Trace amounts of Selenium are necessary for cellular function
Selenium (Se) is an essential nutrient for animals, humans, and microorganisms, but its role in the plants needs further exploration. It is considered beneficial at low levels, but is toxic at higher levels, and there is a fine boundary between these concentrations. New Zealand soils are generally speaking deficient in Selenium.
Although there are areas where livestock poisoning occurs, much evidence has been accumulated to indicate that agricultural livestock is deficient, or bordering on selenium deficiency, in many areas of the world.