# Haystack Fires in Australia: Causes and Considerations for Preventative Management
## 3 Factors Responsible for the Spontaneous Heating of Haystacks
### Moisture Content  
Moisture content during baling is the most important factor that influences excessive heating, resulting in spontaneous ignition. Hay that is baled above the recommended levels of moisture content is susceptible to spontaneous heating where plant sugars are respired into carbon dioxide, water and heat [26]. According to [PERSON] et al. [29], increased bale moisture content usually results in increased bale temperature in storage.  
Generally, hay from the current season, even under optimum conditions, is at greater risk as water activity increases during spring since most storage sugars responsible for heat production have been left unused. Baling lucerne (_Medicago sativa_) at higher moisture contents (>15%) reduces leaf losses during mechanical handling and field curing time; however, moisture content should be below this level for storage. The upper limit of moisture for large round lucerne bales is about 18-20%. Hay baled at greater than 20% moisture content will usually increase storage losses by over 24% [30] as a consequence of the oxidative reaction of spoilage microbes. Table 1 shows the safe moisture contents for storage of various bale types.  
[PERSON] et al. [22] showed that hay baled with the highest moisture concentration exhibited more intense and prolonged heating than drier hay (Table 2). Interestingly the material baled had no influence on this; the pattern of spontaneous heating for bermudagrass was similar to lucerne hay. Bales continued to lose moisture until they reached 12-15% during storage. The final moisture concentration depended upon the environmental and storage conditions.  
The interface between dry and wet hay is ideal for spontaneous combustion due to the presence of optimal heat, dampness and insulation. The green or high moisture region within a bale or stack expands as the water produced through plant and microbial respiration becomes trapped in this area and gradually spreads to the surrounding vicinity. However, combustion can be prevented if the generated heat dissipates anyway.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Bale Type** & **Moisture Content (\%)** \\ \hline Small rectangular bales & 16–18 \\ Round bales (Soft centre) & 14–16 \\ Round bales (Hard centre) & 13–15 \\ Large rectangular bales & 12–14 \\ Export hay & \textless{}12 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Recommended moisture contents (%) for safe storage of various bale types.  
Higher moisture content during storage is highly conducive to rapid microbial growth and development, which in turn results in increased respiration [31] followed by heat generation.  
To better understand the magnitude and duration of heating in hay bales, the heating degree day (HDD) concept is often used as a response variable in hay preservation studies. An HDD \(>\) 30 \({}^{\circ}\)C represents the summations of the daily increment by which the internal bale temperature was greater than 30 \({}^{\circ}\)C [12]. Thus, total HDD during storage serves as a single numeric response variable that integrates both the intensity and duration of spontaneous heating within each bale. Figure 2 represents data from several lucerne hay experiments revealing a positive linear relationship between moisture content and HDD (\(t^{2}\) = 0.902) for lucerne-orchardgrass hays packaged in 0.90 and 1.20 m diameter round bales [10]. It is evident that moisture content is the primary variable causing spontaneous heating within any given bale. Later, when bale diameter was increased to 1.50 m, the positive linear relationship between moisture content and HDD remained consistent, but the HDD response was accelerated with per unit of moisture content [12].