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SELENIUM...THE FACTS
By: James Coomer, Ph.D. |
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Selenium (Se) was first identified in the early 1800s but it's nutritional importance has only been recognized in the later half of this century. Selenium is a micro nutrient element essential in very small quantities. Chemically, selenium falls between tellurium, which is a metal, and sulfur, which is not a metal. In the past, there was controversy over whether selenium was an essential nutrient or a toxic element. In the early 1930s, selenium was identified as the toxic agent causing lameness and death in livestock grazing certain range plants. It was almost 25 years later when selenium was first identified as an essential nutrient.
Selenium Requirement
Tissue selenium and glutathione peroxidase concentrations may be used as indicators of selenium status. Currently, selenium can be supplied to cattle by two methods:
The addition of selenium to livestock feeds is restricted to designated levels by Food and Drug Administration (FDA) regulations. An organic selenium source, currently being used in human mineral supplements, is in the process of being evaluated by the FDA for use in livestock feeds.
Deficiency Symptoms It has been documented that deficiencies in dietary selenium and vitamin E result in increased incidence of mastitis. Supplemental selenium and vitamin E have been shown to lower the frequency and duration of clinical mastitis. Retained placentas in dairy cattle can be caused by numerous factors, one of which may be a selenium deficiency. Retained placenta can have adverse economic effects because over half of the animals affected develop uterine infections which increase the incidence of infertility. Injections of selenium during the dry period have been shown to reduce the incidence of retained placenta.
Metabolic Role of Selenium
Metabolic Role of Selenium Because selenium and vitamin E work together to reduce damage to cell membranes, selenium and vitamin E can partially replace one another. If adequate levels of selenium are available to detoxify peroxides before they can cause damage, then less vitamin E is necessary to repair or stop damage. Conversely, if inadequate selenium is available, then more vitamin E is needed to prevent damage caused by the peroxides that do not get detoxified by glutathione peroxidase. However, neither selenium nor vitamin E can fully replace the other.
Selenium Toxicity Blind staggers or alkali disease is the most common problem associated with selenium toxicity. Symptoms include emaciation, loss of hair, soreness and sloughing of the hooves, excessive salivation, blindness, and death. Restricting access to potentially toxic forage and feeding diets high in protein and sulfur to slow selenium absorption are the most practical means of preventing selenium toxicity. For some livestock producers in the U.S., selenium deficiency is a much greater concern than selenium toxicity. References available upon request.
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