Bitter Harvest (1981)

Ned De Vries (Ron Howard) is the owner of a dairy farm whose livestock falls ill. The veterinarian cannot determine the cause, and the problem escalates. De Vries cleans the pastures of suspected contamination and observes the animals in an effort to explain their symptoms. He discovers that the cattle must have been poisoned, but nobody takes him seriously. Soon, however, his loved ones also become ill, prompting him to launch an investigation on his own. He reaches out to scientists and sends samples to a chemical laboratory. The film is based on true events that occurred in Michigan, USA, in 1973, when several thousand pounds of the chemical PBB were inadvertently mixed into cattle feed sent to farms.