Selenium in agriculture and the environment

Selenium occurs naturally in the soil environment in amounts which may cause nutritional toxicity or deficiency to livestock in areas throughout the world. Normal soils contain between 0.1 and 2.0 μg g^?1 selenium compared with 30–324 μg g^?1 (dry weight) or above from a seleniferous area. Plant selenium concentrations can generally be used as an indicator of the soil selenium status. Selenium accumulator plants may contain thousands of μg g^?1 selenium and grow healthily on highly seleniferous soils. Most plants contain small amounts of selenium; less than 1 μg g^?1 when growing on normal soils. A third group of plants, containing large amounts of sulphur, contain higher amounts of selenium when growing on normal soils.Animals consuming plants containing unusually small or large amounts of selenium over a prolonged period may develop nutritional selenium disorders. Development of sensitive analytical techniques has permitted detection of the very low amounts of selenium present in livestock suffering from selenium deficiency and associated problems. Required amounts of selenium necessary for animal nutrition range from 0.04 to 0.10 μg g^?1 depending on the animal species and the level of vitamin E in the diet. Amounts of selenium causing nutritional toxicity are usually an order of magnitude higher than those producing deficiency.Factors affecting selenium absorption in animals include the form of selenium in the diet and the type of animal. Ruminants moderate their dietary intake by reducing selenium to insoluble forms, which they excrete. Monogastric animals, by contrast, may absorb up to 80% or more of their dietary selenium intake, depending on its biological availability.Although there are areas where livestock poisoning occurs, much evidence has been accumulated to indicate that agricultural livestock is deficient, or bordering on selenium deficiency, in many areas of the world. Overt selenium deficiency in man has been reported in China, indicating the possibility of widespread non-overt sub-clinical selenium deficiency in humans. No definite evidence for human selenium toxicity has been reported.Although anthropogenic selenium contributes to the total selenium in the soil environment, most of it is biologically unavailable. Organisms such as bacteria and fungi reduce biologically available selenium to elemental insoluble forms, whilst others produce volatile organic forms of selenium which are lost to the atmosphere. Some bacteria are able to oxidise colloidal selenium so that it becomes biologically available. Whether the overall amount of biologically available selenium is increasing or decreasing in the soil environment is of vital consequence to agricultural productivity. It is recommended that work should be carried out to quantify amounts of available selenium that are lost from soil systems, and to identify the forms and amounts of selenium that enter the soil environment from natural and anthropogenic sources. Tests to indicate the presence of non-overt selenium deficiency in animals, such as blood selenium measurements, are recommended for areas under suspicion from soil and crop analyses.