Selenium 
Selenium is an essential trace element for ruminants. It is required in cattle for normal growth and fertility and for helping to prevent other health disorders such as mastitis and calf scours. Many pastures are short in natural selenium levels. Therefore, cattle should be supplemented with selenium. The level of selenium in the pasture is dependent on the level of selenium in the soil. Pastures have the lowest levels of selenium occurring in the spring and summer.