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Minimizing Hay Storage Loss From Heating or Fires

Originally written by Tracey Erickson, former SDSU Extension Dairy Field Specialist. 

Successful hay storage is essential to preserving high quality forage, while ensuring desired performance from livestock and deterring economic losses from unwanted hay storage fires. The predominant reason that fires occur in hay is because of excessive moisture in the plant residue that results in heating when it is baled or stacked for long term storage.

Plant cell respiration burns plant sugars to produce energy. This is a normal process as the hay plant tissue dries down and is often referred to as “sweating” or “heating” and occurs until the plant material is less than 15% moisture. When the plant material has more than 20% moisture it can cause the mesophilic bacteria present to grow rapidly, which is encouraged by the excessive moisture present. This produces heat in the bale. The higher the moisture content the longer it takes for the bale to dry down. Correspondingly, the higher the temperature in the bale’s core will be as it works through the cycle of heating and drying. It is important to note that spontaneous combustion of hay bales can occur at interior bale temperatures of 170° F.

Minimizing Heat Damage & Fire Risk

At Harvest

How do we minimize the risk of heating or damage from fire? It is recommended to put up dry hay at a moisture content of 20% or less as this is when mesophilic bacteria growth is minimized, reducing the risk of overheating.

  • Baling when the weather is appropriate for putting up hay is often challenging. However, to achieve the desired moisture content, one should keep the following in mind. If humidity is high or there is heavy dew, the hay will “pick up” moisture content. Often baling later in the day helps minimize extra moisture from accumulating. However, as night approaches it can also increase the moisture present, so constant checking of the moisture content throughout baling is recommended.
  • Use a moisture tester which will give an automatic reading of the moisture content in the field or conduct a moisture test to determine the moisture content before baling.
  • Use equipment that will enhance quicker dry down of the forage. This includes hay rakes and conditioning equipment, tedders and windrow inverters.
  • Preservatives such as propionic acid that are applied at the time of baling can reduce or inhibit the growth of bacteria reducing the potential for excessive heating.

During Storage

Once the hay has been baled it is best to minimize losses or the potential of enhanced heating especially if the hay has been put up at marginal moisture levels (close to 20% moisture).

  • Storing inside is best to minimize losses from weather. In doing so, make sure it is weather tight and has adequate drainage to inhibit water from entering the building.
  • If storing outside is your option, cover the hay with a waterproof type material. To help moisture absorption from the ground, hay should be stored on a bed of gravel or by lifting them above the ground via tires, poles or pallets. If you are unable to cover them, provide enough room between bales to allow for adequate air flow for drying to continue.

Checking Stored Hay Temperature

If stored hay has been put up at a moisture content higher than recommended it will cause heating to occur. Stored hay should be monitored twice daily for a period of six weeks as it continues to dry down.

Checking of stacked bales should be done in teams of two people, along with a life harness being worn by the person on top of the stack. A life harness is recommended as a burned-out cavity can develop in the hay stack, which a person may fall into as they monitor bale stack temperatures. It is also recommended that planks or plywood be placed to help with weight distribution as a prevention measure for falling into a burned out cavity.

To monitor the temperature of baled hay one can use a commercial thermometer or a home fabricated probe can be used to meet your monitoring needs. A piece of 3/8 inch - 8 to 10 ft. iron pipe with a pointed tip can also be used. If you are using a commercial thermometer probe this should be left in the bale for 10-15 minutes to get an adequate reading. If you are using a home-made probe it should be left in the bales for 20 minutes before removing. The guidelines for using a homemade probe is that after removal, if the probe is too hot to hold in your hand, then you have hay that is too hot and should be removed.

The following temperature chart provides a guidelines for actions in correlation to the temperature of the stored hay.

Table 1. Critical Temperatures and Action Steps

Temperature Condition & Action
125 ° F No action needed
150° F Hay is entering the danger zone. Check temperature twice daily. Disassemble stacked hay bales to promote air circulation to cool the hay.
160° F 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° F Hot spots or fire pockets are likely. Alert fire services to the possible hay fire incident. Stop all air movement around the hay.
190° F With the assistance of the fire service, remove hot hay. Be aware that hay could burst into flames.
200° F 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].

“Hot” Stored Hay Hazards

It is important to be aware of the hazards that can exist when hay becomes “hot” or “heated”.

  • Flare-ups can occur at any time once the hay has reached a temperature that is above the danger zone of 150° F. It should be disassembled and allowed to cool. If the hay bale internal temperature has reached 175° F spontaneous combustion can occur once it is exposed to oxygen, thus it is recommended that fire department personnel be present to help with disassembling the hay pile for cooling and that a charged water source / hose be available to help put out fires if they occur.
  • Burned-out cavities can be extremely dangerous and may be present in hay if it reaches a temperature that is conducive to fire. A person may become trapped in a cavity as they are walking over the pile thus, it is recommended to wear a life line with a second person present and to also use boards for weight distribution on the top of a pile.
  • Toxic gases such as carbon monoxide can be present if there is smoldering or burning hay. Hay that has been chemically treated may also emit toxic gas vapors as it burns. This should be communicated to all fire-rescue workers so that appropriate breathing apparatus gear can be worn.

Appearance & Quality

Hay with a brown, carmelized color.
Figure 1. Caramelized or heated hay. Credit: Tracey Erickson

Hay that has heated during storage will often appear brown or caramelized in color (Figure 1). Livestock will often like the caramelized flavor however, nutrients have been lost due to the excessive heating during storage. It is recommended to obtain a feed nutrient analysis prior to diet formulation to determine the quality of the forage.

We are often at the mercy of the weather when putting up hay under ideal moisture conditions. Therefore constant monitoring of hay moisture during baling and the temperature at storage time is essential to having high quality forages available for feeding livestock and for minimization of storage losses.