The author is an extension safety specialist at Penn State University.

The combination of scorching heat and drought conditions heightens the potential for silo gas during harvest. Late-season rainfall on drought-stressed corn in manured fields could produce higher-than-average nitrates resulting in high gas levels in silos.

During the fermentation process, the most concerning gas in upright silos is oxides of nitrogen, commonly referred to as silo gas. The formation of these gases peak in one to two days after filling and can last for 10 days to two weeks after the fresh forage is chopped and blown into the silo. This naturally occurring fermentation process is necessary to preserve the forage so it can be stored long term for livestock feed.

Heavier than air
Silo gas can have a bleach-like odor and under certain conditions can be visible as a fog from a distance (sometimes mistaken for smoke). If the gas is highly concentrated, this fog will appear to be yellow to reddish brown in color, and the silage surface, silo wall, base of the chute and other structures of the silo may be stained (yellow, orange, reddish) from the gas. This gas is heavier than air, therefore, it will settle at the surface of the silage instead of rising to the top of the silo and exiting through the fill door.

The highest concentration of gas is typically located at the silage surface, which is the area where a person will be going if they need to enter the silo for any reason. If a silo door is open near the surface of the silage, the high concentration of gas (heavier than air) could exit the silo through this door and flow down the chute causing it to settle at the base of the silo in the feed room or flow into the barn area. A dangerous buildup of silo gas can occur if there is little ventilation in the barn area.

The presence and concentration of silo gas depends on the storage structure and quality of the chopped forage material. Crops that have received nitrogen fertilizer and those crops that have suffered prolonged drought or especially prolonged drought conditions followed by rain just prior to harvest often lead to high gas production. Stunted corn may be harvested early for silage causing high levels of nitrates leading to higher than normal concentrations of silo gas produced during the ensiling process. Operators need to be aware of this and take precautions.

Precautions include assuring all spaces at the base of the silos are well-ventilated, silo doors are closed well above the level of the silage surface, stay out of the silo for three weeks after filling the silo and always ventilate the silo with the silo blower for at least 20 minutes prior to entry (only effective if the silo is over half full). Consider leaving the lower 10 to 12 inches of the stalk in the field (chop higher than normal) because this part of the plant may have the highest level of accumulated nitrates.

Risk levels differ
Individual reactions to silo gas depend on the concentration of inhaled gas and length of exposure. Very high concentrations of gas will cause immediate distress resulting in a person collapsing and dying within minutes. When gas levels are this high, typically the individual will not be able to withstand the symptoms felt and will quickly vacate the area.

Milder concentrations could cause upper respiratory congestion, watery eyes, cough, difficulty breathing, fatigue, nausea and so forth. If symptoms are mild, an individual may stay in the area to finish the job making the effects of silo gas worse. Effects can last for several hours in the body, causing symptoms to become progressively worse over the course of a day or two.

If a person experiences any of these symptoms when inside or near a freshly filled silo, they should immediately exit to fresh air and discontinue the task. They should immediately go to their doctor or hospital emergency room and report that they have been exposed to "silo gas poisoning."

One after-effect of silo gas poisoning is fluid in the lungs leading to chemical pneumonia and possibly death if not treated promptly. The effects of fluid filling the lungs may not present itself until several hours after the exposure which may be too late.

Occasionally, gas production becomes so great that it is mistaken for a silo fire. People may see "smoke" coming from the silo chute and assume that the silo is on fire and contact the fire department. Upon arrival, the firemen realize that the silo is just "gassing." Remember, it is rare for a silo to begin burning in the first week of filling, and if a cloud is seen escaping the silo, it is most likely due to silo gas. If the fire company is called, make sure firemen use a thermal-imaging camera to identify any excessive heating of the silo. A burning silo will give off temperatures of more than 190 degrees at the general location of the fire as viewed with a thermal-imaging camera.

For more information about silo gas, farm rescue strategies regarding silo fire or silo fire response training for fire departments, contact Dave Hill at the Pennsylvania State University at 814-865-2808 during working hours or 814-404-5441 after hours. The website,, has an excellent resource for fire companies to use in managing silo fires.