Last night I was doing some digging around on facts and figures of hay quality. Scooter Moody (@moodycattle) forwarded me a link to a great fact sheet on hay quality. The topic I was looking at is Moisture content. Have you ever heard of a barn burning because the hay spontaneously combust? Well it’ll combust alright, but it’s more than just an “up-and-decide-to-burn” thing. There’s a whole process behind it.

Since I’m in the middle of the hay fields I figured you might like to know a little more about hay moisture and what I’m talking about when I talk about letting the hay dry or cure. It’s either that or let the barn burn down… I’ve seen it happen. Like I’ve said before, cattle ranching is more than riding horses into the sunset. There’s quite a bit of figurin and thinkin going on inside the head of every cattleman. Sometimes that thinkin is a lil slower than normal.

Moisture content From the Mizzou Fact Sheet

When forage plants are cut, the plant continues to breathe until the moisture content of the plants falls below 40 percent. Dry matter is lost during this process, and in some cases the loss may be as high as 15 percent. Respiration losses, however, are usually about 5 or 6 percent of the total dry matter. In normal hay curing, you cannot eliminate these losses.

When the moisture content of hay drying in the field reaches about 40 percent, further dry matter losses are due to raking and baling. Losses from these operations range from 10 to 25 percent, with most losses averaging about 15 percent. Dry matter losses from raking and baling are especially severe because most of these losses come from the most valuable part of the hay — the leaves.

Using hay crimpers and crushers can greatly reduce dry matter loss. Their use reduces curing time in the swath, exposure to the weather, leaf shattering and respiration losses. All serious hay producers use crimpers and/or crushers.

The key to keeping dry matter losses of hay to a minimum are:

  • bale at a moisture level low enough to prevent excessive heating,
  • Prevent infiltration of moisture into the hay after it has been baled.

When hay is baled, it should not be higher than 18 to 22 percent moisture. At higher levels of moisture, bales lose large amounts of dry matter (Figure 1) caused by excessive heating and molding (Figure 2). In severe cases, spontaneous combustion is possible.

Spoilage loss in bales made from alfalfa-grass at different moisture levelsFigure 2
Spoilage loss in bales made from alfalfa-grass at different moisture levels.

Although there is no danger of burned buildings when you store large bales outside, excessive heating and molding can occur, resulting in large dry matter losses. Therefore, don’t bale hay to be stored in the field until the moisture level reaches 18 to 22 percent. See Table 1.

Table 1
Dry matter losses of alfalfa-grass in big bales under different storage conditions. (Howard David Currence, MU.)

First cutting hay

  • Moisture at baling
    36 percent
  • Dry matter loss
    Inside storage
    17.8 percent
    Outside storage
    26.6 percent

Second cutting hay

  • Moisture at baling
    24 percent
  • Dry matter loss
    Inside storage
    12.2 percent
    Outside storage
    15.2 percent

Heating occurs to some extent in all forage material unless it contains less than 15 percent moisture.

The extent of temperature rise and duration of heat production in hay depends on moisture content. A relative humidity of 90 to 100 percent, which favors mold development, can develop in 20 percent moisture hay that is stored inside. The heat generated by the metabolic activity of the microorganisms and plant respiration increase the temperature of hay (Figure 3). Heat resistant fungi are active when the temperature is between 113 and 150 degrees Fahrenheit.

Causative agents in the heating of hay
Figure 3
Causative agents in the heating of hay.

A large variety and number of microorganisms are associated with plant material in the field, but fungi are the microbes primarily responsible for breakdown of complex carbohydrates. Heating above 175 degrees Fahrenheit results in thermal death of microbes; then heat-producing chemical reactions serve to further increase temperatures. A subsequent rapid oxidation of reactive compounds may cause a further temperature rise to an ignition point of 448 to 527 degrees Fahrenheit. If enough oxygen is present, flames will erupt. The time required for heating to combustion may vary from four to 10 weeks, depending on storage and climatic conditions and on the moisture content of the forage.

The obvious consequence of spontaneous heating of forages is combustion, which has resulted in numerous barn and silo fires. But molding of forages and heating to temperatures below ignition also result in serious losses of forage quality and quantity. Available carbohydrate and protein portions of forages are reduced. Carbohydrates are used in microbial metabolism and subsequent chemical oxidation. Protein is bound in an unavailable form (sugar-protein polymer) through browning or the Maillard reaction.

Moisture levels for safe storage of hay vary with size and density of the bale and type of hay. In general, hay in small rectangular bales should be baled at less than 22 percent moisture to keep molding and heating to a minimum. Large round bales retain internal heat much longer than conventional bales. Therefore, hay should be less than 18 percent moisture before baling in large bales. If you are storing or sheltering some of your big bales, this long-term heat retention affects the proper time to move big bales into storage. See Figure 2. Hay baled with more than 22 percent moisture should probably not be put into storage for at least 30 days. This is especially true if bales are to be stacked several layers deep.

With the threat of barn fires removed by outside hay storage, many operators of large round balers try to bale hay with too much moisture. But excessive heating and molding can cause the loss of as much as one-third of the feeding value of hay baled at 28 percent moisture.