Utah Chapter

Information On Spontaneous Combustion of Hay

Steve Fransen and Ned Zaugg, Washington State University

Fire Risk of Wet Hay
The recent flooding requires close monitoring of stored hay for signs of spontaneous combustion. Dry hay (stored at 15 percent moisture or less) is safe for long-term storage. However, if the hay has become wet the quality has been permanently changed and the potential fire hazard from spontaneous combustion increased.

Checking for Spontaneous Combustion
Chemical reactions and microbial growth in hay occur because of the change in availability of moisture, oxygen, and pressure to create heat to the point of ignition and fire.

There will be early warning signs. Watch for steam rising from bale surfaces and condensing on the roof and eves of the barn. Often molds will start to grow on all these surfaces, too. There will be an acrid, hot, tobacco smell rising from the bales. Even before these visual signs appear, it is wise to take the temperature of the bales in the stack.

If the hay is in round bales, probe the bale ends. If in square bales, probe from the sides. If you do not have a long temperature probe, you can use a crowbar. If the haystack is large, push the crowbar in between bales as deep as you can go. Leave the crowbar there for about two hours. Remove the bar and feel with your bare hands. If the crowbar is easily handled, without feeling heat or discomfort, the hay in that area has not heated yet. 

If the crowbar can only be held for a short time, the hay temperature is approaching 130 Fº. If the bar can only be touched briefly, hay temperatures are about 140 Fº. At 150 Fº, the bar is too hot to hold.

A second approach is to drive a pipe into the stack about 10 feet deep. Follow with a thermometer on a string to the final depth, leave the thermometer there for about 10 to 15 minutes, then pull it out and check the bale temperature. Repeat this same process in a number of places in the stack and repeat daily. The following table shows critical bale temperatures to monitor and actions to consider.

Spontaneous Combustion: how it happens
The process of spontaneous combustion involves both microbial growth and chemical changes and may be slow to develop. The wet hay will first stimulate microbial growth and as these organisms grow they produce heat while drying out the surrounding surfaces of the hay for energy. More drying surfaces produces more microbial growth and different types of microbes live and die as the internal bale temperature climbs.

When the bale temperature reaches about 150 Fº the hay is on a one-way street and going the wrong direction! The larger the haystack and the more densely packed the hay is the longer it may take to show signs of internal bale burning. Internal bale temperature may take several weeks before reaching 150 Fº, but from this point on more heat resistant bacteria, called exothermic bacteria, start a process of chemical change that rapidly increases temperatures to the point of spontaneous combustion.

What You Can or Must Do
When bale temperature rises to 150 – 160 Fº, it is time to take action now. (Fire is likely from 180 to 212 Fº.) Do not walk on the top of a heating haystack because internal burning bales have consumed a quantity of hay and the top, and could collapse sending you into a burning inferno.

First, call the fire department as it is likely already out of control. Remove all animals and equipment out and away from the barn. Prepare a place for storing good hay and a place for hot or burning hay. After the trained personnel from the fire department have arrived, carefully start removing the bales from the barn -- the new infusion of air into the stack may be just the fuel needed for the entire structure to go up in flames.

Store bales individually to allow for maximum airflow and enhance heat loss from the bales. Large, one-ton bales will likely not dry in the center even after stored individually. These large bales will need to be broken apart for more complete drying. Small square bales stacked loosely may dry without splitting, but continue monitoring temperatures until they stabilize at less than 110 Fº.

If you see any flames appear in the stack or the bale as wet bales are removed, douse it well with water to cool and extinguish the fire. If certain materials have been applied to the hay, such as ethoxyquin and BHT (butylated hydroxytoluene), a deadly gas, hydrogen cyanide, will be produced during the burning process.

Hay Quality: some potential problems
A secondary problem with the wet hay is the nutritional value has been reduced from its initial quality. Heated hay may have a large amount of the protein now bound to the fiber and thus unavailable to livestock. Also, unknown pollutants may have contaminated the hay, so toxins could be produced from other microorganisms now living on the hay or toxic substances may have contaminated the hay. It may be best to not use it for feed.

For further information please contact Steve Fransen ( This email address is being protected from spambots. You need JavaScript enabled to view it. ) or Ned Zaugg at ( This email address is being protected from spambots. You need JavaScript enabled to view it. ).

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.

 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.