CDQAP Ruminations: Preventing and Controlling Hay Fires on Dairies

Highlights: What you need to know about preventing hay fires on California dairies…

  • Every year California producers experience significant losses due to hay fires, most often caused by spontaneous combustion during the first six weeks following baling. Hay older than six weeks is also at risk if it has been damaged by rain or flood waters.
  • To prevent fires, as well as avoid losses of yield and quality, typical recommendations include baling conventional three-tie bales at no more than 17% moisture and large (½ – 1 ton) bales at no more than 14% moisture.
  • Hay stacks at risk of fire, including stacks less than six weeks old, those with a high or uncertain moisture content, or those damaged by rain or flood waters, can have core temperatures monitored using simple, inexpensive tools.
  • Hay stacks reaching 130°F should be monitored frequently until temperatures decline, stacks reaching 150°F should be disassembled to cool and those reaching 160°F should be disassembled with the fire service present.
  • In order to be reimbursed for the full value of the damaged hay, it is important to be in compliance with the dairy’s insurance policy requirements.

This article summarizes guidance from a variety of farm extension, fire service and insurance references. Such published information should not replace specific fire prevention and nutritional advice available from a producer’s local extension farm advisor, nutritionist and fire service.

Causes of Hay Fires

Fires can occur in any type of hay storage; in small, large or round bales, stored inside or outside. Hay fires have a wide variety of causes, including electrical shorts, malfunctioning harvesting equipment, cigarettes discarded from employees or nearby road traffic, brush fires and even arson. By far however the most common cause of hay stack fires is spontaneous combustion in hay harvested, bailed and stacked at too high a moisture. Hay fires from spontaneous combustion occur most often within six weeks of harvest but have been reported to occur within several days of baling to more than a year later. Hay bales older than six weeks are more prone to spontaneous combustion if they have become wet from rain or flooding.

Hay Bale Heating and Spontaneous Combustion

Most hay bales containing at least 15% moisture will undergo a period of heating (curing or “sweating”) immediately after bailing. This initially occurs as some of the remaining plant sugars are metabolized, releasing heat energy. At internal bale temperatures below about 110°F to 115°F this heating is usually of little consequence for either forage quality or fire risk.

Hay that is bailed at greater than about 20% moisture however starts to create an environment allowing significant bacterial and fungal growth which generates additional heat. While this microbial growth decreases hay quality, it typically doesn’t substantively elevate fire risk until the bale temperature reaches 130°F.

A single bale in the open rarely heats sufficiently to catch fire. When hay is stacked however, the center bales are insulated, creating the potential for significant increases in temperature. Depending on forage moisture and stack size and density, bales at between 130°F and 150°F may either eventually cool by themselves or continue to experience an explosion of microbial growth. Such a rapid increase microbial fermentation can quickly bring temperatures in the core bales to 170°F to 180°F, the “danger zone” where spontaneous combustion is imminent.

Hay Moisture and Hay Quality   

Aside from preventing spontaneous combustion, hay must be baled within a narrow range of moisture to avoid losses of yield and quality. Producers typically avoid bailing alfalfa below 12% moisture to limit excessive leaf shatter and loss. Conversely, studies predict dry matter loses of one percent for each one percent elevation in moisture above 10%. Bales stored at moisture content of greater than 20% can experience excessive microbial growth and its associated heating. Moist bales experiencing temperatures of approximately 110°F to 125°F are subject to mold, discoloration (“browning”), decreased digestibility, unpleasant odors and reduced palatability. A discussion of hay production practices is beyond the scope of this article, but the University of California – Davis Alfalfa Working Group has exhaustive print and website references. In general, UCD recommends that small two-tie bales be harvested at no more than 20% moisture, larger three-tie bales baled at no more than 17% moisture and very large bales (those weighing between ½ and 1 ton) baled at no more than 14% moisture. The comprehensive UCD reference, Harvesting, Curing and Preservation of Alfalfa, addresses hay moisture management including harvest and storage strategies, economics and mechanical treatments and chemical preservatives. Spontaneous heating temperature profiles for grass hay appear similar to alfalfa.

Prevention of Hay Stack Fires

Hay fire prevention and mitigation strategies relate primarily to:

  1. Ensuring hay is bailed at safe moisture and protected from later water damage.
  2. Monitoring stack temperatures of hay which is at risk of spontaneous combustion.
  3. Preventing exposure to sources of external ignition.
  4. Limiting the risk of spread if a fire does occur.

