Hi, I'm Dr Steve. In most cases of "retained placenta" in cattle , I have clients wait 72hours before becoming worried. THis allows the attachments from the afterbirth (placenta) to loosen from the uterus and often it will fall out by itself. After 72 hours, if it is still hanging there, I will gently pull it out and I give antibiotics like penicillin ( labelled dose) to prevent infection. After72 hours the placenta will smell really bad and rotten, but that is how it loosens the attachments to the uterus.
A lot of my clients will call me to remove the placenta mostly because of its smell. Cattle get alon fine , as a rule,with waiting this long. Occasionally there can be a cow that has more of an infection and will possibly stop eating and act lethargic. If a cow acts this way, then the placenta may have to be emoved sooner and she may need antibiotics. If a cow stops eating, and the placenta needs to be removed sooner than 72 hours, it may be best to have your vet do it so the placenta doesn't tear and leave some of t deep in the uterus. I hope this has helped. If it has would you please rate my answer. Thanks for the question and good luck. I will check throughout the weekend to see if you have more questions.