Spontaneous Combustion of Hay

Steve Fransen and Ned Zaugg, Washington State University

Fire Risk of Wet Hay
The recent flooding on Washington’s west side 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.

Temperatures, quality changes
and action to take with flood-damaged hay

Temperature, Fº Loss of Protein, % Condition and Actions
100 - 120 None No action needs to be take
120 - 130 10 to 30 Monitor bale temps daily
140 10 to 50 Consider tearing stacks down
150 - 160 30 to 80 Dangerous. Chemical reactions are occurring. Call fire department – do not open the stack until they arrive!
160 - 180 90 to 100 Smoldering pockets and hot spots. Gases will ignite once hay is exposed to air.
212   Point of no return.


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 you, could collapse 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 (fransen@wsu.edu) or Ned Zaugg at (zaugg@wsu.edu).

WSU Extension, PO Box 646248, Hulbert 411, Washington State University, Pullman WA 99164-6248, 509-335-2837, Contact Us