by Sally Colby
While this summer’s abundant rainfall was too much for some areas, other regions throughout the Northeast had just enough moisture for a strong second cutting of dry hay. Although any farmer would love to get that additional tonnage from hayfields, late summer days mean fewer daylight hours, less wind and more evening moisture. Ideal conditions for haymaking include a light breeze, relative humidity of less than 50 percent, and no recent heavy rain. Although a field of late-season hay may appear to be extremely dry in the stand, new undergrowth can add significant moisture.
Freshly cut forage continues the respiration process in which plant sugars are producing energy, and this process releases heat. During the drying period when hay is still in the field, the amount of heat produced is minimal, and respiration eventually stops. Additional heating is the result of bacteria, fungi and yeasts consuming sugars from the plant.
The ideal moisture content for baling small square bales is around 20 percent or less. For large round bales, a more desirable moisture level is 16 to 18 percent.
Although hay is not fully cured at 20 percent, bales can be stored and will finish curing in storage at this level. Some producers refer to the continued heating that follows baling as ‘sweating’. Hay fires usually occur within several weeks after harvest, so it’s important to pay attention to the temperature of freshly baled hay whether it’s been stacked outside or packed into a building for storage.
The issue becomes ‘how much heating is okay’? According to the National Ag Safety Database, hay continues to heat to between 130 and 140 degrees F. During this time, the heat kills any remaining microorganisms. Baled hay often goes through several heating cycles during the curing period, with a lower maximum temperature each time. Once the curing period is complete, hay is stabilized close to the ambient temperature.
If hay heats to between 125 degrees and 150 degrees, a chemical process known as the Maillard reaction can occur. It’s much the same as what happens when a cook heats sugar and milk together to create caramel. With hay, it’s the fusion of amino acids and plant sugars, and the result isn’t desirable. Hay changes in color from green to brown and loses significant feed value. In most cases, this reaction doesn’t result in fire, but toward the higher temperature level (150 degrees), fire is possible.
New baled hay that has been stacked outdoors or put into a storage facility should be checked frequently for the first few days, especially if field conditions during cutting, raking and baling were less than ideal. Although many farmers rely on instinct to determine whether hay is becoming too hot in storage, it’s best to use a thermometer to determine the true temperature.
One way to check the temperature in stored hay is by dropping a thermometer into the stack by means of a hay probe. The probe can be made in the farm shop using a three-foot length of one or two-inch iron pipe. Drill a series of holes several inches from one end of the pipe, then hammer that end of the pipe to a point. Insert the pointed end of the probe into the bale, then carefully drop a candy thermometer with a light wire attached to the top down the length of the probe to obtain a temperature reading.
At 150 degrees, hay is becoming dangerously hot and should be checked at least once a day. If the temperature rises to 160 degrees, monitor the temperature every three to four hours and be aware of changes in odor and signs of smoking. At 175 degrees, contact the fire department and make them aware of the situation. Most rural fire departments will have someone who is experienced with hay and may recommend that the hay is either wet down or removed from the barn Between 185 degrees and 212 degrees, the hay is so hot that a fire is highly likely.
If the temperature is becoming dangerously high, be aware of the risks in monitoring. Hot spots can easily ignite if disturbed, and the operator may fall into a pocket that is already close to combustion. Spread planks across the top of the hay to avoid disturbing the hay, tie a rope around your waist before entering the mow, and have at least one other person present when you enter the mow.
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