Beware elevated risk for silo gas


UNIVERSITY PARK, Pa. — A recent death and serious injury resulting from gases inside a Lancaster County, Pa., silo are grim reminders to those working and living on farms that gases produced during the fermentation process can be deadly.

Emergency responders also should be ready to deal with silo gas emergencies, warned Davis Hill, Penn State University senior extension associate in agricultural and biological engineering.

“The danger of silo gas will be with us for the next several weeks, so farmers, their family members and their employees need to understand how and when silo gas happens and what they should and should not do to avoid being overcome or exposed,” he said.

Fermentation process

The conversion of plant material to fermented feed progresses through four phases of fermentation, and this process normally is completed within 21 days. The first phase usually lasts from several hours to a full day and is a time when the plant is still respiring (breathing), producing a large amount of carbon dioxide.

This carbon dioxide, Hill explained, actually displaces, or uses up, any oxygen that is trapped in the pile of silage.

During this phase, being inside the silo is extremely dangerous due to low oxygen levels. Once the oxygen within the pile of silage is used up, bacteria grow and multiply. These bacteria work to convert the sugars within the plant material to acids.

This is the second phase of fermentation, and it usually lasts for one to two days.

“If there are nitrates in the plant material, they will react with these acids to form nitrous acid. As the temperature increases due to the fermentation process, this nitrous acid will turn into nitrous oxide and nitrogen dioxide — silo gas,” Hill said.

During the final two stages, the fermentation process continues, and additional carbon dioxide, nitrous oxide and nitrogen dioxide is produced at lower rates. Eventually bacterial action will cease, and the silage is fermented.

Don’t enter silo

Based on this information, farmers need to take precautions if they feel they have to enter the silo during the two- to three-week period after filling.

Hill strongly recommends not going in.

“Many farmers have moved away from ‘capping’ the silo because of the dangers of exposure to silo gas,” he said. “Capping involves a person going into the silo after the last load is put in to level off the silage pile and cover the surface with plastic. This practice is often done at the very worst time for gas production.”

The low oxygen levels and high nitrogen gas levels can overcome a worker immediately.

Another reason farmers go into the silo during this fermentation period is to hand feed some of the silage. But that practice is especially dangerous because of the large amount of gases released as one digs up forkfuls of the material to throw down the unloading chute.

Run silo blower

If farmers must enter, Hill advised, they should be sure to run the silo blower for at least 30 minutes before entry and during the time they are inside the silo performing their task. The silo blower will be effective at clearing any gases inside the silo and on the silo surface only if there is 15 feet or less of headspace inside the silo.

If silos have more than 15 feet of headspace, he warned, farmers should not trust the silo blower to provide a gas-free space in which to work.

First responders

“If co-workers or family members discover a person lying unresponsive on the silo surface, they must not enter the silo to try to assist — they have to assume that silo gas is present and this condition also can render them helpless. They should make sure the silo blower is running and call 9-1-1,” Hill said.

“When calling 9-1-1, make sure the person on the other end of the phone understands that this is a silo emergency so they can send appropriately trained help.”

Not all local fire companies are familiar with or equipped to manage silo emergencies, and valuable time can be lost if they arrive on the scene and realize that they need additional assistance.

Arm yourself

Farmers should talk to their family members and employees about the dangers of silo gas each year. Detailed information can be found on the Penn State Extension Agricultural Safety and Health website at


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