Trace element toxicity relationships to crop production and livestock and human health: implications for management

In nature, trace element toxicities occur in all living organisms. The consequences of these toxicities have been described in crops, livestock and humans. In some instances, the toxicities are a direct consequence of the organism's position in the food chain and their environment, while in others, they are based upon genetic abnormalities resulting in physiological impairment. Nutrient toxicities in crops are more frequent for manganese (Mn) and boron (B) than for other nutrients. Manganese toxicity is found on acid soils in many parts of the world. Boron toxicities occur in irrigated regions where the well or irrigation waters are exceptionally high in B. Most other nutrient toxicities occur when large amounts of nutrients in question have been added in waste, e.g., sewage sludge. Crops grown near mines and smelters are prone to nutrient toxicities. Generally, the symptoms of toxicity in crops occur as burning, chlorosis and yellowing of leaves. Toxicities can result in decreased yield and/or impaired crop quality. Toxicity levels of trace elements range from 20 to 50 micrograms g-1 for cooper (Cu) and B to several hundred microgram g-1 for Mn, molybdenum (Mo) and zinc (Zn). With the exception of Mo, toxicity of other nutrients can be reduced by liming. Following recommended rates of fertilizers and the safe and controlled use of waste materials, such as sewage sludge and coal fly ash, should reduce metal loading and nutrient toxicity in crops. Use of crop species and genotypes less susceptible to toxicity are recommended where toxicity is suspected. Toxicities of trace elements in animals are caused by the consumption of either feeds or grazing on pastures with high contents of the element in question. Accidental excess applications of minerals in grain mixes or oral ingestions of elements have been described as causing toxicity. Some toxicities, e.g., of Mo result in deficiency of other elements such as Cu. Some of the most toxic elements for livestock include Cu, lead (Pb), mercury (Hg), Mo, and selenium (Se). Under certain conditions, toxicities of arsenic (As), cobalt (Co), fluoride (Fl), iodine (I), iron (Fe), Mn, and Zn have also been reported. Symptoms of toxicity have been described in detail in the animal section. Trace elements, when in excess quantities, accumulate chiefly in the blood, liver, and kidneys. Measures of control for various trace element toxicities include removal of animals from affected areas or removal of the source of toxicity; gastric lavage and the specific use of oral doses of salts and chelates depending upon the element in question. Trace element/metal toxicities in humans are not common under normal conditions. Most toxicities are caused by environmental and/or genetic abnormalities, from excessive intake, by deliberate or accidental over-dose, or from induced deficiencies (e.g. excess Zn causing Fe deficiency). Among the elements causing relatively frequent cases of toxicity are Pb, cadmium (Cd), Hg, Cu, Zn, and Fe. Selenium toxicity is generally limited to those areas/regions of the world, e.g. in certain parts of China, where soils with abnormally high Se content produce food crops containing highly toxic Se concentrations. Effective measures to control metal toxicities include gastric lavage, resuscitation, and the use of chelating agents in the acute phase. Protective legislation against the use of metal alloy utensils used for cooking is the long-term control strategy.