Purchasing or Harvesting Hay       

Regardless of whether a producer purchases some, none, or of all of their hay, there are precautions which help limit the risk of hay fires.

Know the supplier, the hay’s history and age Although short supplies sometimes dictate otherwise, ideally it’s best to have a relationship with a trusted supplier whose harvest and storage practices are known, consistent and reliable. Calls offering one time “cheap loads” from unknown brokers should be avoided. It’s also important to know when the hay was harvested, since hay fires most often occur during the first six weeks after baling. Finally, it’s useful to know the weather conditions under which the hay was harvested and stored.

Know the hay’s moisture An accurate measurement of the hay’s current moisture content is essential. Moisture content of bailed hay will be included as part of a routine nutritional analysis. If a current moisture content is unavailable or in question, producers can use handheld moisture meters to estimate a stack’s average moisture content. Good quality moisture meters are commercially available for around $200 to $300. Because bale moisture can vary from field to field (or even within a field) it’s important to measure at least several bales from each stack. Measurements taken by moisture meters are not as accurate as those provided by the laboratory but they have the significant advantage of providing real-time information in the field for hay that has either been cut and is drying or hay that has already been baled.

Delay final storage of at-risk hay If hay is bailed at a high moisture a producer may consider a delaying its removal from the field and consolidation into larger stacks or barns. Hay baled at more than 22% moisture for instance should probably not be put into final storage for 4 to 6 weeks. Hay venders may agree to leave large orders stored in their own fields until the producer is ready take possession. How this order is insured should be clear to both parties.

Monitoring Hay Stack Temperature    

Once hay has been bailed, the forage moisture content is useful only to inform producers of an increased risk of spontaneous combustion. Measuring stack temperatures, on the other hand, can both alert farmers of a developing critical situation as well as whether precautionary steps need be taken.

Producers should consider monitoring of the temperature of stacks when:

  • Hay was bailed less than six weeks previously.
  • Hay that was bailed at a moisture content higher than 15% to 18%.
  • Hay with a history and moisture content that is unavailable or unreliable.

One strategy for high-risk hay is to measure stack temperature periodically for the first week after baling, looking for a trend of either rising or falling temperature. As core stack temperatures enter the 130°F – 130°F range, the frequency of monitoring will increase, up to several times a day.

Measuring Hay Stack Temperature: Equipment

A number of hay or compost thermometers are commercially available. Accuracy within 5°F is sufficient, but more importantly any thermometer used should be capable of reading up to at least 200°F. Long stem dial thermometers, also sold as “compost thermometers” are reliable but if forced into a bale without a protective probe tend to bend or crimp, severely affecting accuracy. In addition, such thermometers typically are not long enough to reach into the center of a stack.

For stack measurements the most common recommendation is to fashion a hollow probe from a ½ or ¾ inch steel pipe or electrical conduit. Make sure the pipe is large enough to easily accommodate the intended thermometer. The probe should be at least 8 to 10 feet long with one end hammered flat creating a cutting surface. Alternatively, the probe end can be fitted with a machined point. Six to ten ¼ to ½ inch holes are dilled into the sharp end. The probe can be hammered into the stack and an electric or glass thermometer lowered to the bottom with a string or pushed in with a wire. If an armored glass thermometer is used, it should be spirit-filled rather than one using mercury, in case of breakage.  An outstanding description of commercial and homemade hay thermometers is Measuring Hay Temperatures, available from the University of Kentucky.

Measuring Hay Stack Temperature: Procedure and Safety          

Temperature probes should reach deep into the stack. For typical stacks on western dairies this will require forcing a probe 8 to 10 feet towards the stack’s center from either the stack’s top or side. Ideally several locations and depths are probed to locate the warmest area. The thermometer can be lowered to the bottom of the probe with a string or pushed in with a wire. Glass thermometers are read after 10 to 15 minutes; electric thermometers are read after they quit climbing. If the probe is too hot to hold comfortably after removal it is likely the stack has already reached 130°F to 150°F will need to be disassembled. Measuring from the side of the stack is inherently safer then climbing on top, which may collapse if there are smoldering cavities within the stack. If circumstances demand accessing the stack from the top, measurements should be made by a team of at least two adult workers, one person on top fitted with a safety harness linked by a lifeline to the assistant. Plywood or ladders can also be placed on the stack top creating a platform to further protect against falling into cavities or crevasses. An excellent, illustrated description of how to safely measure temperatures in hay stack cores is Control and Prevention of Hay Fires, available from Kansas State University.

