Only the inorganic selenium salts sodium selenite  were available at a cost permitting their use in animal feed. Because selenium is an essential element, sodium selenite is an ingredient in some food supplements.

selenium can be toxic in high doses.

NOTE ::: Huge Differences between Sodium Selenite and Sodium Selenate ?

Sodium selenate is a common ingredient in some insecticides used against mites, aphids, and mealybugs. For most insects, a dose of 10 mg/kg is enough to be fatal. It is also used in some fungicides.

Technical Bulletin

Product Label

selenium can be toxic in high doses.
Recommended minimum of 0.11 mg/kg