Preventing Fires in Baled Hay and Straw

Use the following format to cite this article:

Preventing fires in baled hay and straw. (2012). Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice. Retrieved from


Most hay fires occur within the first six weeks after baling. Understanding the causes of fires in stored hay and learning how to reduce fire hazards will protect your feed supply and could prevent the loss of time and money associated with a fire. 

Causes of Fires in Baled Hay or Straw

Moisture content is the main factor that causes hay and straw to spontaneously combust. Hay fires are more common than straw fires, for reasons involving the type of forage, the moisture content in the stored forage, and heat production.

After forages are cut, respiration of plant fibers (burning of plant sugars to produce energy) continues in plant cells, causing the release of a small amount of heat. When the forages are cut, field dried, and baled at the recommended moisture level (20% or less), plant cell respiration slows and eventually ends. 

When forages are baled at moisture levels of greater than 20%, the right environment is provided for the growth and multiplication of mesophilic (warm temperature) bacteria found in forage crops. Mesophilic bacteria release heat within the bale and cause the internal bale temperature to rise between 130ºF and 140ºF. At this temperature range, bacteria die and bale temperature decreases. Fire risk is greater for hay than for straw because a hay bale’s interior temperature does not cool after the first initial heating cycle. The respiratory heat created by the mesophilic bacteria provides a breeding ground for thermophilc (heat loving) bacteria. Basically, the higher the moisture content, the longer a bale will remain at a higher temperature. For example, a bale with 30% moisture content may have higher interior bale temperature for up to 40 days. When thermophilic bacteria are present, they multiply and produce heat, which can raise interior bale temperature to over 170°F. At these temperatures, spontaneous combustion can occur.

Additional factors that contribute to the risk of hay fires include the volume of the mow or bale stack, bale density, and ventilation or air flow around the stacked bales. Bales with a lower density that are stacked lower and have good air flow and ventilation have a lower risk of overheating.

Decreasing the Risk of Fire

The best way to reduce the risk of a hay fire is to bale hay at a moisture content of 20% or less because at this moisture level, microbial activity decreases. There are several ways of reducing moisture content in baled hay:

  • Baling under appropriate conditions: Weather plays a critical role in achieving the appropriate moisture level in baled hay. The recommended weather conditions for haymaking are a slight wind and a humidity level of 50% or less. Because hay has a higher moisture content in the morning, it is recommended that you bale later in the day. The recommended practice for haymaking is to mow hay in the morning and allow it to dry in the field for a minimum of one full day prior to baling.
  • Using specialized equipment: Another way of decreasing moisture content is to use specialized haying equipment designed to increase drying rates. Such equipment includes tedders, windrow inverters, hay rakes, and conditioning equipment.
  • Using hay preservatives: Hay preservatives, such as liquid propionic acid, applied to the hay during baling inhibit or reduce the growth of bacteria in hay with a high moisture content. 

Another way to reduce the risk of a hay fire is to ensure that stored hay remains dry. 

  • When storing hay inside, make sure the barn or storage area is weathertight and has proper drainage to prevent water from entering the barn. 
  • When storing hay outside, cover the hay with plastic or another type of waterproof material. If you are unable to cover the bales, arrange the bales so that air can circulate between them to promote drying. Bales can be protected from ground moisture by storing them on a bed of gravel or lifting them off the ground on used tires, poles, or pallets.

Monitoring the Temperature of Stored Hay

If you are concerned that hay may have been baled at too high a moisture content, monitor the internal bale temperature twice daily for the first six weeks after baling. For safety reasons, you must work with a partner when checking the temperature of stacked bales. One of you stands atop the bales to measure the internal temperature while the other observes. The person testing the hay should wear a harness and a lifeline that is attached to a secure object. In the event of an emergency, such a system allows the observer to pull the person checking the temperature out of the hay. Due to the potential dangers of this situation, this task should not be assigned to youth workers.

You can use a commercial thermometer to test the temperature of baled hay, but commercial thermometers are not always the appropriate length to monitor the interior zone of baled hay. If a commercial thermometer does not meet your needs, you can fabricate a probe from a 10 ft. length of 3/4 in. iron pipe. Drill eight holes that are 3/16 in. in diameter about 3 in. from one end. Hammer that end of the pipe to form a sharp edge with which to probe. Insert the probe into a hay bale, and use a piece of light wire to lower a thermometer down into the end of the pipe. Alternatively, you may use a piece of 3/8 in. pipe that is 8 to 10 ft. long to test the temperature of hay.

To test the temperature of the hay, place wooden planks or plywood across top of the bales so that the weight of the person standing on the hay is distributed evenly and he or she will be at less risk of falling into a burned-out cavity. Drive a commercial thermometer or a homemade probe into the bale of hay. If you use a fabricated probe, keep the thermometer in the probe for approximately 10 to 15 minutes to obtain the temperature reading. If you use a 3/8 in. pipe, leave the pipe in place for 20 minutes. When you remove the pipe from the hay, if the pipe is too hot to hold in your hand, then you should remove the hot hay.

