The deaths of a father and his two sons in a slurry tank accident in Northern Ireland must serve as a warning to the farming community, a coroner has urged.

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

Northern Ireland's senior coroner John Leckey today described the incident as the worst farming tragedy in the country 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 their farm near Hillsborough, Co Down.

On the second and final day of their 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

Giving evidence, state 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.

But he said he hoped the inquest would again help to spread the message to the farming community about the dangers.

He told Essie Spence, Noel's widow and the mother of Graham and Nevin: "I'm keen for the message to get out into the farming community and the inquest into the death of your husband and sons is one way of getting the message across."

Mrs Spence was accompanied by her daughters Emma and Laura and Graham's widow Andrea for the hearing. Graham, himself a talented rugby player, was a father to two young children.

At one point Mrs Spence Snr asked Prof Crane a question about Nevin's death.

"He seemed to succumb to the gas more quickly than the other two?" she said.

The pathologist explained that may have been because he was being more energetic in the tank and had therefore inhaled more gas.

Jim King, a chemical expert with the Northern Ireland Health and Safety Executive (HSE), said slurry was a very unpredictable substance and it was difficult to know what volume of gases would be released by any mixture.

But he added: "All tanks containing slurry should be considered dangerous."

He reiterated HSE advice that no one should enter a tank unless they had specialist breathing equipment and were trained how to use it.

Stressing the need for ventilation in any shed with a slurry tank, the inspector highlighted that there were no building regulations that required this.

Mr Leckey responded: "It does seem surprising to me there's no building regulations."

The coroner noted that the shed at the Spence farm had "limited ventilation".

"There were no windows in the building and it had only one entrance thereby significantly reducing the potential for weather-aided ventilation," he said.

Mr King said the HSE had developed an 11-point system to advise farmers of the safest way of working with slurry and urged members of the agricultural community to examine it.

Also noting the role of the media, Mr Leckey said: "I think that between us we have a duty to try to spread the message within the farming community in Northern Ireland and further afield."

Mr King's colleague in the HSE, Malcolm Downey, said the dangers had hit home with farmers.

"This terrible tragedy has brought the issue very clearly to the front of the mind of every farmer," said Mr Downey.

"I have spoken to many farmers since that terrible day - September 15 - and every one is very well aware of the dangers of slurry gases and is very concerned and their sympathies are with the Spence family."

Mr Leckey said working with slurry was "potentially highly dangerous" and highlighted the HSE advice that no one should enter a tank without equipment or training.

He added: "However, I recognise that the catalyst for the tragedy that unfolded was an understandable desire to try and rescue the family's collie dog."

The coroner said when news of the incident emerged people could not believe that such an "awful tragedy could happen in the 21st Century".

He said he was concerned at the number of farm-related deaths coming before him of late.

"I am told fatalities in a farming environment are running at one a month (in Northern Ireland) which is a frightening statistic and in any other industry certainly would not be considered acceptable," he said.

Turning to Nevin, Mr Leckey said the rugby world had suffered a great loss.

"This was not just a loss to the farming community but to anyone who loved rugby," he said.

"I know Nevin's loss was felt very keenly in rugby circles throughout the world."

He told the family members: "I hope you feel very proud of what he achieved as a professional rugby player."

Mr Leckey, who found the cause of death for each man as drowning and aspiration of slurry, said an inquest could often bring closure for a family.

But he acknowledged that given the enormity of the tragedy that may not be the case for the Spences.

"It is my genuine hope that through time you may be able to get on with your lives," he told them as he brought the inquest to a close.