North Queensland grazier Peter Finlay did routine checks on his weaner cattle, but two days later, nearly 40 were dead.
"I was pretty gutted because I hate to lose one, but when you lose a whole mob like that," Mr Finlay said.
Carcass testing found blackleg, a bacterial disease contracted from soil, to be the cause of death.
Blackleg is a spore-forming bacteria that primarily affects younger cattle. It is almost always fatal within 48 hours of the onset of symptoms.