Selenium is an element with atomic number 34 and is considered a chalcogen. The ground state electronic configuration is [Ar] 3d¹⁰ 4s² 4p⁴. Selenium has four primary valence states [−2], [0], [+4], and [+6]. The selenium [+6] valence state is difficult to reach, requiring an exceptionally electronegative element, such as fluorine [1] . The ionic radius changes with oxidation state, ranging from 0.198 nm for [−2] to 0.042 nm for [+6]. Other atomic and physical properties are available in Lee [1] and Perrone et al. [2] .

Selenium ionic species typically are: 1) selenate or SeO 4 2 − , 2) Selenite or SeO 3 2 − , 3) elemental selenium or Se⁰, and 4) reduced selenide or Se²⁻. Argillaceous sediments typically have 0.3 to 0.6 mg Se kg⁻¹, whereas sandstone and limestones range from 0.01 to 0.1 mg Se kg⁻¹. Typically soils between 0.05 to 1.5 mg Se kg⁻¹, with a mean of 0.44 mg Se kg⁻¹ [3] Selenium occurs mostly as a secondary constituent of heavy metal sulfides, such as Ag, Cu, Pb, Hg, and Ni [4] . Bacterial derived Se methylation may occur, forming (CH₃)₂Se. Manure may sometimes be a significant selenium source.

Selenium is frequently associated with phyllosilicate minerals and Fe-oxyhydroxides, whose adsorption potential may limit plant availability [4] [5] [6] . Selenate and selenite adsorb onto Fe-oxyhydroxides, with maximal adsorption from pH 3 to pH 5. Thus, with increasing pH levels above pH 5 selenium generally shows more mobility. In more oxic environments ferric selenite (Fe₂(OH)₄SeO₃) forms and in anoxic environments iron selenide (FeSe) may precipitate [5] . Selenite is stable in the most oxic soil environments, whereas selenide (HSe⁻) is stable in the most anoxic soil environments [4] [5] [6] . H₂Se is considered extremely toxic, whereas Se⁰ is relatively nontoxic and is considered an essential element in animal nutrition. Selenium will oxidize to selenate in soils that experience drainage [4] [5] .

Selenium content frequently decreases with increased soil depth, given the affinity of Se for soil organic matter interaction. Conversely, many soils exhibit greater Se content in illuvial horizons that have increased clay and Fe-oxyhydroxide contents [4] . In more oxic soil environments, selenate and selenite bind preferentially with aluminum-octahedral sheets associated with the margins of phyllosilicates, a feature frequently associated with greater Se concentrations in argillic horizons [4] [5] .

Selenium binds strongly to fulvic acid. In a review, Manojlovic [4] described how selenium concentrations frequently dimmish with increasing soil profile depth, a feature attributed to selenium’s tendency to bind to proteins, fulvic acids, and other N-containing compounds. Zhang et al. [7] in a study with rice (Oryza sativa) seedlings documented that fulvic acid amendments negated the growth stimulation potential of selenium. These authors suggested that selenite uptake was inhibited.

Using thermodynamic data from Essington [5] authors of this manuscript constructed a pH – pe activity diagram, where the activity of soluble Se species was standardized to 10⁻⁶ and the water activity was unity (Figure 1). The acid dissociation reactions were: 1) H₂SeO₃ = HSeO 3 − + H⁺ with log Ka = −2.58, 2) HSeO 3 − = SeO 3 2 − + H⁺ with log Ka = −7.29, and 3) H₂Se = HSe⁻ + H⁺ with log Ka = −3.81 [5] . No provision was permitted for adsorption or complexation with fulvic acid, H₂Se volatilization or methylation. SeO 4 2 − and HSeO 3 − and SeO 3 2 − are present in oxic and suboxic soil environments, with HSeO 3 − transitioning to SeO 3 2 − at pH 7.5. HSe⁻ was present in anoxic soil environments. Selenite typically is less abundant than selenate in well-drained soils having a neutral to alkaline pH than in well-drained acidic soils. Given selenite adsorption, the bioavailability for plant uptake is at a minimum below pH 5.

Peak and Sparks [8] investigated selenate adsorption on hematite, goethite, and hydrous ferric oxide. Extended X-ray adsorption fine structure spectroscopy suggests selenate forms inner-sphere complexes on hematite, whereas selenate forms both inner-sphere and outer-sphere complexes on goethite and hydrous ferric oxide. Mathematical models for soil movement of selenate, selenite and selenomethionide based on 1) oxidation reduction, 2) adsorption-desorption, 3) volatilization and Se speciation show respectable predictive capabilities [9] [10] . Hu et al. [11] developed a selenium field trial with rice (Oryza sativa), showing that selenium amended soils reduced plant uptake of lead (Pb) and cadmium (Cd).

