A study reported in the March Journal of Dairy Science confirms that a suppressed immune system causes retained placentas.

The results of the study, led by researchers at the National Animal Disease Center in Ames, Iowa, showed cows with retained placentas had decreased immune-cell function both before calving and one to two weeks after calving.

The results confirm prior research that had suggested the cow must recognize the placenta as a "foreign" tissue in order for it to be expelled, notes Jesse Goff, one of the study’s authors. The research also shows how important it is to bolster a cow’s immune system prior to and at calving.