Robert Davies, 64, was found unconscious at the bottom of a slurry pit inspection chamber in December 2005 and died the next day in hospital.
A jury at the inquest at Barnstaple County Court returned a verdict of accidental death.
The Health and Safety Executive (HSE) said the death highlighted the dangers of working in confined spaces on farms.
Mr Davies managed farms on the Portledge Estate at Abbotsham, near Bideford.
Brain damage
Health and safety inspector Jo Fitzgerald said evidence suggested he had climbed into the underground chamber to investigate a leak or check a valve.
She said slurry appears to be passing through the chamber and Mr Davies was overcome by the fumes.
The inquest heard he was discovered unconscious by herdsman David Barker who saw a ladder protruding from the top.
Mr Davies died in hospital the following day from anoxic brain damage due to breathing toxic gases.
The HSE said there would be no prosecution as a result of the investigation.
Ms Fitzgerald said: "This incident highlights the need for everyone in the industry to think carefully about whether there are potentially dangerous confined spaces on their farms."
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