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Montana State University Communications Services

Keeping Hay Fires from Spontaneously Combusting

by Dennis Cash and Rob Johnson

MSU Extension Specialist and MSU Ravalli County Agent

7/21/99  - BOZEMAN – A hay fire isn't a minor incident. It can damage property and threaten life.

It is not uncommon for a fire to occur when transporting hay, like the fire July 18 in Helena. Such fires are most likely started by overheated exhaust pipes or a thoughtless passerby throwing a cigarette out a car window.

However, hay fires on farms are much more likely to be caused by spontaneous combustion. Such fires damage or destroy buildings and equipment in addition to the hay consumed. In 1998, the Montana Fire Marshall's office recorded 148 agricultural product fires that resulted in one injury and over $797,000 of damage. The reporting system does not say how many of these were hay fires.

Archive photo by Carol Flaherty of a fireman dragging a hose to spray a hay fire in Utah.

 Oddly enough, wet hay is more likely to lead to a spontaneous combustion fire than dry hay. If hay is put into a barn or stack when it has more than about 22 percent moisture, not only does the hay lose forage quality, but it has an increased risk of spontaneous combustion.

High moisture hay stacks can have chemical reactions that build heat. Hay insulates, so the larger the haystack, the less cooling there is to offset the heat.

When the internal temperature of hay rises above 130 degrees Fahrenheit (55 degrees C), a chemical reaction begins to produce flammable gas that can ignite if the temperature goes high enough.

Hay fires generally occur within six weeks of baling. Heating occurs in all hay above 15 percent moisture, but generally it peaks at 125 to130 degrees F, within three to seven days, with minimal risk of combustion or forage quality losses. Temperature within a stack then declines to safe levels in the next 15 to 60 days, depending on bale and stack density, ambient temperature and humidity, and rainfall absorbed by the hay.

To avoid hay fires, small, rectangular bales should not exceed 18 to 22 percent moisture, and large round or rectangular bales should not exceed 16 to 18 percent moisture for safe storage.

In addition, you should check your hay regularly. If you detect a slight caramel odor or a distinct musty smell, chances are your hay is heating. At this point, checking the moisture is too late, and you'll need to keep monitoring the hay's temperature.

What do you do if you suspect that your hay is heating?

A simple probe inserted into the haystack can accurately monitor temperature. You can make a probe from a 10-foot piece of pipe or electrical tubing. Sharpen the pipe or screw a pointed dowel to one end, then drill several 1/4-inch diameter holes in the tube just above the dowel. Drive the probe into the hay stack and lower a thermometer on a string into the probe. The thermometer should be left for 10 minutes in several areas of the stack to ensure an accurate reading.

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.

Take precautions. Pockets may have already burned out under the hay surface.

Before entering a barn, place long planks on top of the hay. Do not attempt to walk on the hay mass itself. Always tie a rope around your waist and have a second person on the other end in a safe location to pull you out should the surface of the hay collapse into a fire pocket.

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.

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.

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Send questions or comments to Carol Flaherty, MSU Communications Services, Bozeman, MT 59717 or email Flaherty at

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