A retained placenta — properly, retained fetal membranes — is when a cow fails to pass the afterbirth in the normal window after calving. It's common, especially after a hard or assisted calving, and how you handle it matters: the instinct to pull it out is exactly wrong and can do real harm.
What's normal Most cows pass the placenta within a few hours of calving, and certainly within 24 hours. If membranes are still hanging or retained beyond ~24 hours, it counts as retained.
Who's at higher risk - Cows that had a difficult or assisted calving, twins, or an abortion/premature calving. - Milk-fever / low-calcium cows (calcium is needed for the uterine contractions that expel it — another reason transition management matters). - Cows with selenium/vitamin E deficiency in some regions. - Over-fat cows and those with a rough transition.
What to do — and not do - Do not manually pull or tear the membranes out. This is the big one. Forcibly removing a retained placenta damages the uterine lining, can cause bleeding, and dramatically raises the risk of severe uterine infection (metritis). Let nature detach it. - Trim any long, trailing membrane that's dragging on the ground (cut it to hock level) so it stays cleaner and the cow doesn't tread on it — but leave the attached part alone. - Watch the cow closely, because the real danger isn't the membrane itself — it's the infection it can lead to. Take her temperature daily. The membranes will usually slough away on their own within 2–11 days. - Call the vet if she becomes sick — fever, dullness, off feed, a foul-smelling discharge = metritis developing, which needs treatment. A cow that's bright and eating with retained membranes often just needs monitoring; a cow that's gone off is the one that needs intervention.
Why it matters beyond the calving A retained placenta on its own is a nuisance; the metritis it can trigger is the costly part — it delays the cow getting back in calf, cuts milk, and in bad cases threatens her life. The whole management is aimed at preventing that infection: keep it clean, don't pull, and treat the cow (not the membrane) if she gets sick.
Prevention It mostly comes back to the transition period: control calcium (prevent milk fever), avoid difficult calvings with good calving management and sire selection, keep the calving environment clean, and ensure adequate selenium/vitamin E where soils are deficient. (See "Transition cow management" and "Preventing milk fever.")
Log it Record retained placentas by cow — repeat offenders and a herd cluster both point to transition or mineral problems worth fixing at the herd level.
Sources Merck Veterinary Manual — Retained Fetal Membranes in Cattle. AHDB Dairy — Fresh Cow Disorders. Penn State Extension — Retained Placenta Management.