Historically, when hay was stacked loosely (without binding), it may have dried more extensively than modern stacks because of the time required for hand labor. When small bales became the most common method for harvesting hay from 1950 to 1975, the bales were hand-stacked, weighed 40 to 80 pounds per two-string bale and density was 8 to 11 pounds per cubic foot.

Shewmaker glenn
Emeritus Professor and Forage Specialist / University of Idaho

As machine stacking became commonplace, larger and denser bales were desired, and they could weigh 80 to 120 pounds per two-string bale.

During the same time, commercial dairies wanted premium hay with high leaf attachment. The 3x3, 3x4 and 4x4 (foot) large rectangular bales have densities of 14 to 16 pounds per cubic foot, nearly double the density of the old two-string bales.

In the last 25 years, there may have been more hay fires than in the past because high moisture is retained longer in the more dense and massive types of bales.

Types of moisture
Free water, often called “dew moisture,” can be readily evaporated from forage. Physically trapped water is contained inside plant cells.

A fraction of the water is bound water that cannot be removed without destroying the forage. Free and physically trapped water can eventually be evaporated from the forage given sufficient solar radiation, relative humidity and time.

The term “stem moisture” is used to describe the moisture that remains in the plant stem after leaves have dried during the curing process.

Windrowed hay, given ample time to field-dry, will eventually dissipate stem moisture to a level safe for baling.

Dew moisture on the outer surface of the stem and leaf is the result of relative humidity and temperature forming condensation.


Dew moisture is rapidly removed by sunlight or a light breeze and usually remains on the hay for only a few hours each day.

The process of plant and microbial respiration
Heat is generated in hay bales when microbial growth uses carbohydrates as an energy source. Plant and microbial respiration converts carbohydrates and oxygen into carbon dioxide, water and heat.

This results in a loss of weight (dry matter yield) and increased bale temperature.

This rise in bale temperature can have a detrimental effect on forage quality by binding proteins to fiber and rendering these proteins and nonstructural carbohydrates unavailable to the animal.

There is a strong correlation between bale size or density and heating. Large rectangular-ended bales are susceptible to greater storage losses and risk of spontaneous heating than small rectangular-ended bales formed at the same moisture.

Monitoring moisture in the windrow
Weather and soil conditions affect drying rates of hay, so producers should monitor these conditions to help them make management decisions.

The most important factors affecting hay curing are solar radiation, which provides the energy to evaporate water from forage; air temperature; relative humidity; wind speed and soil moisture.

Moisture and temperature changes in a bale or stack
Water is one of the end products of respiration, so the higher the moisture content of hay, the higher the microbial respiration and subsequent moisture production in hay after baling.

This process is why farmers commonly report that hay “sweats” or feels wetter after they get it into the stack than it did when baled.

Water vaporized by the heating will be in airspace within the stack as it moves out, increasing humidity levels.

Equilibrium moisture in alfalfa is around 15 percent for humid regions or seasons and about 10 to 12 percent in arid regions. The final moisture content of hay should not be more than 12 percent to avoid mold.

Spontaneous heating and combustion
Hay fires from spontaneous heating usually occur within six weeks of baling, but they may also occur in cured hay when external moisture is added, or more oxygen is allowed inside the stack.

The moisture content of a bale shouldn’t be higher than 18 to 22 percent for small rectangular bales, 14 to 18 percent for large round bales or 14 to 16 percent for large rectangular bales.

However, these are not absolute numbers because other factors affect spontaneous heating.

Higher moisture levels allow microbial activity to increase, which causes hay to heat and mold, resulting in the loss of dry matter and usable protein.

Usually the temperature will peak from three to seven days after baling. Temperature should decline in the period from 15 to 60 days to non-damaging levels, depending on relative humidity, the density of the bales and the amount of rainfall the bales soak up.

The longer it takes for temperature to decline, the more damage is done to hay, and the more likely a fire may result.

Monitoring moisture and temperature in bales and stacks
If the moisture level of hay at baling is more than recommended for storage, check the moisture and temperature at least twice daily for abnormal heating.

If the hay temperature reaches 130ºF, you could be on the way to a hay fire, so close attention needs to occur.

The best option is to arrange for someone to feed the bales as soon as possible. The bales could be placed to allow increased air circulation and cooling.

If hay is stacked tightly or in a barn and temperature climbs to 150ºF or more, call the fire department and be prepared to inject water to cool any hot spots before moving the hay.

Don’t move the hay without fire suppression equipment if the hay is smoking, you can see charred centers within bales, or if you can smell a strong odor.

If additional oxygen reaches areas near high-temperature hay, the hay can burst into flame.

Steps to reduce hay fires
Forage producers, hay brokers and forage consumers need to avoid situations that increase the risk of hay fires and take necessary precautions. The following are some guidelines to minimize potential of heat-damaged hay and spontaneous heating:

1. Record day of cutting and drying conditions in a journal.

2. Monitor hay moisture prior to raking, tedding and baling.

3. Record the day of raking, tedding and inversion, as well as baling and drying conditions.

4. Test the first three bales from a field with a hand-held electronic moisture probe in two places on each side or ends of the bale (total 12 readings) and note the low, high and average.

If the average moisture is below the maximum for the type of bale and forage, continue baling.

5. Observe the baler moisture meter if available, and the bale pressure indicator. When anything changes, get out and re-test more bales.

6. Record date, time, weather conditions and moisture average and range in your journal.

7. Mark any bales with higher moisture than desired, and notify the person stacking the hay to place those bales separate from the dry bales with space between bales to facilitate air movement and drying.

Try to feed these bales as soon as possible. Do not store any bales with moisture over the recommended level for that bale type in a large stack or hay shed.

8. Review and comply with your insurance carrier’s requirements for fire insurance coverage.

9. If moisture is marginal, stack large bales in a single column with no more than 500 tons per stack. Under these circumstances, allow at least 100 feet of separation to the nearest adjacent stack.

Lay out the stack so it can be quickly broken apart with a loader. Avoid mixing different lots of hay because of marketing and hay fire issues.

10. Obtain a bale core sample, double sack the core sample in two Ziploc plastic bags and submit it to a NFTA-certified lab for moisture and forage quality analysis.  FG

References omitted due to space but are available upon request. Click here to email an editor.

—Excerpts from 2013 Idaho Hay and Forage Conference proceedings

Glenn Shewmaker
Extension Forage Specialist
University of Idaho