WHY HAY SPONTANEOUSLY COMBUSTS and ways to prevent it…

Thursday, June 21st, 2012 | Filed under Handy Tips

I was to have hay delivered on Monday from my grower.   He said it was a beautiful first cutting orchard grass with just the right amount of protein to sugar.  I was very excited.  I immediately called the trucking company to set up a delivery for Monday.

On Monday morning, I called the trucking company to see what time they would arrive – which is when I heard the words you never really want to hear….

Them:  “Oh, haven’t you heard?!”

Me:  What?!!

Them: “Glen’s barn burned up.  All of it – gone.  Nobody was hurt and all the animals are OK but he lost the entire barn, his squeeze and a few trucks…”


My grower’s barn… I took this off of the news feed.



My grower’s fire didn’t happen from spontaneous combustion.  He saw his squeeze throw a spark.

But… when anyone hears of a hay barn fire, they always ask:

“Was it new, just put-up hay?”  And then they calculate the last rains, trying to rule out the moisture factor in newly baled hay.

For those of you who don’t have large quantities of hay around your place, you’ve probably never even considered the danger of newly baled hay…

Neither had I.

But then someone told me about spontaneous hay combustion.


Moisture.  If the hay is baled before it is totally dry, it can blow up.  Literally.

I found this article that had much good information.  If you’d like to read it, here is the link.

Causes and Prevention of Spontaneous Combustion of Hay
Lester R. Vough
Forage Crops Extension Specialist
University of Maryland

Spontaneous combustion is always a possibility with stored hay but particularly if hay was baled
too wet or too green.

Hay that was too wet from rain or dew or that was not allowed to dry sufficiently in the field will
go through a curing process (sometimes referred to as a sweat) in storage. During the curing process,
heat is produced. This heat buildup is caused from live plant tissue respiration coupled with bacteria and
mold activity. Plant respiration converts plant sugars to water and carbon dioxide, increases neutral
detergent fiber (NDF) and acid detergent fiber (ADF) and decreases the net energy content of the hay.

Plant respiration slows as moisture content decreases but does not stop until plant moisture is 20% or
less. Mold organisms grow in hay having 20 to 35% moisture content. As with plant respiration, molds
likewise consume plant sugars, producing water and carbon dioxide, causing loss of dry matter,
digestible nutrients and net energy. The production of water through plant and mold organism
respiration can actually increase the moisture content of hay in storage (sweating) if the moisture is not
able to escape from the bale, mow (the pile of hay in the part of a barn where hay is stored), or stack.

Another photo from the news story of my grower’s barn burn.

YIKES!… it goes on to say:

Although most problems with spontaneous combustion begin occurring within two weeks after
hay has been placed in storage, combustion is possible for two months. Growers, especially those who
know that the hay was a little too wet or green when it went into storage, should start checking for
temperature rises within two days following storage and monitor on a daily basis for at least 10 days to
two weeks. Depending upon storage conditions and initial moisture content of the hay, it may be
necessary to continue monitoring for up to two months. But generally, any problems that result from
storing hay with an excessive moisture content are most likely to occur during the first month of storage.



Know where your hay comes from and trust your grower/baler.

Or, get a hay thermometer and test your new batch.  Especially if it has rained recently and the hay is newly baled.

Watch for the following temperatures:
–150 degrees F (65 degrees C) is the beginning of the danger zone. After this point, check temperature daily.
–160 degrees F (70 degrees C) is dangerous. Measure temperature every four hours and inspect the stack.
–At 175 degrees F (80 degrees C), call the fire department. Meanwhile, wet hay down and remove it from the barn or dismantle the stack away from buildings and other dry hay.
–At 185 degrees (85 degrees C) hot spots and pockets may be expected. Flames will likely develop when heating hay comes in contact with the air.
212 degrees (100 degrees C) is critical. Temperature rises rapidly above this point. Hay will almost certainly ignite.

You can get hay thermometers at several online farm supply stores.  I found this one at Gemplers.

This one is a temp and moisture tester.



Did you know that some growers who don’t want to wait for the hay to dry – or a sudden, impending rain storm is coming to ruin the crop – will add preservatives to the hay to dry it?!  I didn’t either.

Here is what I read in the article:

Hay preservatives like propionic acid aren’t necessarily going to be a good fit for every hay-making operation. But carefully picking the situations where you use these products could be beneficial, say growers and university forage specialists.

“We’ll only use it (propionic acid) on high-quality alfalfa we plan to sell to dairies and only if we think there’s a chance the hay will get rained on before we can get it baled,” says Gary Carmichael, owner of Carmichael Farms, LLC, Evart, MI.

Along with the alfalfa, packaged in 3 x 3 x 8′ square bales, he also grows grass and alfalfa-grass mixes on 2,000 acres. Markets for the grass and alfalfa-grass hay include alpaca, horse and beef cow operations.

The major advantage of using a hay preservative, says Carmichael, is that he can start baling when hay dries to the 24%-moisture level without worrying about bales heating or developing mold when they go into storage.

“If we can get several consecutive days of good drying weather, we’ll let the hay dry down to 15% on its own before baling,” he says.


This kinda scared me – made me certainly not want to feed hay with preservatives.  Of course, I would never know if they used preservatives or not.

Here is what happens when preserved hay catches fire:

Spontaneous Combustion in Hay Stacks
William T. W. Woodward1
June 1, 2004

Hay treated with preservatives may produce hydrogen cyanide gas at 240 degrees F, so extreme caution should be taken when fighting a hay fire if hay has been treated with such preservatives. Hay treated with preservatives containing ethoxyquin and BHT (butylated hydroxytoluene) produce hydrogen cyanide gas at around 240 degrees (115 degrees C). This gas is deadly. Additives containing primarily propionic acid to not produce hydrogen cyanide during a fire.



So after reading the next article, I ran down to the barn and opened the hay barn doors…  Basically, the article says that air flow certainly helps dry the wet.  Duh… But it isn’t just air flow throughout the barn- it is stacking.

You want to have the hay stacked so the air can flow through it and no moisture can grow into a fire.


How to start a safe stack…with airflow. Not great to stand on, but better for flow.




I found this blurb at then end of this article.  Kinda scary… but all very interesting.

In the past, farmers sprinkled salt on wet hay as it was stacked to prevent spoilage, but salt does not prevent spontaneous combustion. Dry ice, liquid nitrogen or carbon dioxide gas pumped into the hay can prevent combustion by eliminating the oxygen from the hay mass. Hay fires from spontaneous combustion occur infrequently in the arid western U.S., but can be a hazard for new hay or old stacks. Good storage practices will avoid spontaneous combustion and ensure higher quality hay.

Hay stacks are so gorgeous when they are in your own barn… ;)


HORSE AND MAN is a blog in growth… if you like this, please pass it around!




Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

HORSE AND MAN is a blog in growth... if you like this, please pass it around!

2 comments have been posted...

  1. Esther Long

    My dad salted the hay down in the mow… it was loose hay, though, and not baled. We never had a fire.

  2. Maggie

    I can remember seeing a barn burn when I was a young girl. Watching the hay bales explode was so scary. Once it starts, the whole thing is going to burn, no stopping a hay fire. Hope you can find an alternative hay source. Hope Glen’s barn etc were insured.

Post a comment!

Your email address will not be published. Required fields are marked *