According to Bob Schultheis, natural resource engineering specialist, University of Missouri Extension, proper harvesting and storage practices will reduce the possibility of hay fires and reduce the associated costs.

Schultheis says that hay fires usually occur within six weeks of baling because the most common cause is excessive moisture.

“You can reduce fire and mold risk by baling small square bales at 18 to 22 percent moisture content and large round bales at 14 to 18 percent moisture content. Higher moisture levels increase microbial activity and also results in loss of dry matter and usable protein, which can reduce the feeding value of the hay by as much as one-third," said Schultheis.

Heating in hay bales will occur to some extent in all forages over 15 percent moisture content, with a peak in temperature three to seven days after baling.

"It takes 15 to 60 days for the hay temperature to decline to non‑damaging levels, depending on outdoor humidity, density of the bales and amount of rain the bales soak up. The longer it takes for the hay temperature to decline, the more damage is done to the hay," said Schultheis.

New hay that is stacked in the field or placed in a barn should be checked at least twice a day for abnormal heating. If storing hay inside, be sure the barn roof and plumbing does not leak, and that surface water cannot run into the barn.

If the hay temperature reaches 130 degrees Fahrenheit, move the hay to allow increased air circulation and cooling. If the temperature climbs above 150 to 175 degrees, call the fire department and be prepared to inject water to cool hot spots before moving the hay.

"Don't open the barn door if the hay is smoking. The added oxygen can cause the hay to burst into flame," said Schultheis.

Hay temperature can be easily checked using a garden-composting thermometer.

According to Schultheis, a probe can also be built using a 3/8-inch diameter pipe with a pointed tip screwed to the end and holes drilled in it. A thermometer can then be inserted into the pipe and retrieved and read after 10 to 15 minutes.

For details on making and storing quality hay, contact the nearest MU Extension center and ask for MU guide sheet G4575, "Making and Storing Quality Hay." That publication is also available online at http://extension.missouri.edu.