Agricultural Sciences

Beware of silo gas -- it could be common again this summer

UNIVERSITY PARK, Pa. -- A unique set of circumstances that could lead to a heightened threat of deadly gas again is being created in silos across the Northeast, according to a farm-safety expert in Penn State's College of Agricultural Sciences.

The phenomenon may have started with the scorching heat wave the region experienced in early July -- which has some areas on the edge of drought conditions -- according to Davis Hill, senior extension associate in agricultural and biological engineering. It could develop if the region receives normal amounts of rainfall through the rest of the summer.

"There is now a lot of drought-stressed corn, particularly on manured fields," he said. "If this crop receives sufficient rainfall later in the season, there will be a potential for higher-than-average nitrates to build up in the corn plants just prior to harvest. This condition can lead to high gas levels in silos."

During the fermentation process of silage, a number of gases are given off, Hill explained. Of particular concern is a family of gases called oxides of nitrogen -- often referred to as "silo gas."

"The formation of these gases peaks in one to two days after filling and can last for 10 days to two weeks after the fresh, green forage is chopped and blown into the silo," he said. "This is a naturally occurring process and is necessary to ferment the forage so it is usable feed for livestock and for long-term storage."

Hill said that sometimes gas production is so great that it is mistaken for a silo fire.

"Farmers and passersby may witness 'smoke' coming from the silo chute and believe the silo is on fire," he said. "There has been at least one instance where a fire company was called to a farm for a 'silo fire' just two days after the farmer finished filling the silo.

"Firemen proceeded to pump water into the top of the silo, only to learn later that the silo was just gassing off."

Farmers and fire personnel need to realize that it would be nearly impossible for a silo fire to start so soon after filling, Hill noted. "This is why we always talk of attempting to locate the actual fire location within the silo before any attempts of extinguishment proceed," he said.

Silo gas sometimes has a bleach-like odor and under certain conditions can be visible as a fog from a distance (thus the mistake for smoke). If the gas is high enough in concentration, 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, which means it will settle at the surface of the silage instead of rising to the top of the silo, exiting through the fill door. This is an important factor, Hill pointed out.

"The highest concentration of gas will be at the surface of the silage, which is where a person will be going if he or she needs to enter the silo for any reason," he said. "Also, if a silo door is open near the surface of the silage, the high concentration of gas -- being heavier than air -- could exit the silo through this door, flow down the chute and settle at the base of the silo in the feed room or in the barn area.

"If there is little ventilation in the barn, a dangerous buildup of silo gas can occur, which can affect livestock or people who enter the area."

The presence and concentration of silo gas is dependent on the storage structure and the quality of the forage material that is chopped. Those crops that have received nitrogen fertilizer (corn) 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.

That could happen this summer, Hill worries. "It appears that this year, with the long droughty period that much of the state endured -- which stunted the corn crop -- there will be more corn harvested for silage, and that will be done fairly early," he said.

"The high levels of nitrates in this crop will lead to higher-than-normal concentrations of silo gas produced during the ensiling process. Operators need to be aware of this and take precautions."

These precautions include assuring all spaces at the base of the silos are well ventilated and that silo doors are closed well above the level of the silage surface. Farmers should 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 (however, this is only effective if the silo is over half full).

Also, consider leaving the lower 10-12 inches of stalk in the field (chop higher than normal) as this part of the plant may have the highest level of nitrates accumulated.

Individual reactions to silo gas depend on the concentration of gas that is inhaled and the length of exposure, Hill said. Very high concentrations of gas will cause immediate distress, which will result in a person collapsing and dying within minutes.

"When gas levels are this high, normally the individual will not be able to withstand the symptoms felt and will vacate the area quickly," he said.

"More mild concentrations could cause upper respiratory congestion, watering eyes, coughing, difficulty breathing, fatigue and nausea. If symptoms are mild, an individual may stay in the area to finish the job at hand. This can make the effects of silo gas worse, as these effects can last for many hours in the body, causing symptoms to become progressively worse over the course of the next day or two."

People experiencing any of these symptoms when inside or near a freshly filled silo should immediately exit to fresh air and leave the task for another day. They also should go immediately to their doctor or the hospital emergency room and report that they have had a serious "silo gas poisoning" exposure.

One aftereffect of silo gas poisoning is fluid in the lungs leading to chemical pneumonia and perhaps death if not treated promptly. The effects of fluid filling the lungs may not present itself until several hours after the exposure—and then it may be too late.

Hill reminds farmers that it is rare for a silo to begin burning in the first week of filling. 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 don't just start pumping water into the silo.

"Ask them to use a thermal-imaging camera to try to identify any excessive heating of the silo," he urged. "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."

Several technical experts are available throughout Pennsylvania to help farmers and firefighters think through the many management strategies when dealing with silo fires. This emergency information can be obtained by calling (814) 865-2808 during business hours or (814) 404-5441 after hours.

A Penn State website,, also offers information for fire companies to use in managing silo fires.

During the fermentation process of silage, a number of gases are given off. Of particular concern is a family of gases called oxides of nitrogen -- often referred to as Credit: Penn StateCreative Commons

Last Updated July 23, 2012