Selenium uptake by forage, grain and oil seed crops supports animal and human nutrition. Song et al. [12] observed that selenium uptake in rice (Oryza sativa) is greater than for soybean (Glycine max) and corn (Zea mays); however, soybeans exhibited greater seed transfer factors. Rice selenium concentrations were 0.20 mg·kg⁻¹ (root), 0.18 mg·kg⁻¹ (leaf), 0.12 mg·kg⁻¹ (stem), and 0.04 mg·kg⁻¹ (grain),

respectively. Bitterli et al. [13] observed that across different plant species Se accumulations vary widely, even with specified soil Se concentrations. Transfer of Se from soil into plants results primarily from the Se species solubility and selenium transfer factors were between 0.01 and 100 with few exceptions. Where selenium was soil incorporated, the selenium transfer factors were one or more orders of magnitude higher than selenium transfer factors derived for “native” selenium.

Wang et al. [14] selected soils having similar concentrations of total Se, bio-available selenium and related soil parameters and cultivated wheat (Triticum aestivum), rice (Oryza sativa) and canola (Brassica napus). The plant selenium concentrations differed significantly between crop species. Wheat seeds exhibited significantly higher Se concentration than rice seeds and canola seeds; however, wheat stem and root selenium concentrations were smaller than those from canola and rice. Yang et al. [15] confirmed that sodium selenite and Se-enriched mixed fertilizer amendments supported improved selenium contents in soybeans (Glycine max). Foliar selenium applications provided greater soybean selenium contents than the soil amendments. Soybean cultivars exhibited different selenium accumulations, demonstrating cultivar differences. In China, Fang et al. [16] analyzed rice (Oryza sativa) samples from different locations and noted that Se levels varied considerably. The mean content of Se was 0.02 mg·kg⁻¹. In a subsequent field trial involving rice the Se content of rice could be significantly increased by 194%.

Selenium competes with sulfur and is transported by the sulfate transporter. Selenium hyperaccumulator plants typically accumulate more than 1,000 mg Se kg⁻¹, whereas non-accumulators accumulate less than 100 mg Se kg⁻¹ [17] . Plants that generally accumulate between 100 and 1000 mg Se kg⁻¹ are termed secondary accumulator. Grasses and field crops are typically non-accumulators, whereas brassica, broccoli (Brassica oleracea) and aster are secondary-accumulators. Selenomethionine (SeMet), Se-MeSeCys (Se-Methylselenocysteine), selenite and selenate are the primary selenium species in plant materials [17] .

Soil is the major source of Se for vegetation, including pastures and forage [18] . Plants absorb selenium in the form of selenate or selenite; however, the distribution of available and unavailable forms of selenium influence plant uptake patterns. Soil management therefore plays a significant role in optimizing selenium uptake and subsequent plant growth. The usage of inorganic fertilizers with sulfur, over application of acidic fertilizers and soil compaction decrease selenium plant availability. Management practices that increase soil acidity and decrease aeration promote insoluble complexes between selenium and other substrates. Selenium soil deficiency is usually addressed by applying selenium containing fertilizers or selenium in feed additives to livestock.

The intensity of selenium uptake in plants is largely determined by plant genetics, with significant influences conferred by rainfall, soil pH, soil redox status, abundance of Fe-oxyhydroxides, clay content, and microbial activity [4] . Phosphorus has been shown to synergically augment selenium uptake [4] . Plant uptake of selenium results in the metabolic formation of selenoamino acids, with a particular abundance of selenomethionine (SeMet), selenocysteine, Se-methyl selenomethionine and Se-methyl-selenocysteine. SeMet is a main constituent of cereals ranging from 46% - 82%, 55% - 87%, 50% - 81% and 63% - 72% of the plant selenium in corn, rice, wheat, and soybean [19] [20] . Surai [21] demonstrated that cereals adapted to selenomethionine become an important selenium source for livestock. Selenomethionine has been shown to have higher concentrations in the seeds and roots compared to culms (stems) and leaves [22] .

Crush et al. [23] investigated foliar micronutrient (B, Co, Cu, Fe, Mn, Mo, Se, and Zn) concentrations applied to eight perennial ryegrass (Lolium perenne) cultivars cultured across various regions in New Zealand. Selenium perennial ryegrass concentrations ranged from 0.026 to 0.036 mg Se kg⁻¹, 0.007 to 0.009 mg Se kg⁻¹, 0.017 to 0.026 mg Se kg⁻¹, and 0.070 to 0.109 mg Se kg⁻¹ across four New Zealand regions. The concentration differences were attributed to cultivar, site, seasonality, and nitrogen amendment rates. Wu et al. [24] investigated selenium uptake and accumulation in tall fescue (Festuca arundinacea) and white clover (Trifolium repens). Selenium tolerance appeared to be related to selenium plant uptake exclusion. The assimilation of selenium into proteins is correlated with selenium toxicity. Owusu-Sekyere et al. [25] used nutrient solution cultures to demonstrate that alfalfa (Medicago sativa) exhibited increased soluble sugars and starch contents at various selenium concentrations and the plant tissue selenium concentrations were nitrogen dependent. Selenium accumulation may have increased nodulation.