Temperature Thresholds and Interventions

The table below lists ranges of internal hay stack temperatures and their corresponding interpretation and recommended actions. Repeated measurements are more useful then single measurements, since they reveal trends of either rising or falling temperatures.


Temperature Interpretation and Action*
110 – 130 °F Hay may feel warm to touch and may be associated some loss of forage quality. There is minimal increased risk of fire, but bales can be periodically checked.
130 – 150 °F Stacks may continue to heat to dangerous levels or may cool by themselves. Monitor internal stack temperatures frequently until they fall.
150 – 160 °F Without intervention, stack temperatures are likely to rise. Disassemble hot stacks to allow them to cool. Move bails away from buildings and equipment.
160 – 200 °F High risk of fire. At about 175°F fire may already exist and flare up when bales are moved. Have the local fire service present to help coordinate stack disassembly.

*This table represents composite information collected from the National Resource Agricultural and Engineering Service (NRAES) and extension bulletins from Kentucky, Minnesota, New York, Ohio and Virginia. These recommendations should not replace real-time, on-site guidance from a producer’s farm advisor, nutritionist and fire service.

The take-home message from the table above is that hay stacks that reach 130°F should be monitored frequently for increasing temperature, stacks that reach 150°F should be disassembled to cool and those reaching 160°F should be disassembled with the fire service present. Under no circumstance should workers climb on top of stacks to measure temperatures if flames are present or smoke is seen or smelled.

Hay Storage (General Safety) 

Thoughtful placement and management of hay stacks can prevent some hay fires but also limit involvement of other stacks, equipment and structures if a fire does occur. If possible:

  • Situate hay stacks away from ignition sources such as roadsides, workshops and power lines.
  • Situate hay stacks at a safe distance from structures such as barns and employee residences.
  • Stack size and spacing should adhere to insurance company requirements. The State Fire Code requires a clear distance of 20 feet between stacks but an insurance policy may require more spacing. See the Regulatory Compliance and Recovery and Insurance sections below.
  • Do not routinely park or store vehicles, fuel, machinery, tractor implements or other equipment close to stacks.
  • Establish and maintain fuel breaks around stack storage area that is clear of vegetation. The wider the break the more effective it will be containing a fire. One rule-of-thumb is that a fuel break should be at least as wide as the stack is tall.
  • Perform mechanical weed control (mowing deck, fail mower) prior to establishing stacks to prevent sparks from blades or chains starting a fire.
  • For ground prone to saturation, concrete or gravel storage pads can limit water from wicking up into the stacks.
  • If wet weather is likely during hay storage, protect open-air stacks with tarps and situate stacks in areas that will not flood.
  • For hay barns: limit wetting of hay by addressing leaky roofs and spouts and ponding from runoff.

Hay Storage (Regulatory Compliance)       

The California Fire Code (CFC, in the California Code of Regulations, Chapter 37) has requirements applicable to stacks of combustible fiber agricultural products such as hay, straw and cotton. Aside from simply meeting regulatory compliance, limiting the size of stacks and their proximity to one another will reduce losses should a fire occur. Regulations in the State Fire Code relative to hay stacks include:

  • Individual stack size is limited to 100 tons (200,000 pounds), equivalent to 2,000 100-lb, three-string square bales or 200 large one-ton square bales.
  • Stacks will be separated from one another by at least 20 feet of clear space.
  • Stacks will be separated from structures by a clear horizontal distance equal to at least the height of the stack.
  • State regulations do not require a permit for hay storage, but there may be additional applicable local laws or regulations.

Employee Awareness and Monitoring of Stacks   

Besides monitoring the temperature of new, freshly bailed hay, dairy employees should be aware of the signs indicating that stacks are heating to dangerous levels. These signs include:

  • Mold growth in or on bales, seen when they are broken apart to be fed.
  • Brown or caramelized “burned” spots in or on bales, seen when they are fed.
  • Hard, sharp, glass-like concretions of melted silica and minerals, called “clickers”, seen when bales are broken apart to be fed.
  • An unusual odor, described as a musty, burning, pipe tobacco or composting smell.
  • Steam or smoke rising from haystacks.
  • Slumping or collapsing of sections of the haystack.

Dairy employees should be trained and encouraged to report any of these warning signs. Signage reminding them of what to look and smell for, as well as notices of a strict ban on smoking, can be posted in hay storage areas.

Responding to Hay Fires

In spite of taking proper precautions producers may still be required to respond to hay stacks on fire. The following are actions producers can take to assist firefighters.

