A mysterious brown gas filmed escaping from a silage clamp near Kidwelly has been identified as deadly nitrogen dioxide (NO2).

Dairy farmer Anthony Gibbon posted a video on Twitter showing clouds of gas lingering around the bottom of his silage clamp.

The silage, harvested as winter fodder for his herd of 330 dairy cows, had been cut less than 24 hours earlier.

The grass had not been tedded to prevent it getting too dry as temperatures had been extremely high at the time.

His video provoked a great deal of response on Twitter, including from those who identified the gas.

“This is nitrous oxide, directly related to residual nitrogen combined with some other nasties, like soil contamination,’’ wrote one follower.

“Later cuts are always at more risk. Don’t ever breathe it in, it will kill you, and your cows. Once the gas is gone, the silage is generally safe to feed.’’

Nitrogen dioxide is extremely dangerous and can kill animals and humans. It is produced from free nitrates and is generally residual fertiliser nitrate.

It is more likely to happen when silage is made well but where there is residual nitrogen in the crop.

It is more commonly associated with first-cut silage and linked to extreme patterns of weather which prevents nitrogen being used efficiently – this can result in excess nitrate in a plant.

Nitrogen dioxide is best prevented by pre-testing grass just prior to cutting – anything above 2 per cent nitrate in a pre-cut fresh matter sample is high.

If the grass is ready to cut and conditions are right, there may be no other option but to harvest but farmers should be aware of the potential issues and take care.