Contact with deadly silo gases continues to occur wherever silos
exist. Although such contact may not occur as often as with
other kinds of hazards in agriculture, one should not be lax
with safety during the ensiling process. These gases are and
will continue to be a very real hazard for as long as silage
remains a common livestock feed. It is important that every
farm worker understand the dangers associated with silo gases
and learn how to deal with them.
Silo gas is formed by the natural fermentation of chopped silage
shortly after it is placed in the Silo.
Though a variety of gases are released during this process,
the type of silo in which the forage is stored is important
in determining which gas will be predominant. For instance,
in sealed silos both nitrogen and carbon dioxide gases are created
but carbon dioxide is produced in far greater amounts. This
is desirable because high carbon dioxide levels help to maintain
high quality silage
At the same time, however, this odorless and colorless gas is
dangerous. This gas replaces the silo's oxygen, and in high
concentrations, it gives a person little warning that he is
about to be overcome by a lack of oxygen. Sealed silos are usually
designed in such a way that entering them is unnecessary.
A variety of gases are also formed in conventional or open-top
silos with nitrogen dioxide being the most abundant. This highly
toxic gas is characterized by a strong bleach-like odor and
low lying yellow, red, or dark brown fumes.
Unlike carbon dioxide, nitrogen dioxide levels reach a peak
about three days after harvesting and rapidly begin to decrease
thereafter, particularly if the silo is ventilated. The gas
actually starts forming within hours of the material being ensiled. After
two weeks it is unlikely that more gas will be produced, although
some hazard remains if the gas has not been able to escape the
Nitrogen Dioxide is harmful because it causes severe irritation
to the nose and throat and may lead to inflammation of the lungs.
However, what makes this gas especially dangerous is that low
level exposure to it is accompanied by only a little immediate
pain or discomfort though death can and has occurred immediately,
after contact with high concentrations.
A farmer might breathe the gas without noticing any serious
ill effects and then die in his sleep hours later from fluid
collecting in his lungs.
Many victims can suffer relapses with symptoms similar to pneumonia
two to six weeks after the initial exposure. For these reasons,
it is extremely important for anyone who is exposed to this
gas, even a short time, to seek immediate medical attention.
Like carbon dioxide, nitrogen dioxide is heavier than air. Because
of this, as it is produced it tends to settle right on top of
the silage or flow down the silo chute and collect in the adjoining
feed rooms or other low lying areas near the base of the silo.
Gas may even flow into the barn itself and become trapped in
corners, under feed bunks, or lie low against the floor. The
threat that this poses to livestock is a serious one.
Some environmental conditions, particularly drought, cause nitrates
to accumulate in plants fertilized with nitrogen, even at recommended
rates. By taking precautions, outlined below, you can fertilize
at recommended rates for maximum production of silage crops
without fear of excessive nitrogen dioxide production.
Proper fertilization combined with good cultural methods such
as proper weed, insect and disease control, reduces the chances
of nitrogen dioxide gas production when you ensile the crop.
While growing the crop:
- Apply adequate nitrogen, but don't over do
- Follow the recommendations on soil analysis reports.
- Use balanced N-P-K fertilizers; add minor elements if
- Use disease and insect resistant varieties and/or spray
to control insect and disease damage to leaves and roots.
- Keep fields relatively free of weeds. Weeds can make
silage dangerous even though there is no excess nitrates
in the corn itself.
- After a drought, plants rapidly take up nitrates following
rain. So, harvest the crop before fall rains, or wait at
least five days after a rain.
- Plants damaged by hail or frost should be harvested immediately
before they take up nitrates. To reduce the amount of nitrate
going into the silage, cut higher than normal (10 to 12
inches). Most nitrates are in the lower stalk.
