Selenium (Se) is an essential trace mineral important for immune function and overall health of cattle. Optimal immune function is critical for calves undergoing the stresses of weaning, relocation to feedlots, and commingling with animals of different origins. The majority of Oregon-grown calves enter the feedlot. Even with good vaccination programs, producers often encounter significant health issues in the feedlot, including mortality. Reducing these losses by enhancing immune function would have a significant economic impact for cattle producers.