Hay and fire are a bad combination for more reasons than you might think. “There are several factors that make hay fires difficult to extinguish,” says Jon Barry, a forester for the University of Arkansas Extension System Division of Agriculture, and a volunteer firefighter in southwestern Arkansas. “Round hay bales in particular are pretty tightly compressed,” Barry adds. “The hay baler assembles the bale in layers like a roll of paper.” Because the hay is tightly compressed, a round bale doesn’t burn intensely or quickly. Instead, a smoldering _ re burns up into the bale. The bale has to be taken apart to get to the _ re so it can be put out, and all of it has to be put out or it will _ are up again.

There are several approaches, depending on where the bale is and the terrain. “Grass is a tough plant and that gives round hay bales a distinct tough grain pattern almost like wood grain,” Barry says. “When we are trying to get into the bale to find the smoldering area, we have to work across the grain, and that’s hard work. Since we are going to spray water all over and into the bale, it is a total loss to the farmer anyway so nothing is lost by tearing the bale apart.” Square bales aren’t as tightly compressed and tend to burn quickly, although stacked square bales can burn very intensely.

Another cause for hay fires is fermentation caused by hay that’s too moist. That moisture will stimulate growth of microbes in the hay, causing heat, says Dirk Philipp, assistant professor for the University of Arkansas System Division of Agriculture. “Even at recommended moisture levels, hay will slightly increase in temperature, as microbes respire for a couple of weeks and slightly increase bale temperature — up to around 120° F.” If moisture is left unchecked, temperatures will rise, with temperatures reaching 180° F or more and creating a risk of spontaneous combustion.