FROM THE FARM REPORT: SILENT BUT DEADLY – WHAT ARE SILO GASES & HOW TO HANDLE THEM?

Although it is hard to write this without including a pun, silo gases should not be taken lightly. Even brief exposure to silo gases can be fatal, making them one of the greatest health risks on-farm. There are two main silo gases to be aware of: carbon dioxide (CO2) and nitrogen dioxide (NO2). The considerable drought conditions across the northeastern US and Canada have increased the risk for nitrogen dioxide (NO2) production from ensiling, and it has already been observed in farms across New York and Vermont.

What is Nitrogen dioxide (NO2) and where does it come from?

Nitrogen dioxide (NO2) is a yellowish-brown gas that smells like bleach. It is heavier than air and tends to hang close to silos. NO2 is deadly: even brief exposure can cause death or permanent lung damage. NO2 is produced from nitrate during ensiling by bacteria belonging to Enterobacteria and Clostridia, which are undesirable for optimal silage fermentation. Nitrate can be converted to Nitric Oxide (NO) which is then converted to NO2 when exposed to oxygen. One of the main causes of NO2 production during ensiling is from excess nitrates in forages.

Where do the excess nitrates come from?

During normal plant growth, nitrates are used for protein synthesis. However, there are several stressors that will alter plant growth including drought conditions, excessive fertilization, and damage caused by hail or frost. Although nitrate uptake will continue at a slower rate, protein synthesis is often stopped during these instances which leads to an accumulation of nitrates. Additionally, plants will rapidly up-take nutrients when prolonged drought conditions are followed by notable rain events. Considering these conditions have been observed in Clinton County, there is a reasonable risk for the 2025 corn silage season.

Can we stop the production of NO2 after harvesting?

Generally speaking, this is not possible. Trying to prevent NO2 production post-harvest is like trying to alter kernel processing scores after harvesting – not doable. However, good management practices at the time of harvest can help minimize this activity (example: good packing density, timely and proper bunker covering, etc.).

How do we mitigate the risks?

If there is suspicion for increased risk of NO2 production in forages by way of drought conditions, etc., it is strongly recommended to take some precautionary steps. During harvest and packing, ensuring proper management practices can potentially mitigate the risk by minimizing the growth of undesirable bacteria that convert nitrates to nitrite (precursor to NO2 before oxygen exposure). For instance, enterobacteria are generally outcompeted by lactic acid bacteria under optimal conditions. Although this will not halt NO2 production entirely, ensuring good ensiling management practices could potentially help. This includes achieving optimal packing density, proper bunker covering, and inoculant usage.

If forages have already been harvested and ensiled, the next steps involve avoiding the silos to allow for gases to dissipate naturally. The risk for NO2 production is highest during the early phases of ensiling. Some sources cite 12-60 hours, while others caution 10 days post-harvest. Therefore, it is strongly recommended to avoid bunkers for the first 10 days post-harvest. If working with tower silos that are attached to feed rooms, keep these doors shut and ventilate the rooms for at least two weeks. Do not enter silo rooms during this period.

What are the warning signs for NO2 exposure? What should we do after exposure?

Remember that NO2 is visible. Do not approach any bunkers or feed rooms that have a low-hanging yellowish-brown gas. However, sometimes the gas can be present with minimal visible signs. Step away and seek fresh air immediately if you smell bleach or start to experience even the slightest throat irritation. Since the gas rapidly converts to a caustic form upon contact with lung tissue, it is critical to seek medical attention after suspected exposure to NO2. Permanent lung damage can occur within minutes of exposure.

It has already been 10 days since harvesting, is the risk gone?

Yes and no. In short, the biggest risk for NO2 exposure is within the early phases of ensiling. However, there could be some lingering gases beyond 10 days. See previous precautions above.

Furthermore, excessive nitrates in forages pose a significant health risk to animals. Not all nitrates will be converted to NO2. Upon feed-out, remaining nitrates in silage are also converted to nitrites in the bloodstream post-absorption which can bind to hemoglobin and reduce the oxygen carrying capacity. Acute nitrate toxicity can cause serious issues to health and performance. If there is suspicion of higher nitrates in forages, or observed NO2 production after harvest, it is strongly recommended to test the silage for nitrate levels. Adjust inclusion rates depending on the nitrate levels. For reference, nitrate levels above 1.50% DM are toxic for animals. Prolonged ensiling times can reduce nitrate content, and it is recommended to ensile forages for one month prior to feeding. Regardless, nitrate levels should still be evaluated prior to feed-out if issues are suspected.

What about carbon dioxide (CO2)? Is this silo gas a significant risk, and what steps can be taken?

Although carbon dioxide (CO2) is not as dangerous as NO2, it is still a notable risk. CO2 is odorless and colorless (hence, silent but deadly). Under some conditions, this gas can be observed around silos as a “shimmer” or “haze”, much like watching heat rising from an open oven. CO2 can originate from several different types of bacteria and yeasts including lactic acid bacteria (the “good guys”). Generally, the risk for CO2 from silage to pose a health hazard is low but not null. Similar advice applies, although CO2 exposure risk increases with silage stored under closed conditions (example: tower silos with attached feed rooms). If this applies, take appropriate precautions. Regardless of silo type, always listen to your body. Step away from the silos if begin you feel light-headed or start to cough.

Take-home messages, and looking forward

Although flatulence is funny and gas jokes are entertaining, silo gases are nothing to laugh at. Aside from the frustration of watching dry matter losses occur in real-time, the two main silo gases pose significant health risks. Looking forward to the next seasons, mitigation of NO2 production from ensiling involves steps in the field to manage nitrogen. Considering the drought conditions this season, followed by some periods of notable rainfall around harvest, I am particularly concerned about the level of nitrates that could be present in forages from the 2025 growing season. As a reminder:

Although this will not fully eliminate the risk, following good ensiling management practices can help mitigate risks.

Avoid approaching bunkers/ silos for at least 10-days post-harvest.

Stay away from bunkers if you observe a yellowish-brown gas, or a haze/shimmer.

Get to fresh air and seek medical attention immediately after suspected exposure to silo gases.

Listen to your body! If you are coughing or have even a slight throat irritation after approaching a bunker, you could be exposed to silo gases.

If you suspect issues in your forages at the time of harvest (example: drought, hail, frost, etc.), remember the risks beyond silo gases:

Excessive nitrates in feed are toxic to animals.

Allow at least one month of ensiling to help reduce nitrates.

Test your silage for nitrate levels prior to feeding.

Adjust the ration accordingly.

One of my favorite safety-related quotes is along the lines of “if it is not worth doing safely, then it is not worth doing at all.” In other words, take these risks seriously. Everyone deserves to go home at the end of each day, silage-making is not worth risking your life.

— Ricky Scuderi

Research Scientist,

Lallemand Animal Nutrition

rscuderi@lallemand.com