Q: A friend recently told me that stored hay can self-combust. I’d never heard of this before. I keep my hay in my horses’ barn, above the stalls, and now I’m terrified it’s going to catch fire! Should I be concerned, and how can I store hay and keep my horses safe?

A: It is, indeed, true that under certain conditions, hay can self-combust. In fact, it’s a major cause of barn fires along with electrical malfunction, poor housekeeping, and careless work habits.

The No. 1 cause of hay self-combustion is that it was put-up (stored) with too high of a moisture content. Whether you should be concerned will depend on the conditions under which your hay was baled.

Hay shouldn’t be baled and placed into storage with a moisture content more than 20%. Most fires where hay self-combusts occur within two to six weeks of storage where bale moisture content exceeds this level.

When grass is cut for hay, the cells within the grass continue to respire—a process that produces oxygen and some heat. If hay is given adequate time to dry in the field and baled with low enough moisture content, this process will stop. However, it will continue after baling and in storage in bales with a higher moisture content.

Baling hay with a moisture content more than 20% creates an ideal environment for mesophilic bacteria growth. These bacteria thrive in warm environments and can release more heat within the bale. This can cause the internal temperature of the bale to rise to 130ºF to 140oF, at which point the mesophilic bacteria die off. This itself isn’t enough to cause combustion, although it will negatively impact the nutritional value of some more heat-sensitive nutrients. The greater concern though is that this raised temperature creates a breeding ground for the heat-loving thermophilic bacteria. These bacteria can cause a further temperature increase to over 170oF, which can lead to self-combustion.

While storing hay in smaller stacks with good air flow will help reduce combustion risk, the best option is to ensure that hay is baled with less than 20% moisture. To help achieve this, growers should put up hay when there’s a slight wind and a humidity of 50% or less. Baling later in the day when there’s less dew will also help.

Many hay growers use equipment designed to increase air movement through the hay while it’s drying to help decrease moisture content. Hay preservatives such as proprionic acid can also be used to help reduce bacterial growth on high-moisture hay.


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