The acceptable range is 1:1 to 7:1 cal/phos ratio. Cows nursing need more phos. Heavy milking cows need greater amounts.
Selenium is very important to the immune system. It helps use vitamin e, and is used to repair damaged tissue, i.e. post calving rebuilding of uterine tissue. Issues of cows not cleaning, not breeding, calves with pneumonia/scours has been linked to lack of selenium.
Jason Trowbridge Southern Angus Farms Alberta Canada