The deaths of a father and his two sons in a slurry tank accident is a warning to the whole farming community, a coroner has ruled.

Ulster Rugby player Nevin Spence, 22, his elder brother Graham, 30, and their father Noel, 58, were knocked out by poisonous fumes before drowning in less than four feet of slurry near Hillsborough, County Down.

Senior coroner John Leckey described the incident as the worst farming tragedy in the region in recent times.

The inquest heard that the last occasion three people died in a farm-based accident in Northern Ireland was 20 years ago.

The tragedy unfolded last September as the men tried to save a family dog from the half-full tank.

At the inquest in Belfast, Mr Leckey was told by experts that one of the colourless gases emitted by slurry – hydrogen sulphide – had the same devastating effect on the body as the better-known hydrogen cyanide.

"There isn't anyone in the country who isn't aware what a dangerous substance cyanide is and we are talking about the same thing," said Mr Leckey.

Pathologist Professor Jack Crane said the gas had not killed the men, but rendered them unconscious. He said they then drowned when they collapsed into the slurry.

"I don't think the initial concentration of hydrogen sulphide was high enough to cause their death because they were still breathing when they entered the slurry," explained the pathologist.

Mr Leckey said he felt the tragedy had reinforced the need for farmers to take precautions when working with slurry. He said he hoped the inquest would help to spread the message.