The placenta (or afterbirth) is the name given to the membranes that transfer nutrients from the cow to the calf before the calf (fetus) is born. These membranes and blood vessels are made by the calf and connect to the blood supply in the uterus of the cow or heifer. The cotyledons (buttons) of the placenta “hook up” to the caruncles of the uterus. It is across this thin connection between the membranes of the cow and the membranes of the calf that essential materials pass to the developing calf. These essential materials include oxygen, blood sugar (glucose), amino acids (the building blocks of proteins), fats, calcium, phosphorus, vitamins, trace minerals and all the other essential components of life.
When the calf is born the placenta normally detaches within a few hours and is expelled. That is why it is referred to as the “afterbirth”. In most all situations, the placenta is expelled within 12 hours of the birth of the calf. If it is not expelled by 24 hours it is defined as a retained placenta or the cow is said to “have not cleaned”.
Why is a retained placenta a problem?
In and of itself the retained placenta is not a problem. However, the retention creates a number of potential problems. The retained placenta “hangs” out of the vulva and as the cow walks the placenta slides back and forth through the external opening of the vulva and vagina and “sucks” material into the uterus. The vulva of a cow is located just below the anus, so fecal material and fecal microorganisms are pulled into the uterus. Also, when a cow lays down the placenta will hang out further and lay in the dirt or mud, which is loaded with bacteria. These bacteria set up an active site of infection in the uterus and this can have serious consequences for the cow.
The local infection in the uterus can cause the animal to become ill (fever, weight loss, etc). Sometimes the infection is so bad the cow can actually die.
When the uterus becomes infected and inflamed, it takes much longer for the cow to “clean” and to be ready for the next breeding season. A retained placenta usually causes the cow to have an increased time from calving to the conception of the next calf. It is not uncommon for a cow with a retained placenta to delay the next pregnancy for 2-6 months. Obviously, a two-month delay will mean a late calving date in the following year. A six-month delay may result in an open cow next year at pregnancy checking time. Another result of retained placenta can be tetanus. The tetanus organism is commonly in the soil or in the feces and when it gets into the uterus it can set up an infection and result in tetanus (lockjaw). Tetanus can be fatal in cattle and at the minimum requires long term therapy (1-3 months).
Source: UC Davis Ag Extension