Hay is a forage crop that has been cut and dried in the field, a process known as curing, which transforms high-moisture green plants into a shelf-stable feed. Freshly cut plant material contains a high percentage of water, typically ranging from 75 to 85 percent, which must be reduced significantly for safe storage. The time hay can lay before baling is not a fixed period but depends entirely on achieving a specific moisture threshold. The goal of field curing is to quickly lower the plant’s water content to halt biological activity before weather or field conditions degrade the feed value. The duration hay spends in the field reflects how long environmental conditions and field management take to bring the moisture level into the correct range.
The Critical Role of Moisture Content
The single most important factor determining when hay is ready for baling is its internal moisture content. Hay must be sufficiently dry to prevent the growth of aerobic microorganisms, such as molds and bacteria, which cause spoilage and generate heat during storage. If the moisture level is too high, these microbes consume the plant’s nutrients, leading to a significant reduction in feed quality.
The safe moisture percentage varies based on the size and density of the final bale package. Small square bales, which are less dense and allow for greater air circulation, can generally be baled at a higher moisture content, typically between 18 and 20 percent. Large round and large square bales are much more tightly compressed, retaining heat and moisture more effectively. For these larger packages, the maximum safe moisture level is lower, usually 15 to 18 percent for large round bales and 12 to 16 percent for large square bales, to mitigate the risk of excessive heating.
Producers use precise methods to measure this percentage rather than relying on feel alone. The most accurate method involves the microwave test, where a small sample of hay is weighed, dried, and then re-weighed to mathematically determine the exact moisture percentage. Electronic moisture meters equipped with probes are also widely used, providing quick readings directly in the windrow, although their readings can sometimes vary and require proper calibration.
Environmental Factors Influencing Curing Time
The speed at which cut hay reaches the target moisture level is governed by atmospheric conditions. Solar radiation is the primary driver of drying, supplying the energy needed to evaporate water from the plant tissue. Warmer ambient temperatures increase the rate of evaporation, shortening the time hay must remain in the field.
Relative humidity (RH) plays a counteractive role; air saturated with water vapor has a limited capacity to absorb moisture from the hay. When the RH is consistently high, such as above 90 percent, hay may never dry down to a safe baling level. Wind speed accelerates the process by continuously moving the moist air layer away from the hay’s surface, allowing drier air to pull moisture from the forage.
Precipitation and heavy dew are the most significant weather events that reset the drying timeline. Even a small amount of rain can leach water-soluble nutrients, and the absorbed moisture must be re-dried completely before baling can resume. Heavy dew occurs when the air temperature drops to the dew point overnight, depositing substantial moisture on the hay and often delaying the start of the drying day until late morning.
Mechanical Steps to Prepare Hay for Baling
Field operations are performed while the hay is laying to ensure uniform drying and maximize exposure to the sun and air. The first step involves laying the hay out in a wide swath immediately after cutting, covering nearly the entire width of the mower. This maximizes the surface area exposed to solar energy and significantly reduces the time needed for initial moisture loss compared to narrow windrows.
Tedding involves machinery that fluffs and spreads the cut forage, often turning wetter material from the bottom of the swath to the top. This practice is effective for speeding up drying time, especially in heavy crops or when the hay has been rained on. Tedding is best performed when the hay moisture is still relatively high, ideally above 40 percent, as aggressive handling of drier forage leads to excessive leaf loss.
Raking is the process of gathering the widely spread hay into a narrow windrow, the final shape necessary for the baler to pick up the material efficiently. This step should be timed carefully, typically when the hay has dried to about 30 to 40 percent moisture, to minimize leaf shattering. Raking at this stage ensures the outer layer of the windrow is the driest part, while the slightly wetter inner core is rolled to the outside to continue curing.
Consequences of Delay: Quality Loss and Spoilage Risk
Allowing hay to lay too long in the field, even after it has dried, or baling it with excessive moisture carries severe consequences for feed quality and safety. If the hay is left on the ground too long, natural plant respiration continues until the moisture content drops below 40 percent, consuming readily digestible carbohydrates. This process burns up sugars that would have been used for animal energy, resulting in a dry matter loss that can range from 3 to 15 percent.
This loss of dry matter simultaneously increases the percentage of indigestible fiber. Baling hay when it is over-dry causes a different, but equally significant, loss of quality known as leaf shatter. The leaves contain the highest concentration of protein and other valuable nutrients, but they dry much faster than the stems.
Once leaves become brittle, the mechanical action of raking and baling causes them to break off and be left behind in the field, drastically lowering the nutritional value of the final bale. The most concerning risk is baling hay that is too wet, as it creates an environment for rapid microbial growth within the dense bale.
This biological activity generates heat. If the temperature inside the bale reaches 150 degrees Fahrenheit, the Maillard reaction begins, binding protein to carbohydrates and making the protein unavailable for digestion. If the temperature continues to climb, reaching between 150 and 160 degrees Fahrenheit, the potential for spontaneous combustion becomes a hazard.