Why Do Hay Bales Catch Fire?

Almost every farmer has a story about their own hay catching on fire or a friend or colleague’s doing the same. For the most part, your hay will seem fine then one day it starts emitting steam and then smoke before it ignites. Why do hay bales catch fire?

Hay bales will catch fire due to the build-up of mould and/or bacteria. The mould and bacteria stems from hay bales with excessive moisture in them either from the time of baling or they get rained on. Stacking hay insulates it, which can increase heat to such a high degree that the entire stack of hay can burst into flames.

This guide to hay bale combustion will paint a clear picture of why hay bales catch fire and what you can do to preserve your hay.

Can Hay Set Itself On Fire? Why Do Hay Bales Spontaneously Combust?

Spontaneous combustion, at least to humankind, may be a myth, but that doesn’t mean this form of combustion cannot happen at all. Hay bales do unfortunately spontaneously combust all the time, mostly due to farmers who don’t understand what causes hay to ignite in the first place.

Let’s discuss those causes now.

Bacteria Buildup

Hay is made from a plant, and plants contain a simple sugar known as glucose. When bacteria have a source of glucose to feed on, that’s exactly what will transpire. Your hay bales are thus a smorgasbord of delectable glucose to bacteria.

The bacteria will take the glucose and use it for energy. This conversion process causes the glucose to catabolize, which is a reaction driven by enzyme catalysts.

Essentially, catabolism takes larger molecules and makes them smaller, causing the smaller molecules to release their energy.

Then the bacteria will respirate, oxidizing the glucose until it’s energy, water, and carbon dioxide. The energy is released as heat.

If bacteria accumulate to a high enough degree, then a bale of hay absolutely could spontaneously combust. The hay can just be sitting there, untouched since when you initially stored it in your shed, yet it will still catch fire.

Can hay set itself on fire
Excess Mould and bacteria build up caused a fire

Cutting and Then Storing

When you cut your crop, its important to monitor the stalk for moisture before it is baled.

After being cut, the plants that comprise the hay bale are still actively respirating. This is the second time we’ve touched on respiration, so let’s explain what it is now.

Respiration is a plant process in which a plant takes the sugars that were a by-product of photosynthesis and combines the sugar with oxygen to make energy.

Regarded as the inverse of photosynthesis, respiration is critical to plant survival.

Just as catabolizing can cause heat to release from hay, so too can respiration. When respirating hay is placed beside more respirating hay, you can see very easily how spontaneous combustion could occur.

High Moisture

Hay bales are supposed to have under 20% moisture on average. This isn’t just a suggestion, but a recommendation if you want your hay to be salable.

When hay gets soaked in the rain and then placed back in a storage shed or barn without being dried, fungus such as mould and mildew can develop in the warm, dark, moist crevices of the hay bale.

See our article on can hay bales get wet?

The problem is only exacerbated if you’re storing a lot of hay in one area. The mold and mildew will thrive. The fungi might reproduce, and each time this happens, the mold and mildew give off heat.

This heat can eventually cause a fire.


The last cause of spontaneous hay bale combustion is stacking hay. Not only does stacking hay worsen the above issues, but the tall piles of hay also create insulation. To insulate is to trap in heat, and that too might be the catalyst for a fire.

Now obviously it is fine to stack your hay bales but be sure to monitor it’s moisture content before storing them away. Hay within the recommended moisture range will be fine to stack in you hay shed.

What Should You Do When Your Hay Is on Fire?

Once your hay bales ignite, there’s nothing you can do. You should never try to go into the shed, barn, or storage area where the hay is to save it. You’d risk your life to do so, and no hay is worth that!

You also should not attempt to put out the fire yourself. You’re wasting precious seconds by doing so. In the meantime, the fire can spread beyond the shed and to other areas of your farm.

What you should do when your hay catches on fire is call the fire department immediately. They’ll arrive at the premises and put out the fire.

Some of the hay may survive, but since it was blasted with high-pressure water, it’s no longer saleable. You might be able to use it as feed around the farm, but most of the hay will have to be disposed of.

More so, you’ll have to rebuild a shed or barn to store your hay.

Although it can be a huge financial hit, if the fire was only contained to the shed with the hay and none of your other crops or livestock were harmed, then you should still consider yourself lucky. It could have been a lot worse!

How Do You Keep Hay Bales from Catching Fire?

One hay fire on your farm can be debilitating to your bottom line. The customers who ordered hay are going to have to wait for their orders to be fulfilled, which can cause them to abandon your business. Plus, you’ve lost inventory.

The following tactics and tips can help you prevent instances of your hay bales catching on fire.

Cure Hay, Then Bale It

Curing hay refers to drying it. You can use a multitude of methods for drying, everything from a hay tedder to a hay conditioner and even natural sunlight if you have the time.

What matters less than the method you use is that the hay is adequately dried before you bale it and stack it up.

As you’ll recall, hay that’s been recently cut but not baled is still undergoing respiration. When the hay generates heat and is then baled with more hay that’s producing heat, the risk of fire is a lot greater. 

Store Your Hay Under Cover

It’s difficult to prevent your hay from being exposed to any moisture, as weather events such as fog and dew can induce moisture that affects the wetness of your hay.

That said, you must protect your hay when it rains and snows, as being soaked will make the hay especially likely to ignite.

We wrote a great post about what to do with hay when it rains. In that article, we recommended structures you can store the hay.

Any structure with overhead protection is best. If you can afford to, build sidewalls, as rain and can still blow in from the sides if the structure only has a roof, one wall, and a floor.

See our article on how to store hay.

Test Moisture Content

Even if you don’t suspect that your hay may be moister than normal, it’s still a good idea to make a habit of testing the hay’s moisture content every few days to every week.

We’d suggest a long-pronged electronic moisture tester for the job. The length of the prong enables you to get good readings from the centre of the bale.

To get an accurate reading from the electronic moisture tester, it helps to test hay in large volumes. Smaller volumes of hay may skew the accuracy of the reading.

If you don’t want to test it yourself, be sure to send us in a sample and we will test it for you! Follow this link with instructions on how to test your hay.

Don’t Stack Hay

To reiterate the point from before, stacking hay insulates the bales. You’re at a higher risk of losing all the bales to spontaneous combustion than you are if you don’t keep the bales so close together.

If your bales have tested with higher moisture content, stack them apart to enable them to breath for a while.

Use Drying Agents

Should the hay test for more than 20 percent moisture content, you need to dry it if you hope to keep it on your farm. You can use measures such as pumping carbon dioxide gas, liquid nitrogen, or dry ice into the hay.

If you sell your hay as feed, keep in mind that customers may refuse to purchase hay that’s been treated with the above methods.

Know the Signs of Combustion

Hay might seem like it combusts spontaneously, but certain signs can act as a giveaway if you know what to look for.

Visible steam may rise from the hay as its moisture evaporates. The storage area may have condensation on surfaces even though the shed or barn is dry. You can even smell mould, as it has a tobacco-like odor that’s quite acrid. 

If any of these signs are present in your hay, we suggest taking it out of your shed and stack them on their own for a while until it settles down.


Hay bales catch fire due to mould and mildew and bacterial build-up, usually caused by excess moisture. Now that you understand why hay can combust, you can actively prevent fires so you can sell more hay and keep your farm operational!


  • Neville Janke

    Neville Janke is a qualified agronomist and Horticulturist with over 20 years of experience guiding farmers in the Agricultural and Horticultural industries. With this experience, Neville has been helping long-term users of Hay and grain to experience the Feed Central way of sourcing quality Feed for hungry cattle.

Share This