Spontaneous combustion in haystacks topped 500 in NSW last season and here we are again - baling time.

Complex biological, chemical and physical reactions influence heating in hay.

Biological respiration of the plant material and bacterial and fungal activity cause initial heating.

This can lead to a build up of flammable chemicals which can ignite when exposed to air.

Hay that has been baled too moist is the most common cause of overheating.

Other reasons include high levels of sugar in drought-stressed crops leading to increased microbial activity and heating, poor calibration of moisture meters leading to inaccurate moisture readings, and a lack of effective hay conditioning.

In addition to the risk of fire, heating can reduce hay’s nutrient value, causing losses in protein digestibility and energy content.

To minimise the risk of heating it is important to reduce moisture content.

Due to their large volume and surface area, large round and square bales need lower moisture content than small ones.

Small rectangular bales should contain 18 to 20 per cent moisture, round bales 14 to 18pc and large rectangular bales less than 14pc.

Most fires last season were in large rectangular bales stacked more than three bales high.

For cereal hay the node is the last part of the plant to cure.

To check if the nodes are fully cured, take a plant sample with an intact node, place the centre of the node against the tip of your thumbnail with the stem sideways and bend the stem over the thumbnail.

If the stem splits cleanly in half on both sides, your sample is fully cured - a fully cured stem will snap cleanly in half through the middle of the node.

If the stem only half splits, that is, splits on the outside of the bend but not the inside, and is green on the inside the sample is not cured.

If the sample just bends without splitting it is even further from being cured.

When making lucerne, vetch and clover hay, use the thumbnail test for legumes.

Take a sample of the cut crop.Scrape your thumbnail along the stem. If a strip of the stem or the colour peels off, the plant is not cured.

If the thumbnail does not scratch the stem, the sample plant is cured and should be ready for baling.

For more information about baling hay and silage, the Australian Fodder Industry Association offers a good website.