Summer hay season can be a stressful time of year.  Farmers are praying for rain, but not so hard that it flattens the hay or knocks the kernels off the grain and then hoping it will come at just the right time.  Once the cutting has begun, the praying turns to few breakdowns, fast parts pick-ups, and sufficient dew so that they can get water back on the fields for the cutting to grow.  You would think that as soon as the hay is in the stackyard, a farmer would finally stop worrying about that crop of hay, but the truth is, there is plenty more to worry about after the hay has been put up.

Old-time farmers used to say, “Bales in the hay yard are as good as money in the bank!”  They knew that having feed put up for the winter months was just like saving up dollars for a rainy day.  Available pasture or range grazing is never a given but feed standing in the stackyard is.

Farmers and ranchers depend on that hay to have the nutrition their cattle need to maintain their condition, be reproductively healthy, as well as capable of producing sufficient milk to raise their next calf.  Worrying about the condition of their hay seems like a reasonable thing to be concerned about even when the job seems done.

Aside from the quality of the hay, there are some years farmers have to watch the temperature of their haystack because getting too hot might just bring the whole pile down.

There is a delicate line of when is the right time to put up hay.  Too dry and it falls apart and all the nutritionally dense leaves just fall off.  Baled too wet and mold can develop, microorganisms will deplete the nutrition, and the risk of combustion is a serious concern.  Farmers and ranchers are well aware of what is too wet to bale, but all too often, conditions outside their control force their hand.  With hay ready to nearly dry enough and a storm on the horizon, farmers might just take that gamble to bale it a little wet and pray it all turns out ok.

It sounds wrong to blame spontaneous combustion in a haystack on conditions being too wet, but that is exactly the cause.  When there is too much moisture in hay, two things happen that lead to conditions that can result in spontaneous combustion.  First, plant cellular respiration, how a plant uses oxygen to convert sugars to energy for its growth, will continue to happen until the moisture content is down to 20%.  As respiration happens, energy, heat, water and carbon dioxide are produced.

As the stack of hay begins to “sweat” from both the high moisture already in the hay and the added moisture from the continued cellular respiration, the hay becomes an ideal place for various microorganisms that thrive in warm, wet conditions.  The microorganisms will break down the hay, a chemical reaction that gives off heat.     

The exothermic reaction, or heat producing reaction, of both the respiration and the microorganism activity drives the internal temperatures of the hay into dangerous numbers and conditions. The perfect storm exists when a patch of hot, wet hay sits among tightly packed dry hay that won’t let the heat escape.  

The hay will go through several stages as the temperature continues to rise.  First as the hay begins to warm around 115-120 degrees fahrenheit and is wet, mold and a musty smell will begin to develop.  As the temperatures continue to push passed 130 degrees fahrenheit, the sugars in the plant will begin to caramelize, giving the hay a brown color.  As the thermometer moves beyond 150 degrees fahrenheit, the hay will begin turning black and combustion is possible.

If a farmer or rancher is able to catch a bale of hay in this condition, there is little to do to keep a bale from eventually igniting.  What can be done is to move the bale to where it won’t catch other hay or structures will also catch on fire.  But when the hot spot of a bale is tucked inside the bale on the inside of the stack, it is often too late to do anything before the fire will spread.

No farmer or rancher counts on a stack of hay burning up when they bale up a cutting a day early to beat the forecasted rain.  But it’s a realistic possibility.  It might seem like just a little extra moisture to begin with, but it could be the beginning of a much bigger and devastating problem to come.  Checking moisture content and internal temperatures could be the difference of a haystack going up in smoke.