Plants can absorb selenium from the soil (selenium mainly present in inorganic forms: selenite or selenate), however selenium is generally not considered essential for plant growth. Nevertheless, if selenium is present in the soil, plants can take it up and convert the selenium to seleno-cysteine (se-cys), selenomethionine (SeMet) and other organic Se compounds, which are essential to animals.
Grains and grasses often have very low selenium levels as they depend on the selenium levels in the soil (which varies greatly). That’s why in some countries (especially NZ), sodium selenate is frequently included in the fertilizer regimes, to enrich soils with selenium to enhance the SeMet contents of grains and grasses. New Zealand has poor soil selenium levels, thus we also have low selenium levels in grasses, grains and vegetal sources. So, selenium must be added to animal diets.