Silo gas: A threat to farmers

By Gail Lapierre
Vermont AgrAbility Project Outreach Specialist
University of Vermont Extension

Silo-filler's disease, caused by exposure to silo gas, is a real risk to farmers this year due to the dry weather. Although New England has been fortunate to have not had the drought conditions that the Midwest has suffered, it has been dry enough to increase nitrates in corn. These high nitrate levels mean a greater potential for silo gas to form from fresh stored silage.

Workers can be exposed to silo gas around horizontal silos and bagged silage as well as in upright silos. Inhaling even a small amount can result in serious, permanent or fatal lung injury. Luckily, the disease can be prevented through proper work practices.

What is Silo Gas?

In a dry year, there will be increased nitrates in the corn. Within a few hours of ensiling, fermentation begins. Some bacteria use the nitrates in the corn instead of oxygen for fermentation, forming nitric oxide, a non-lethal gas.

This gas combines with oxygen in the air, producing nitrogen dioxide (N02), which is heavier than air and toxic to humans and animals. It has a yellowish-reddish-brown color and a bleach-like smell. However, with so many odors around the farm, farmers should not rely on odor alone to alert them to its presence.

Carbon dioxide (CO2) also is formed in the process but not often to lethal levels. Silo gas--the combination of NO2 and CO2--forms within a few hours of ensiling and continues to be formed for up to three weeks after the last silage is added to the silo.

Silo-filler's disease results from exposure to silo gas. The NO2 combines with water in the lungs and forms nitric acid, which is very corrosive. Once exposed to the gas, a person can become helpless in as little as two to three minutes.

Symptoms of silo-filler's disease include coughing, burning, shortness of breath, chills, fever, headache, nausea and vomiting. Symptoms may take from three to 30 hours to develop after mild exposure to silo gas. The slow, progressive inflammation of the lungs causes a buildup of fluid in the lungs, which often is fatal.

Relapses often occur in two to six weeks. The second occurrence may be milder or more severe than the first episode.

Prevention starts in the field. The highest level of nitrates in the corn plant is in the lowest part of the stalk. To reduce the nitrate level in forages for silage, farmers should raise the cutter bar when harvesting, leaving 10 to 12 inches of stalk in the field.

Other tips include:

Anyone exposed to silo gas should see a doctor immediately. Remember, this can be fatal.

In addition, high nitrates in corn can cause health issues with livestock. Before feeding, farmers should work with their feed dealer and have the corn tested. The University of Vermont's Agricultural and Environmental Testing Lab will do nitrate testing for $10 per sample. For information on submitting a sample, visit or call (802) 656-3030.