Selenium (Se) belongs to a group of micronutrient elements required in very small amounts by animals and humans for the basic functions of life. The properties that allow Se to be biochemically active at dietary levels of less than 1 part per million (ppm) also give it the potential for toxicity at excessive concentrations. The range of nutritional activity for Se is about one hundred-fold. That is to say, Se deficiency can occur at dietary Se levels of 0.04 ppm and toxicity can potentially become a problem at 4.0 ppm in the diet. This is not particularly unusual for trace elements or vitamins. For example, copper has a four-fold margin of safety in sheep (a particularly sensitive species) and less than a ten-fold margin of safety in cattle.

Selenium is not equally distributed in the earth's crust. Consequently, there are widespread areas of the world and the United States that have Se deficient soils and smaller areas that have very high levels of Se in the soil. These latter areas can have Se concentrations high enough to be potentially toxic. Plant and forage species do not require Se as animals and humans do. Therefore, many of the deficient regions have ample grazing for wild and domestic animals; but, the low amounts of Se in the plants of these deficient regions limit animal health and productivity. Efforts to ameliorate the effects of Se deficiency have been undertaken in many parts of the world over the past 3 decades. In China, barefoot doctors have dispensed Se supplements to citizens in the Se-deficient areas of that country to prevent disease in humans (Keshan Disease). Finland uses mandatory Se fertilization of its agricultural lands to supplement both the animals and the humans through the food chain. In other parts of the world, animals are supplemented with Se injections, Se salts in feeds, and heavy Se-containing boluses, to name a few of the methods.

In the United States, supplementation of Se to animals is regulated by the Food and Drug Administration (FDA). Selenium is the only essential nutrient that does not have GRAS status (generally regarded as safe). Therefore, Se supplementation for livestock and poultry is regulated by the FDA as if it were a drug, while Se supplements for humans are available over the counter. In 1974, the FDA approved Se supplementation at 0.1 or 0.2 ppm of the diet for certain domestic animals, recognizing that Se was an essential nutrient. In 1987, the FDA increased the allowable Se supplementation level to 0.3 ppm for all major food-producing species. In 1993, the FDA issued a stay order of the 1987 amendments. This stay legally limited Se supplementation to 0.1 ppm for all domestic species, except turkeys (0.2 ppm) and growing swine (0.3 ppm). This decrease in allowable Se supplementation was done despite a wealth of scientific data that placed the dietary requirements for Se at 0.1 to 0.5 ppm depending on the species and state of production of the various animal species. This decrease in allowable Se by the FDA stemmed from local problems noted in wildlife in 1983 through 1985 at the Kesterson evaporation ponds in Merced County, California. There was no connection between Se supplemented to animals and the high levels of Se in the evaporation ponds. The accumulation of Se and other toxic elements was associated with Se rich soils and rocks in the Panoche hills west of the San Joaquin valley and the irrigation and drainage practices upstream from Kesterson. Had Se supplementation to livestock and poultry never been allowed in the United States, the problems at Kesterson would still have occurred. Recognizing that regional problems of Se toxicity could not be solved by limiting allowable supplementation of an essential nutrient to animals, Congress passed two laws in 1994 that reversed the FDA's decision to decrease the allowable level of Se supplementation.

The various actions of the involved parties created a tremendous amount of controversy both in the popular press and in the scientific community. Conjecture and opinion replaced facts on many occasions. The need for interdisciplinary exchange of scientific data was apparent. These were the reasons this symposium was organized. The goal of this scientific meeting was to bring together experts from all the involved disciplines to examine and discuss the effects of Se both as a nutrient essential for life and as a potential toxicant for animals and man. We hope that readers of these proceedings will gain insight into the role of Se in nature and realize that we need to continue to add to our understanding of the various roles of Se.

The University of California's Division of Agriculture and Natural Resources supplied the majority of support and funding for this meeting. Schering-Plough Animal Health and BioTech Associates Limited, Inc. were also very generous in supporting these efforts. We would again like to thank these institutions for their support.

The scientific meeting was preceded by a day long trip to the Kesterson area to view the site where wildlife losses due to Se toxicity occurred. It was very informative for attendees to see the local area where the problems had occurred and to be able to put this area into context with the overall issue of Se in the environment. The scientific meeting itself brought together a very diverse group of experts on Se and, while we cannot replicate the richness of the discussions that occurred, it is our hope that these written proceedings will be valuable to the reader.

John Maas
Chair, Organizing Committee

Back to "Table of Contents"