soil in relatively insolu- ble, inert forms. Some soil bacteria can convert the elemental selenium to selenite and selenate forms, thereby making it available to plants (Sarath- chandra and Watkinson, 19811. In addition to the biological processes in the soil, chemical oxidation of selenium compounds may increase the availability of the element to plants.