Nine additional states have mild to moderate deficiencies. Generally speaking, the soils of most of Canada, the Great Lakes region, the Northeastern states, Atlantic seaboard areas, Florida and areas including the western half of Oregon and Washington and the northern half of California, plus an area including the panhandle of Idaho and the western edge of Montana are low in selenium. Scattered across the Great Plains, South, and the Southwestern United States are localized areas where soil is high in selenium, and local accumulator plants contain 50 ppm or greater. Many of these sites are found in South Dakota, just 100 miles from here in Northwest Iowa. Alkaline soil is more conducive to plants which accumulate selenium.
Plants grown on acidic soils are less likely to take up toxic amounts of selenium.
I hope the reader understands that the amount of selenium which a horse or mule can receive from his forage depends on the availability of selenium in the soil, the pH of the soil, the species of plant growing on this soil, growing conditions, and grazing conditions. There are certain plants which require selenium for their growth, and these plants are named obligate selenium accumulator or indicator plants. These plants may store up to 10 times the amount of selenium in the soil and become very toxic to the horse or animal which ingests it. A second group of plants known to store high amounts of selenium are called secondary selenium accumulator plants. They do not require selenium for their growth, but will store it if weather conditions are favorable for their growth on high selenium-available soil.