The following temperature chart outlines further actions that may need to be taken depending on the temperature of the hay.

Critical Temperatures and Action Steps
Temperature (ºF) Condition and Action
125 No action needed.
150 Hay is entering the danger zone. Check temperature twice daily. Disassemble stacked hay bales to promote air circulation to cool the hay.


Hay has reached the danger zone. Check hay temperature every couple of hours. Disassemble stacked hay bales to promote air circulation to cool the hay.
175 Hot spots or fire pockets are likely. Alert fire services to the possible hay fire incident. Stop all air movement around the hay.
190 With the assistance of the fire service, remove hot hay. Be aware that hay could burst into flames.
200 or higher With the assistance of the fire service, remove hot hay. Most likely, a fire will occur. Be aware that hay could burst into flames.

(Source: National Resource, Agriculture, and Engineering Service [NRAES])

Hay Fire Hazards

The following three hazards exist from hay fires: 

  • Flare-Ups: When the internal hay bale temperature is between 150ºF and 170ºF, the potential exists for spontaneous combustion, and the hay should be moved to allow it to cool. If the temperature is at the higher end of the range, moving the hay could expose it to oxygen and cause flare-ups. Contact your local fire department and have charged water hoses available. 
  • Burned-Out Cavities: These cavities form when temperatures deep within stored hay reached high temperature levels and the hay has burned. A person can become trapped in a burned-out cavity if he or she is walking over the top of the hay pile. Due to the risk of a person falling into a burned-out cavity, at least two people should investigate a hay mow.
  • Toxic Gas: Toxic gases such as carbon monoxide can be released by smoldering and burning hay. Chemically treated hay may emit additional toxic gas vapors. A trained fire-rescue worker with a self-contained breathing apparatus (SCBA) should be called to assist at the scene in either situation.

When a Fire Occurs

In the event of a fire, or even when hay is smoldering, contact the fire department immediately. Your next action step and main priority should be to protect human life. Remember that you can replace hay, buildings, and equipment, but you cannot replace human life.

Before taking any action to fight a fire, consider other valuable actions you can take to address the situation prior to the arrival of fire fighters, including the following:

  • Account for all personnel on your farm or ranch operation.
  • Check the area for flammable products. If any are present, immediately leave the area and upon the fire fighters’ arrival, make them aware of the flammable products.
  • Determine whether electricity needs to be turned off in buildings.
  • If the hay fire is located inside a building that houses livestock, consider personal safety before relocating livestock to an area away from the structure.
  • Remove any extra vehicles or machinery from the area around the fire to clear space for the fire service equipment.
  • Stage bale-moving machinery out of the immediate fire area, but have it available to help move bales, as directed by fire fighters.
  • Retrieve material safety data sheets (MSDSs) for any chemical preservatives that may have been used on the hay and that fire fighters will need to review.

Moving hay bales is hot, smoky, and physically demanding work that can cause injuries, exhaustion, smoke inhalation, and heart attacks. Individuals involved at the scene need to be monitored and should receive medical attention should they exhibit signs related to any of these health concerns.

Things to Remember:

  • Most hay fires related to moisture levels occur in the first six weeks after baling.
  • When baling hay, keep moisture levels at 20% or less.
  • Keep baled hay dry by covering it or storing it inside.
  • Monitor internal bale temperature on a regular basis.
    • Youth workers should not be given the task of checking hay temperatures.
  • If you store uncovered bales outside, arrange bales so that air can circulate around them.
  • The use of ventilation changes based on the temperature of the hay. At lower temperatures, increased ventilation around the bales will help the hay return to an acceptable temperature. If hay temperatures reach 175ºF, stop ventilating hay because the increased air flow could feed a fire.
  • Maintain MSDSs for any crop preservatives that may have been used on the hay, and have the MSDS readily available for fire service personnel.


Use the following format to cite this article:

Preventing fires in baled hay and straw. (2012). Farm and Ranch eXtension in Safety and Health (FReSH) Community of Practice. Retrieved from





Gay, S., Grisso, R., Smith, R., & Swisher, J. (2003) Hay fire prevention and control. Virginia Cooperative Extension. Retrieved from….

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Lemus, R. (2009) Hay storage: Dry matter losses and qualify changes. Mississippi State University Extension Service. Retrieved from

Murphy, D. & Arble, W. (2000) Extinguishing fires in silos and hay mows. New York: Natural Resource, Agriculture, and Engineering Service (NRAES).


Reviewers, Contributors, and Summarized by:
Linda M. Fetzer, Pennsylvania State University –
LaMar J. Grafft, East Carolina University
Dave E. Hill, Pennsylvania State University  (Has since retired)
Dennis M. Murphy, Pennsylvania State University – (Has since retired)
Cheryl Skjolaas, University of Wisconsin
Aaron M. Yoder, University of Nebraska Medical Center –