OMAFRA Report: Silo gas and nitrate poisoning risk after rain

By Christine O’Reilly

A weekly report prepared by the staff of the Ministry of Agriculture, Food and Rural Affairs (OMAFRA). If you require further information, regarding this report, call the Elora Resource Centre at 519-846-0941. Office hours: 8:30 a.m. to 4:30 p.m.

For technical information, call the Agricultural Information Contact Centre at 1-877-424-1300 or visit the OMAFRA website:

Welcome rain events across the province have interrupted a dry spring.

The five to seven days after rain ending a severe dry period increases the risk of nitrates in forage crops. Nitrates are of concern because they can increase silo gas production and cause nitrate poisoning in livestock.

Silo gas

Silo gas (nitrogen dioxide/N2O) is produced almost immediately after filling a silo.

It has a bleach-like odour and may be visible as a reddish-brown haze, however, it’s not always visible.

Nitrogen dioxide is heavier than air, therefore it tends to be located just above the silage surface and on the ground around the silo. It may flow down silo chutes and into feed rooms. Tower silos are at greater risk because the silo gas is contained at the silage surface level, and operators often enter the silo after filling to level silage and set up the unloader.

It is difficult to predict when silo gas will be produced, so always take precautions following harvest.

People exposed to silo gas are at risk of severe respiratory distress, permanent damage to lungs, and even sudden death.

When inhaled, nitrogen dioxide mixes with body moisture to form nitric acid which causes severe burning of the lungs and the rest of the respiratory system, resulting in pulmonary edema. Victims often collapse. Other people attempting a rescue can also be overcome.

Do not enter a recently filled silo under any circumstances without a pressure-demand remote breathing apparatus.

The confined space often contains lethal concentrations of hazardous gases.

People exposed to silo gas should get immediate medical attention.


Handheld multi-gas detection monitors can help prevent disaster.

Handheld detectors monitor environmental gases constantly, are compact in size and sound an alarm when a dangerous gas level is detected. They can be equipped with a sampling hose and pump used to monitor the atmosphere of a confined space outside harm’s way.

When purchasing a gas monitor, note whether the unit can be calibrated at the farm or has to be serviced

Silo safety tips

– Do not allow children or visitors near the silo for 3 weeks after filling;
– provide sufficient feed room ventilation to exhaust any silo gas that might have spilled down from the silo;
– check with your local fire department to see if pressure-demand remote breathing apparatus is part of their emergency equipment;
– during filling, adjust the distributor as needed to level the silage. Do not level the material by hand;
– if it is necessary to enter the silo when filling is complete, do so immediately following the last load, on the same day. Remember to leave the blower running while inside;
– oxygen-limiting silos are a special case and should never be entered;
– a top unloader can ventilate a silo effectively. However, if it becomes necessary to service a defective unloader, assume that gases are present; and
– if someone collapses inside a silo, begin ventilating with the forage blower immediately and contact your local fire department.Never attempt to rescue someone yourself.

Nitrate poisoning

Nitrates (NO3-N) in forage are converted to nitrites (NO2-N) in the rumen.

Normally, the nitrites are quickly converted to ammonia (NH3-N) by rumen bacteria and are absorbed into the blood stream to be excreted with urine.

When there are high levels of nitrates in the feed, the rumen microbes cannot keep up with nitrite production.

The nitrites form methemoglobin in the blood, which reduces oxygen-carrying capacity.

Signs of acute nitrate poisoning in animals include staggering, vomiting, laboured breathing, blue-grey mucous membranes, and death (typically within three hours).

Chronic nitrate poisoning often appears as reduced weight gain, early-stage abortions, and premature births.

Testing the forage is the only way to know whether the level of nitrates may pose a problem. Most laboratories that conduct feed and forage analysis offer a nitrates test.

Be sure the sample is representative of the feed, and it should be frozen to keep the nitrate levels from changing between the farm and the lab.

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