While rare in most cowherds, retained placentas jeopardize reproductive efficiency and can rob operations of profit potential ...

The placenta is normally expulsed within six hours ... In most instances upon successful birth; the cow will vigorously lick the calf removing mucus from nostrils; this process stimulates breathing.

If it is not shed after twelve hours, then the cow has a case of retained placenta and requires veterinary intervention to treat.