  • At the first indication of fire (smoke or flames) call 911. Depending on how rural the dairy’s location is, response from the local fire service may take 10 to 30 minutes.
  • Account for all dairy employees and organize their response. Do not allow employees to move burning or smoldering bales until the fire service is present.
  • If there are flammable or hazardous materials close to the fire (fuel, herbicides, pesticides), evacuate the area.
  • Move any threatened vehicles and equipment away from the fire.
  • Determine what, if any, danger exists for livestock. If needed (and if it is safe for employees) organize a calm, orderly animal relocation to a safe area of the dairy.
  • Firefighters rarely have experience moving livestock. If circumstances require firefighter involvement, the producer’s direction and leadership is usually in order.
  • Clear a path for emergency vehicles by ensuring there are no gates, vehicles or equipment blocking roads leading to the involved area.
  • For hay stored outside: If needed create a fire break by disking or mowing a 15-foot wide perimeter around the stack. If a water supply and hose is available, wet the surrounding vegetation.
  • For hay stored in a barn: Turn off electricity to the building. If needed and a water supply and hose is available, wet the roof of adjacent structures downwind of the fire.
  • When fire service arrives, guide them to the fire. Fire engines are heavy; keep them on roads during wet weather or they can easily become mired in soft ground.
  • Inform the Fire Service Incident Commander of the following:
    • If all employees are safe and accounted for.
    • If livestock are in need of protection or rescue.
    • If the hay was treated with preservatives. Some preservatives release toxic gases during combustion which can dictate fire tactics and respiratory protection. Retrieve Material Safety Data Sheets (MSDS) for preservatives.
    • What water sources are available on the dairy.
    • What heavy equipment, particularly hay squeezes and front-end loaders, can be made available to assist in stack disassembly.
  • Firefighters typically have no experience in moving hay stacks, so using farm employees may expedite disassembly of burning or smoldering stacks. Those employees will need to be monitored and protected, so coordinate with the incident commander.

The primary functions of the responding units are to ensure human safety and prevent the fire from spreading to other stacks, equipment, structures and properties. With people safe and the fire contained, firefighters will wrestle with how to best extinguish the burning stacks. For safety, resource and water conservation reasons, the fire service may want to allow large stacks to burn to a more manageable size. This plan may be opposed by local air quality regulators who wish to have the fire extinguished as quickly as possible. Extinguishing a large fire may require more than a million gallons of water, so local water quality regulators may also be concerned about runoff containing partially combusted materials and fire retardants.

Recovery and Insurance Issues       

Contact your insurance company as soon as possible and keep them advised of the fire’s status. Your company representative may advise the fire service of actions which could jeopardize reimbursement. Record photos or video of damaged feed, equipment and shelters for insurance purposes. Collect receipts and records of expenditures related to the fire.

Hay that has been damaged by flame, smoke or water should be considered a complete loss. Under no circumstances should hay exposed to water containing fire retardant, which contains detergents and nitrate, be fed to livestock. Damaged hay could potentially be used for erosion control. Hay heated sufficiently to change color from green to brown indicates a chemical reaction has occurred, fusing plant sugars and amino acids to fiber creating an indigestible compound. Virginia State University recommends that the degree of heat damage can be quantified by sampling for acid detergent insoluble nitrogen (ADIN).

The fire service and insurance company may conduct investigations to determine the cause of the fire and potential contributory negligence. Insurance companies typically have monetary “stack limits” paid out per stack. If the size, and thus the value, of the stack exceeds the stack limit, the difference will not be covered by the company. This is why it’s important to implement farm policies which ensure that all insurance policy requirements, including stack size and spacing, are observed. Your insurance carrier for instance may for instance recommend or require stack separation of 100 feet compared to the 20 feet of separation required by the State Fire Code.

Preventing Other Types of Feed Fires       

Besides hay in stacks there are other types of feed which may ignite.

Bailer Fires

Not only hay, but bailers and tractors can burn as well. Bailer fires often result from friction “hot spots” created by inadequately maintained equipment, such as worn bearings on conditioning rollers or loose belts. An extension bulletin, Preventing Baler Fire is available from Ohio State University.

Silage Fires

Just as hay can heat up when it is too wet, so can silage spontaneously combust when it is too dry and aerated. Upright silos are not the norm in California, but silage piles can also ignite. While not as dangerous to fight as upright silo fires, extinguishing horizontal silage fires in trenches or pits can be similarly prolonged and frustrating. There are extension publications for farmers and technical references for fire fighters focused on silo fires that also contain useful information for addressing silage fires in piles or pits.