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Wet hay may lead to barn fire: Experts suggest testing
Ohio Farm News
By Steve Bartels

A local hay producer called last week, concerned that the hay he had made seven days before, might burn the barn down. He had made hay in large, round bales and put it in the barn.

He had used the traditional feel, twist, and smell method to decide if the hay was dry enough to bale. He baled the hay even though he thought it might have been a little damp. After he put the hay in the barn, he began to worry and got a hay moisture tester.

He had also taken temperature readings from the center of several bales. He wanted to know how hot the hay would get before it started to burn, and how long it took to get through the sweat.

If hay is put into a barn when it has more than about 22 percent moisture, not only does it lose hay nutritional value, but also it may spontaneously combust. This is caused by a chemical reaction that builds heat.

The hay actually insulates, so the larger the stack, the less cooling there is to offset the heat. When the temperature of the hay rises above 130 degrees Fahrenheit, a chemical reaction begins to produce a flammable gas that can ignite if the temperature continues to rise.

Heating occurs in all hay that is baled above 15 percent moisture and is normal, what we commonly call the sweat. The normal process usually peaks at 125 to 130 degrees within three to seven days, with little risk of fire. Temperatures within the stack then decline over the next 60 days.

In an article published by Montana State University a couple years ago, Dennis Cash and Rob Johnson said, if a hay fire is going to occur, it usually occurs within the first six weeks after baling.

Hay moisture should not exceed 18 to 22 percent for small rectangular bales and large round or rectangular bales should not exceed 18 percent moisture, Montana State suggested.

If you donít own or canít borrow a hay moisture tester from your neighbor, you can use your microwave and a scale that weighs in ounces to determine the moisture.

You can go to and search for forage moisture. It will give you a detailed description of the process in Fact Sheet AGF-004-90. You can also get it at county OSU extension offices.

If you smell a slight caramel odor, or a distinct musty smell, itís too late to worry about a moisture tester. You need to monitor the hay temperature regularly. Montana State suggested taking a 10-foot pipe or electrical conduit. Sharpen the pipe, or screw a pointed dowel to one end, then drill several quarter-inch diameter holes in the pipe just above the dowel.

Drive the probe into the haystack and lower a thermometer on a string into the pipe and donít disturb it for at least 10 minutes.

If the temperature is 150 degrees, check it every day until it declines below 130. If it is 160 degrees, Cash and Johnson recommend checking the stack every four hours.

At 175 degrees, call the fire department, remove the hay from the barn and wet it down. At 185 degrees, hay is likely to bust into flame when it comes into contact with air. If the hay reaches 212 degrees, it is almost certain to ignite.

This farm news was published in the June 14, 2006 issue of Farm World, serving Indiana, Ohio, Illinois, Kentucky, Michigan and Tennessee.