File photo. Fire and Emergency is urging farmers to make sure hay is dry before baling and stacking - and to check later if temperatures are building up in the bales. Photo: RNZ / Cosmo Kentish-Barnes
Farmers are being urged to ensure their hay is completely dry before it's baled and stored over the holiday period.
Fire and Emergency national wildfire manager Tim Mitchell said some farms have had a very wet few weeks and conditions were not great for hay baling.
He said hay must be completely dry before its baled, stacked and stored for use as winter feed.
"There's little bacteria that live or are within that grass or on that straw and if that moisture is still there when it gets baled, they continue to work, and during that process that builds up heat within the hay bale itself.
"If that heat can't escape - which obviously if it's compressed into a bale it's really difficult - and then if you stack all those bales together it makes it even harder.
"What can happen is that the heat can build up to a point where it reaches ignition temperature and then it spontaneously combusts.
"Then not only do you lose the hay but, depending on where you've stored it, you could lose the hay, you could lose the hay barn, the shelter belt, the other things around it as well."
Mitchell said every year Fire and Emergency was called out to fires started by hay bales which spontaneously combusted.
He said most hay bale fires started between two and seven weeks after storage and farmers should be checking their bales during that time.
"Use a steel rod to check the hay is not overheating," he said.
"Hay should be stacked loosely to improve airflow and keep the bales cool.
"Ideally store them away from things that could feed a fire after it starts - such as sheds, hedges and trees."
Mitchell said there were also warning signs to keep an eye out for if hay is stored in a shed, such as steam condensation on the roof, mould growth on or inside bales, and acrid fumes or hot humid air at the top of the stack.
"It's better not to get to that point," he said.
"If everyone takes the extra time to dry the hay out properly, hopefully we'll see far fewer haystack fires this summer."
Mitchell urged farmers to avoid the pressure to get everything done before Christmas if the conditions were not right.
"I know it's tempting at this time of year - we all want to get things squared away before Christmas," he said.
"This can put pressure on people to get the work done quickly and miss some crucial steps.
"But getting bales into the shed before they're completely dry is a recipe for all the bales going up in flames."