Be Alert for Hot Hay

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Already this year we've had reports of first crop baled hay that has become excessively hot after storage. With the recent humid weather, forage has dried slow in the field and baled hay was no doubt harvested at higher than optimum moisture levels in some cases.

Although moderate temperature elevation is normal for baled hay put into storage, excess moisture in forage can result in hay heating to levels capable of spontaneous combustion. Whether hay actually burns depends mainly on the stack size and the materials surrounding the pile.

If the heat developed in the pile can readily escape, the hay or straw may only turn brown and become moldy. If there is enough hay or straw around the hot spot to prevent the escape of heat, and there is a slow infiltration of damp air, the hot spot will burn. Chopped materials stored in a pile are more likely to spontaneously ignite than the same materials in baled form. Keep this fact in mind as straw is chopped and piled into mows later this summer.

Heating of wet hay or straw occurs in three stages. First, carbohydrates combine with oxygen to yield carbon dioxide, water, and heat. At this point, temperatures may reach 100 degrees F. Next, heat is produced by bacteria and fungi pushing temperatures to 170 degrees F. In the third stage, oxygen combines with highly oxidizable material produced during second stage heating. This process starts at about 130 degrees F and may reach 190 degrees F. At this point, the situation becomes critical. Temperatures will continue to rise to the kindling point if enough oxygen and moisture are present to generate heat faster than it can escape. When temperatures reach 130 degrees F, begin monitoring piles daily. If contacted, most volunteer fire departments will check pile temperatures as a service. A steel rod can initially be used to check pile temperatures by driving it into the pile, waiting for fifteen or twenty minutes, and then pulling it out. If the rod is too hot to hold in your hand, the situation is critical.

Although we can't change the weather, there are some things that can be done to help insure hay will cure and store properly. If you know that hay will be baled, field drying will be faster and more uniform if forage is laid in a wide swath rather than narrow windrow. The key is to rake hay when moisture levels are at 40 to 50 percent to inhibit leaf loss. Many producers will rake in the early morning following the day when hay has dried to less than 30 percent.

Commercial drying agents in the form of potassium carbonate work extremely well but are only cost effective where a substantial acreage is baled each year. Buffered propionic acid is an effective preservative and allows for baling at somewhat higher than normal moisture levels. The keys with propionic acid are to make sure you apply rates that are proven to be effective and to not "push your luck" on the upper end moisture content which is usually 25 percent.

Finally, hay that is borderline in moisture content needs to be stacked where air and moisture can escape. If possible, stack bales on edge with a minimum number of layers. Be aware of forage and straw moisture contents as we progress through the harvest season. Also remember that weeds can significantly increase the moisture content of otherwise dry material. Take time to monitor suspect piles and contact your local fire department if there is reason for concern.