Importance of Hay Moisture

Host: Joanna Coles, Warren County Extension Agent for Agriculture & Natural Resources
Guest: Brandon Bell, Metcalfe County Agriculture and Natural Resources Agent
Topic: Importance of Hay Moisture
All hay baled at moisture contents above 15 percent will undergo some elevation in temperature in the first two to three weeks following baling. This heating is referred to as “sweating” or “going through a sweat” and is due to plant respiration and microbial activity. Even relatively dry hay (15 to 20 percent moisture content) will heat enough to cause measurable dry matter losses (4 to 5 percent). A small amount of heating (up to 130°F) does no real damage to hay quality but serves to dry out the hay by evaporating some of the moisture it contains. Under Kentucky conditions, hay will generally reach an equilibrium moisture content near 15 percent. When moisture contents of hay are greater than 15 percent at baling, some changes in hay color occur along with microbial growth and heating. The green color of moist baled hay typically changes to various shades of brown, depending on the extent of heating. In general, some loss in dry matter and quality is associated with these changes initially occurring after baling. A rule-of thumb useful in estimating yield loss of round bale hay is that 1 percent of original yield will be lost for each 1 percent moisture that is lost as stored hay reaches its equilibrium storage moisture. For example, if hay is baled at 20 percent moisture and then dries to 14 percent during storage, it will suffer a dry matter yield loss of about 6 percent. Baling above 20 percent moisture without taking steps to reduce the microbial activity responsible for this heating may result in substantial quality loss. The extent of fungal growth during hay storage is a function of moisture content. Under normal conditions, the evaporation of water helps to dissipate heat generated by microbial growth on moist hay. The extent of heat damage can be assessed by looking at the color change during hay storage. Heating of moist hay causes a chemical reaction that fuses plant sugar and amino acids into an indigestible product called the Maillard product. Protein bound up in this process is called heat-damaged protein and is unavailable for animals. When hay heats sufficiently to cause a very dark brown to black color, its protein may be nearly indigestible.

[@KennyTrussell]

Very informative. Thanks!