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Heating can be due to excess surface moisture (rain, etc) being trapped inside at baling, or plants being too green (internal moisture) at baling, or a combination of both.
Initial heating occurs because living processes continue for some time after plants are cut. The heat may dissipate within days, depending on the drying or storage conditions, or may continue to rise over a period of weeks.
The degree of heating reached depends on the type of crop, its maturity and moisture content, the density, volume and storage system of the feed, the bacteria and fungi present and the external conditions of atmospheric temperature, humidity and wind.
To minimise the likelihood and danger of hay heating, it should be baled at specific moisture contents.
Due to their large volume-to-surface area, round bales should be baled at a lower moisture content than small rectangular bales. Large rectangular bales should be even drier.
Hay with a moisture content slightly above the suggested levels will usually heat to some extent, then eventually cool to the ambient temperature.
The temperature will rise substantially if hay is baled with moisture contents well above (>3%) the suggested level, particularly the large rectangular bales.
''Dry'' hay that subsequently becomes moist with dew or rain then is stacked into tight confines such as a hay shed before drying will often also heat.
How much will depend on the factors mentioned earlier.
If in doubt, stack the hay bales loosely to allow heat to dissipate.
When hay is stored in a shed, there are often signs that it is heating significantly.
Steam condensation on the steel roof, mould growth on and inside bales, acrid fumes and hot humid air at the top of the stack are indicators of heat generation inside the stack.
Most fires resulting from spontaneous combustion start between two and seven weeks after storage.
For further information, please contact the Southern Rural Fire Authority on 0800-732-732.
Information for this article was obtained from State of Victoria, Department of Primary Industries, Agriculture Notes, February 2006.
• Sally Chesterfield is the Southern Rural Fire Authority's community and education co-ordinator.
Suggested moisture contents (%) for safe storage of various forms of hay:
Moisture content range
• Small rectangular bales: 18-20%
• Round bales: 14-8%
• Large rectangular bales: Under 14%
Guide to judging moisture from the appearance and handling characteristics of hay:
• 50-60%: Little or no surface moisture, leaves are limp, juice shows on stems or leaves if rubbed or pressed hard.
• 40-50%: No surface moisture, parts of leaves are brittle, moisture may be seen in stems twisted in a bundle, but the hay is still tough.
• 30-40%: Leaves begin to rustle, no show of moisture unless rubbed very hard. Moisture shows in stems scratched with a fingernail or less easily, when twisted in the hands.
• 25-30%: The hay rustles, a bundle twisted in the hands snaps with difficulty, it shows little sign of moistness. Thick stems may show moisture if scraped or split open with a fingernail.
• 20-25%: The hay rustles readily, stems snap when twisted, leaves may shatter. There are few moist stems, nodes or joints are shrivelling. The bark on stems cannot be raised with a fingernail.
• 15-20%: The hay fractures easily. Bundles snap easily when twisted. It is difficult to see any moisture and leaves shatter readily.