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Fire Hazard in Wet Bales

Written collaboratively by Alvaro Garcia, former SDSU Extension Agriculture & Natural Resources Program Director, and Fernando Díaz.

Baled stored hay can get wet during spring as a result of melting snow or rainwater. Stored forages with high sugar content (i.e. hay baled at high quality-early vegetative stage) usually have higher nutritive value. At the same time, they could be at greater risk of achieving higher temperatures upon exposure to air particularly when they are not stored at the right moisture. As spring progresses, higher ambient temperatures together with air infiltration favor the growth of bacteria and molds that further build-up heat. If air continues to penetrate the mass, oxidation by these microorganisms may continue until spontaneous combustion occurs. For this reason, bales stored with excessive moisture are also more susceptible to heating as they constitute and ideal substrate for microorganisms.

Heat Production

Ideal growth conditions for mesophile bacteria are achieved when hay reaches approximately 68°F. These bacteria continue to proliferate and generate heat until inhibited at approximately 113°F. If the cycle stopped at that point there would be no risks of heat damage and/or fire hazard. The problem is that other microorganisms can take over at those higher temperatures and continue to generate heat. Thermophile bacteria, frequent in moldy hay, start growing at these moderate temperatures, reaching their optimum growth between 131 and 149°F. Thermophilic actinomycetes (i.e. Saccharopolyspora rectivirgula) is one of the bacterial group most common in moldy hay. The optimal growth temperature for S. rectivirgula is 130°F, but some strains are able to grow at elevated temperatures (145 °F). In addition, it is one of the major agents responsible for farmer’s lung disease, a form of hypersensitivity pneumonitis. Although a third group of bacteria labeled “hyperthermophiles” (growth temperatures of 176 to 212°F) has been described in certain extreme environments, the author is unaware of them having been isolated in stored forages

If air infiltrates the baled forage not only bacteria generate heat but also heat-resistant molds. Although the fungi causing heat in hay are considered aerobes, many are capable of growth at low oxygen concentrations.

In hay at or below 18 percent moisture, bacteria will have trouble proliferating but molds will not. In general, common storage fungi (Aspergillus, Mucor and Absidia) growth in stored hay occurs at 50 – 104°F with optimal conditions between 77 – 95°F. However, some temperature tolerant fungi (i.e. Emericella nidulans and Aspergillus fumigatus) can grow in high temperature environments, of up to 122°F and 140°F, respectively. Mold growth can be recognized initially by the presence of the mycelium (white growth in the bale), and their spores (whitish dust) once they arrive to the reproductive stage. Mold proliferation further decreases the nutritive value of the forage by using up the remaining sugars and even some structural carbohydrates. In addition, some are capable of producing toxins that are deleterious to livestock.

Oxygen Infiltration

Moisture concentration at baling is the most important factor affecting spontaneous heating, but it can also happen however in hay that gets wet during the next season. Hay that was baled under optimum conditions the previous year, is at greater risk when water activity increases in the spring, since most storage sugars responsible for heat production had been spared. Baling alfalfa at higher moisture contents (greater than 15%) reduces leaf losses during mechanical handling and field curing time. These two advantages can be an incentive for farmers to harvest hay at higher moisture concentrations.

Air presence though is the key component for heating to occur. Bale temperatures above 160°F can stimulate heat generating oxidative reactions that further increase temperatures. If there is enough oxygen under these conditions spontaneous combustion may occur. Normally, spontaneous combustion is generated near the outside of the bale or haystack because oxygen concentration is higher.

Research conducted at Kansas State University to investigate changes in temperature during bale storage showed that hay baled with the greatest moisture concentration exhibited more intense and prolonged heating than drier hay (Table 1). The researchers also reported that the pattern of spontaneous heating for bermudagrass was similar to alfalfa hay. During storage, bales continue to lose moisture until they reach 12-15%, and the final moisture concentration will depend on weather conditions and storage structure.

Table 1. Heating of Bermudagrass hay made at five concentrations of moisture.