Edens and Sefton [26] showed that selenium is a key trace element in animal nutrition, whereas Mayland [27] demonstrated that animals require diets containing 0.1 - 0.3 mg·kg⁻¹ selenium for adequate growth and development. Forage and grain crops are the major dietary source of Se. Beef cattle requirement is 0.1 mg Se kg⁻¹ in the livestock diet to meet daily requirements [28] . Soils with potentially high levels of sulfur may lower forage selenium contents [17] [28] . When soils are selenium deficient, producers supplement selenium in livestock diets to alleviate white muscle disease and improve calf disease immunity [28] . Timmerman and Omaye [29] provided a substantial review of selenium essentiality in livestock and biochemical pathways for selenomethionine and selenocysteine to produce selenoproteins.

Selenium plays a substantial role in antioxidative enzymatic systems, including the synthesis of glutathione peroxidase. Glutathione peroxidase is a key enzyme that reduces adverse oxidation reactions and improves immunity [4] . Selenium was shown to have a complementary role to Vitamin E, preventing dietary hepatic necrosis and exudative diathesis in rats and chicks [30] . More recently, Lyons et al. [17] documented that selenium increased the quality and efficiency of egg, meat, and milk production.

Optimal levels of selenium are required in feed rations; however, there is a small concentration range between deficiency and toxicity. Doucha et al. [31] detailed various conditions and health issues associated with low and high levels of selenium in humans, livestock, fish, and birds. Selenium deficiency is reported to increase susceptibility to various diseases in animals. Symptoms of selenium deficiency in livestock are reduced appetite, infertility, and muscle weakness [28] [32] . Feed rations with either organic or inorganic selenite and selenate are provided to poultry, pigs, and cattle to minimize deficiencies and to improve reproductive performance and growth. [31] .

Plants will either have low, adequate, or high levels of selenium depending on the soil type. Plants could be deficient in selenium when growing on soils having unavailable forms of selenium, such as selenium-iron complexes. These not readily available soil complexes may be either inorganic (SeO₄, SeO₃, Se⁰) or organic (selenomethionine) [33] . Soils with concentrations above 3 - 15 mg·kg⁻¹ generally have plants that are toxic to livestock. Incorporating these toxic selenium plants in livestock diets frequently results in health defects and death. Selenium toxicity is characterized by hair and hoof loss, impaired vision, embryonic deformities, and infertility [27] . Séboussi et al. [34] supported the usage of selenium fertilized forage to improve the performance of lactating dairy cows. de Abreu Faria et al. [35] observed tropical pastures and their potential for improved animal performance with selenium soil amendments.

Pfister et al. [36] studied if sheep and cattle could discriminate between forages and feeds with different selenium concentrations. The selenium concentrations ranged from 0.8 to 50 mg·kg⁻¹ in wheatgrass (Thinopyrum intermedium), 1.4 to 275 mg·kg⁻¹ in alfalfa (Medicago sativa), and 4 to 4455 mg·kg⁻¹ in western aster (Symphyotrichum ascendens). Selenium concentrations exhibited no influence on sheep or cattle preference. When given selenium containing pellets, initially cattle and sheep responses were variable, but they subsequently adjusted their intake to avoid excessive intake of Se. Juszczak-Czasnojć and Tomza-Marciniak [37] noted that beef cattle from conventional farms had significantly (p < 0.05) higher serum Se concentration than those on organic farms.

Mehdi and Dufrasne [38] noted selenium is an antioxidant and the nutritional requirements of selenium in cattle are estimated at 100 μg·kg⁻¹ DM (dry matter) for beef cattle and at 300 μg·kg⁻¹ DM for dairy cows. Rations high in fermentable carbohydrates, nitrates, sulfates, calcium, or hydrogen cyanide negatively influence the use of dietary selenium. Selenium supplementation may reduce the incidence of metritis and ovarian cysts during the postpartum period. The addition of selenium yeasts in the foodstuffs of cows significantly increases the Se content and the percentage of polyunsaturated fatty acids in milk compared to the addition of sodium selenite. Enrichment of selenium in the diet did not significantly affect the slaughter weight and carcass yield of bulls. The impact and results of selenium supplementation in cattle depend on physiological stage, selenium status of animals, type and content of selenium and types of selenium administration [28] [38] .

Selenium is an important nutrient, protecting plants as an antioxidant. In Missouri, selenium is a soil nutrient that is frequently deficient, especially in beef cattle (Bos taurus) production. Therefore, there is a need for selenium in livestock rations.

The greatest selenium concentrations in soils are derived from felsic materials (rhyolite). All other soils possessed parent materials derived from limestone residuum, loess, and alluvium. Soil having fine textures (clayey) had significant selenium concentrations, whereas soils having coarse textures (sandy) exhibited the smallest selenium concentrations. Selenium was associated with Fe-oxyhydroxides, typically Fe-oxyhydroxides associated with the clay separate.

The authors declare no conflicts of interest regarding the publication of this paper.

Aide, M., Braden, I., Nakasagga, S., Sargent, K., Siemers, S., Snider, M. and Wilson, M. (2022) Selenium Concentrations in Southeastern Missouri Soils and Its Impact on Livestock Nutrition. Agricultural Sciences, 13, 1363-1378. https://doi.org/10.4236/as.2022.1312083