what are you feeding? any silage? or silage wrapped hay?

Could be dealing with Listeria!

Put deceased calf in garbage bag and take to your vet for tissue samples to send to ISU vet diagnostics lab.

MAKE SURE YOU ARE WEARING RUBBER/ LATEX GLOVES WHEN HANDLING DEAD FETUS....  ESP AFTER 5 DEADS......  IF WEARING LEATHER GLOVES, THEN DESTROY IMMEDIATELY.  

take an electric prod to buzz your vet...  After 2 cases, either of my 2 vets would be in my driveway - no questions asked.