Map of Selenium Status in US & Canada
This is a very old map (Kubota et al., 1967). While the areas showing deficient may still be correct I'm aware that there have been other areas deficient in selenium that have beenidentified in the last decade(s) that are not represented on this map. Please see Selenium in Counties of the Conterminous States
"Selenium deficiency is a major problem for livestock or wildlife in at least 37 states and costs beef, dairy, and sheep producers an estimated $545 million in losses every year. " soil scientist Gary S. Bañuelos
Concentrations of selenium in the soil depends on the rocks from which the soil was derived. The Northwest, Southeast, and Great Lakes states have low (<.05 ppm) soil selenium concentrations because the soils in those areas were derived from volcanic deposits or well-washed coastal deposits. Soils originating from cretaceous shale, such as found in South Dakota, Montana, Wyoming, Nebraska, Kansas, Utah, Colorado, and New Mexico, tend to have high (2 to 10 ppm) soil selenium concentrations.

Forms of selenium commonly found in soils include: selenides, elemental selenium, selenites, selenates, and organic selenium, for absorption by plants selenate and selenite are the most available forms. Selenate and selenite are generally found in alkaline, well-aerated soils. The insoluble selenide and elemental selenium are the least available and are found in acidic, poorly aerated soils.
(Reference and sources - Selenium Related Disorders in Washington Livestock N.L. Gates and K.A. Johnson,Washington State University)

Cattle consuming high levels of organic selenium will accumulate selenium in skeletal muscle. Selenium is stored in muscle primarily as selenomethionine, a selenium containing amino acid. Muscle serves as an important storage area for selenium. In a study conducted in Canada, skeletal muscle selenium content was examined in cattle from different geographic locations. Selenium deficient areas produced beef with much lower selenium content than cattle produced on seleniferous areas in Manitoba and Saskatchewan. A similar study tested skeletal muscle selenium concentrations in ewes grazing forages with known selenium content. These studies indicate that much of the variation of selenium content of skeletal muscle can be attributed to selenium levels in feed influenced by geographic location. The aforementioned studies correlate to a generalized map of the United States and Canada showing selenium distribution in crops (see above map). 
[Reference & sources : Korry J. Hintze, Graduate Student, Animal & Range Science Dept., NDSU, Fargo - Greg P. Lardy, Assistant Professor, Animal & Range Science Dept., NDSU, Fargo - John W. Finley, Research Scientist, USDA Human Nutrition Center, Grand Forks  - 1998 North Dakota State University * Dickinson Research Extension Center]


Additional references and sources:
UC Davis: Copper deficiency in cattle and other ruminants in California
Cornell: Selenium and Livestock Metabolism, Toxicity, and Deficiency
Soil Sulfur Amendments Suppress Selenium Uptake by Alfalfa and Western Wheatgrass
Kubota, J., W.R. Allaway, D.L. Carter, E.E. Cary and V.A. Lazar. 1967. Selenium in crops in the United States in relation to selenium-responsive diseases of livestock. J. Agric. Food Chem. 15:448.Lakin, R.W., and D.F. Davidson. 1967. The relation of the biochemistry of selenium to its occurrence in soils. Selenium in Biomedicine: A Symposium, Westport, CN: AVI.
 

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