Moisture (%)
Storage Temperature
  Maximum Minimum 30-day Average 60-day average
32.5 143.2 88.5 113.9 103.6
28.7 139.1 88 113.5 103.5
24.8 129.6 86.5 108.1 100.2
20.8 110.3 86.2 99.9 95.7
17.8 104.4 86 96.6 93.7

Source: Coblentz et al. (2000).

Heating Factors

Factors that contribute to increased heating include:

  • Bale density: Increased bale density increases spontaneous heating due to larger quantity of material in each bale and reduced heat dissipation. Bale density is affected by the baling machine, the experience of the operator, and the hay type.
  • Bale size: the capacity of the forage to dissipate heat is associated with the surface-core distance, both for an individual bale and a stack of bales. In addition, large bales generally have higher densities. Large round or rectangular bales are at greater risk of maintaining high core temperatures than small rectangular bales. Regrettably this does not mean that small rectangular bales are not at risk of excessive heating as this will depend of the heat developed in the center of the stack.
  • Moisture uniformity within bales: wide swaths lead to more uniform drying, which can reduce the presence of wet or green spots in the windrows at baling. The interface between dry and wet hay is an ideal area for spontaneous combustion to occur.
  • Use of additives/preservatives: The two most commonly used preservatives for wet hay are organic acids and bacterial inoculants. Organic acids, mainly propionic (pure or buffered as salt) limit temperature build-up in wet hay by inhibiting the development of molds, yeasts and bacteria. Their effectiveness depends upon application rate and moisture content of the hay. Purdue University research tested propionic acid addition to hay with 32% moisture at baling and after 5 days of storage. Maximum storage temperatures were 124, 127, 115, 104 and 84 °F for propionic acid concentrations of 0.02, 0.2, 0.5 and 1%, respectively. Propionic acid application rates recommended for controlling heat production in large bales generally range from 20 to 40 pounds/ton of wet forage. Due to high inclusion rates and cost, several researchers conclude that acid preservatives may not be economical unless used to avoid rain-damaged hay. In general, bacterial inoculants developed to improve the fermentation of silage (i.e. lactobacillus) or specifically for hay (i.e. bacillus) have been ineffective reducing storage temperatures in wet hay.
  • Weather conditions: Ambient temperatures also play a role as they can allow for heat dissipation from stored hay (cold weather) or compound the problem (warm weather). Although this is confounded by the fact that it can be hay that was harvested during the spring-summer of that year and didn’t go through the cooling-off period of at least one cold season. Heat build-up in stored forages depends on ambient temperature, relative humidity, and air movement.
  • Storage site: During storage, bales continue to lose moisture until they reach 12 - 15%. The final moisture concentration will remain relatively constant unless water is absorbed from the ground, rain, or humid air. Storage structures need to be well ventilated to allow moisture to escape.

Fire Prevention Tips

Key points to remember to prevent fire hazard in hay:

  • Bale hay at the proper moisture content. Make sure moisture is right before baling; avoid overcast, high dew point days. Windy, dry days greatly speed-up drying; on the other hand, these days are when more leaf losses occur. Make sure that moisture content is under 20 and 17 percent for small and large bales, respectively. The maximum moisture content at baling can be increased to 23 percent when the hay is treated with an effective preservative.
  • If moisture content is slightly higher do not store bales until it drops to that moisture to avoid temperature build-up in the core of the stack.
  • Kept the bales under roof or under a tarp. For higher heat dissipation stack the bales with an air gap between loads.
  • Store hay on well-drained surfaces, and raise the bales to prevent direct contact with soil moisture or pooled water.
  • Do not store high-moisture baled hay with dry hay or straw.
  • Monitor temperature during storage (Table 2).

Table 2. Suggested action steps for heating wet hay.

Temperature (°F) Action Additional measures
150 Monitor temperatures daily None.
160 Monitor temperatures every 4 hours Separate bales if temperatures continue to rise.
175 Call the fire department Wet the hay. Separate bales from buildings and other dry hay.
185 Call the fire department Wet the hay. Do not move separate bales as air can start the fire!
212 Call the fire department Fire is imminent. Wet the hay. Do not move or separate bales as air can start the fire!

Source: Modified from Woodward. (2004).

More in depth information about this article can be found at the ResearchGate website.