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PREFACE Although Se is not considered to be an essential element for plants, it is an im- portant element for animal and human nutrition. Past research in the plant and soil sciences has shown, however, that Se is somewhat unique among the essential nutrients provided by plants as part of animal diets. In some areas, native vegetation can con- tain levels that are toxic to animals, whereas in other locations, feeds can be defi- cient in Se, also causing animal health problems. Selenium levels in plants have received considerable attention from animal, plant, and soil scientists since the early 1930s. Attention was refocused on Se in the environment in the early 1980s, when sub- surface agricultural drainage waters were used for the creation and management of wetlands in Kesterson Reservoir at the Kesterson National Wildlife Refuge in Califor- nia. Studies at the Kesterson reservoir have shown that Se was bioaccumulated in plants and animals at levels that could adversely affect wildlife. These findings re- quire a re-evaluation of human activities, which may redistribute excess Se within our environment and have implications for biological life. To provide a forum where current knowledge about Se in the environment might be explored as it relates to crop production, a symposium on "Selenium in Irrigated Agriculture" was organized and held at the 1986 American Society of Agronomy annual meeting. Several individuals from outside ASA were invited to join Society members and presented papers on 2 December in New Orleans, LA. This sympo- sium was cosponsored by Society Divisions A-5 (Environmental Quality), S-2 (Soil Chemistry), S-4 (Soil Fertility & Plant Nutrition), and S-7 (Forest & Range Soils). Subsequent to the symposium, manuscripts were written and constitute Seleni- um in Agriculture and the Environment. Seven of these papers are reviews, which help summarize the current state-of-knowledge regarding Se. Four are research papers, which report findings of recent studies, three of which particularly pertain to Kester- son Reservoir. This publication does not attempt to review Se in animal nutrition but will provide a perspective of how the presence of Se in our environment relates to the plant and soil sciences, animal nutrition, and wildlife. We are grateful to the authors, particularly those from outside ASA, who par- ticipated in the symposium and prepared papers comprising this special publication. We also wish to thank the many scientists who contributed suggestions for the sym- posium and who reviewed manuscripts during the preparation of this publication. The Editorial Committee L.W. JACOBS, chair, Michigan State University, East Lansing, Michigan A.C. CHANG, University of California, Riverside, California R.H. DOWDY, Agricultural Research Service, USDA, St. Paul, Minnesota R.C. SEVERSON, U.S. Geological Survey, Denver, Colorado L.E. SOMMERS, Colorado State University, Fort Collins, Colorado V.V. VOLK, Oregon State University, Corvallis, Oregon The Organizing Committee R.C. SEVERSON, U.S. Geological Survey, Denver, Colorado L.W. JACOBS, Michigan State University, East Lansing, Michigan vii