Recent hot, dry weather may increase the risk of silo fires, warns a Penn State University farm safety expert.

Many drought-stressed fields, with crop low in moisture to begin with, will dry down quickly once harvested, says ag engineer Dave Hill.

"Internal combustion of silage material can occur if the silage is put in when it's too dry for the silo,” he says. “For anything to burn, you need three ingredients – a heat source, air and fuel."

With silage, heat is generated as it goes through the fermentation process, Hill says. Proper moisture levels help keep the material from getting too hot.

"Air is trapped in the chopped forage during harvest and when blowing the material into the silo. The drier the material, the more air that is trapped; conversely, the wetter the material, the less air that is trapped."

The fuel is the forage material itself. Generally, it’s not a good source of fuel from a burning standpoint, because even forage that’s too dry for good silage is too wet to burn quickly. That's a good thing to keep in mind when discovering a silo fire, Hill notes, because when a silo burns, a farm operator can lose a tremendous investment and be faced with an unmanageable cost to replace ruined feed.

"If you have a 20’-diameter by 60’-high silo that contains 400 tons of corn silage, and if you had to purchase that 400 tons of feed, it would cost you nearly $20,000 – $50 per ton," he says. "Good hay-crop silage would be considerably more.

"The key to managing a fire inside a silo is locating the fire area and controlling that area without affecting the rest of the material. It’s better to lose a few tons than a few hundred tons."

Limiting the fire while protecting the unaffected silage is difficult but not impossible, Hill says. "The earlier you detect a fire, the easier it is to control, so it's important to regularly monitor your silos for a good three weeks post-harvest. This is a critical time when natural fermentation and heating are taking place inside the silo."

Another common cause of spontaneous combustion in silos is putting new silage on top of old silage, which can be quite dry.

"Remember, the dryer the material, the more air that can be trapped in that material. When fresh material is put on this older material, the natural heating that the new material will go through could be too hot at this location. It also will not pack down as tightly, leaving more air. This would be the first place to look if a fire does occur.

"Likewise, if you know that some of the material you are blowing into the silo is drier than ideal, you might make a mental note of where in the silo that is placed. If your silo was two-thirds full when you put fresh material in it, and a fire is discovered a few weeks later, you and the fire company should concentrate efforts to determine if there is more heating going on at this two-thirds level."

Silo fires occasionally can start from the outside. These external blazes most frequently start in the chute from either a shorting-out electrical wire or from an adjacent barn fire. Dried material in the chute then can catch on fire and burn through one or more wooden silo doors.

A silo fire usually is discovered when smoke comes from the top of the silo, when charred silage or burnt silo doors drop down the chute or when a burning smell is evident. The initial decisions made can mean the difference between salvaging a viable crop or ruining it, according to Hill.

"Remember, a fire inside a stack of silage in a silo does not have adequate air to burn aggressively. This means you don't need to panic. The fire is not going anywhere in a hurry, so you have time to evaluate what you have, to report it to your local fire company and seek out additional expertise if needed."

Information about managing silo fires is available at PAgricultural Rescue Training. Hill tells of some of the silo fires he helped diagnose in the February 2011 Hay & Forage Grower story, Silo Fires: Not Too Hot To Handle.