- Be alert for silo gas odours and/or yellowish-brown or
reddish fumes in or near the silo. Silo gas is heavier than
air and will displace oxygen. The greatest danger from nitrogen
dioxide gas from silage is during the first 12 to 60 hours
- However, take care to avoid possible exposure for10 days
after filling the Silo, and when opening the silo for feeding
- Do not enter the Silo for 4 to 6 weeks after filling
- Ventilate the silo room adequately for at least two weeks
- Open the windows and outside door of the silo room and
use fans if necessary.
- If a silo must be entered, do it immediately after blowing
the last load into the structure ensuring the blower is
running for 15 to 45 minutes prior to entry. Leave the blower
running for ventilation while anyone is inside.
- Running the Silo blower for 15 to 45 minutes before entering
the silo and while you are in the silo will help remove
silo gas, but this is not a substitute for a self-contained
- Never enter a silo if you are alone, without a selfcontain
breathing apparatus and a life- line, especially during
the danger period when gases may still be forming.
- Get to fresh air immediately if you start coughing or
experience throat irritation.
- Seek medical attention if you suspect you have been exposed
to silo gas.
- Keep all doors closed and the roof panel open for more
effective ventilation. This eliminates the danger of, simply
moving the gas out of the silo and down the feed chute.
- There have been reports of livestock being killed from
silo gas flowing down the chute and entering the barn. Provide
good ventilation to the silo room to dissipate dense gases
as they cascade out of the silo during fermentation.
- If the silo adjoins a barn (or other building), use barn
portable barn exhaust fans to blow air into the feed room
to overcome some of this hazard. Barn air would then be
expelled through the feed room rather than the reverse.
- A box duct connected over the inside of the fan and extending
down to 150mm above the feed room floor will ensure that
the heavier -than-air Silo gases are effectively removed.
The bottom of the silo chute must be closed off so that
the fan will draw air from the stable and not from the silo.
- Lock up all unloading mechanisms.
- Always wear a self-contained breathing apparatus if you
must enter the silo within four to six weeks after filling.
- Have three people outside to help if needed. Keep a hatch
door open near the level of the silage within the silo.
- Post all appropriate warning signs. Oxygen-limiting
silos require a sign that warns people of the absence of
oxygen. People need to be told to stay away from these areas
and never enter them.
- Barricade enclosed silo areas to prevent children and
strangers from entering the silo room and silo.
- During the dangerous loading and fermentation period
provide fencing around the Silo base to keep children and
animals a safe distance away from toxic silo gases.
- Air purifying respirators remove contaminants from
the air, but can be used only in an environment that
has enough oxygen to sustain life.
- Do not use air purifiers to provide protection from
the dangers of oxygen limited environments such as
- There are two general types of air purifying respirators,
those with mechanical filters and those with a chemical
cartridge. Mechanical filter purifiers are not for
use around toxic gases.
- Chemical cartridge respirators protect against certain
gases and all but the most toxic organic vapors.
- Powered air purifiers can be mechanical filter,
chemical cartridge, or a combination of both. They
cannot be used in oxygen-limited environments.
- Gas masks are more effective than chemical cartridge
respirators against high concentrations of toxic gases.
Gas masks should not be used in oxygen limited environments.
- Supplied-air respirators are for use in oxygen-deficient
areas such as manure pits; Silos containing silo gas,
airtight silos, or bins containing high moisture grain.
- The two main types of supplied-air respirators are
the hose mask with blower and emergency air supply,
and the self-contained breathing apparatus (SCBA)
The information and recommendations contained in this publication
are believed to be reliable and representative of contemporary
expert opinion on the subject material. The Farm Safety Association
Inc. does not guarantee absolute accuracy or sufficiency of
subject material, nor can it accept responsibility for health
and safety recommendations that may have been omitted due to
particular and exceptional conditions and circumstances.
Copyright © 2002 Farm Safety Association Inc.
22-340 Woodlawn Road West, Guelph, Ontario (519) 823-5600.
Disclaimer and Reproduction Information: Information in
NASD does not represent NIOSH policy. Information included in
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