Summary

All silages produce lots of gas during the early stages of ensilage, although most of it is relatively harmless under the circumstances in which it is produced. However, under certain unusual circumstances oxides of nitrogen can be produced in significant quantities, and farmers should be alert to this.

Large quantities of gas are produced in the first days of silage fermentation. This occurs for all silages, but is likely to be greater where filling and sealing of the silo is protracted or where wet grass which will undergo extensive fermentation is ensiled. Sometimes where bunker silos are extremely well sealed, or where bales are individually ensiled in bags, the gas production is evident by the plastic sheet or bag inflating. However, in most silos and wrapped bales the gases produced continuously and quietly leak past the plastic, such that their production is not obvious.

Carbon dioxide (CO₂), which is produced by respiration before the silo is sealed and by fermentation after sealing, accounts for most of this gas. Where it accumulates in silos and causes the plastic sheet to inflate, it can be released by removing the material that is sealing the plastic sheet (e.g. sand-bags) from a corner of the silo, thus letting the accumulated gas escape. The main caution required in doing this is to stand upwind while the gas escapes for, although carbon dioxide is not toxic (it is the gas we ourselves normally exhale), inhalation of large quantities can be dangerous because it means that oxygen, which we need, is not being breathed in.

Gases that are oxides of nitrogen can also be produced from within the silo during the early days of ensilage. In most cases the quantities seem to be small, and they get carried away in the stream of escaping carbon dioxide and are lost to the atmosphere. However, in some unusual circumstances significant amounts of nitrate in grass can be converted via nitrite to ammonia, nitric oxide (NO - a colourless gas) or nitrogen dioxide (NO₂ - reddish-brown heavy gas). The latter two gases can react readily with water to form nitrous and nitric acid, respectively. Thus, by dissolving in the aqueous lining of the respiratory tract, these gases are capable of inducing severe respiratory irritation. Although examples of this occurrence have been occasionally reported from tower silos in North America and parts of Europe over the years, reports in Ireland have been rare.

Last May, an Irish farmer ensiled an intensively managed permanent grassland sward during dry, warm and calm weather. It was precision-chop harvested without additive and filled two adjoining concrete-walled silos. On the outer side of each silo was a lean-to shed. Ten calves were accommodated in one of these sheds. The day after the silos were filled, and while the plastic sheeting was being covered with tyres and sealed, the farmer noticed a yellowish haze around the silo. Later he entered the lean-to in response to hearing an abnormal level of coughing by the calves. He found them coughing, choking, standing with tongues fully extended and frothing from the mouth. The farmer detected a strong bleach-like smell and noticed a yellow-brown haze. He himself found it difficult to breathe and was uncomfortable for a few days afterwards. He released all the calves immediately into an open yard. Six calves recovered quickly while four others recovered a number of days later following veterinary intervention. A day later, two adult wood pigeons were found dead beneath their nest which was located on the wall-plate between the lean-to building and the silo.

Investigations carried out at that stage indicated that the yellow-brown gas seen while the silos were being sealed and again the following day, and which had distressing effects on the farmer and some of his stock, was nitrogen dioxide.

Toxic gaseous oxides of nitrogen can be formed during the early stages of fermentation of unwilted grass silage. Under certain unusual conditions, these gases can pose a threat to humans and livestock in the immediate vicinity of the silo. In the above case, the quick action by the farmer (who had never encountered such a case before) prevented fatalities, but the incident underlines the need for vigilance and common sense near silos. Ideally, livestock should not be accommodated in buildings adjoining silos during silo filling or during the following week. If animals must be housed near such silos, they should be inspected very often in the first few days after ensiling.

If any farmers notice coloured silage gasses being produced from recently ensiled forage on their farms this year, the authors of this article would be interested in hearing about it.