Does Hay Moisture Increase After Baling?

Hay moisture can absolutely increase after baling, and understanding when and why it happens is one of the most important things a hay producer or livestock owner can learn. Moisture rises inside a bale through two main routes: redistribution of moisture already trapped in the forage at baling time, and absorption of water vapor from the surrounding air. Both can push a bale that tested “safe” at the baler into a danger zone for mold, heating, and even fire within days or weeks of storage.

What Happens Inside a Freshly Made Bale

When hay is compressed into a bale, the stems and leaves are not uniformly dry. Thicker stems hold more moisture than thin leaves, and the outer windrow often dries faster than the inner layers. Baling locks these differences together under pressure. Once the bale is tied and sitting in a field or barn, moisture migrates from wetter zones toward drier ones. This internal redistribution does not add new water to the bale as a whole, but it can raise moisture in specific areas well above the average reading you got from a probe at the baler.

Round bales show this effect in a pronounced way. Density varies across a round bale because the forming chamber compresses some zones more than others. Those highly compacted zones resist airflow and hold moisture longer, making them difficult to dry down to safe levels even when forced-air drying is used.

1ASABE Technical Library. Numerical simulations and experimental measurements on the distribution of air and drying of round hay bales

The practical result is that a round bale reading 16% average moisture at baling might have pockets at 22% or higher. Those wet pockets become the starting point for microbial activity and heating, even though the bale as a whole appeared to be in a safe range.

Moisture Absorption From the Air

The second, and often less appreciated, source of post-baling moisture gain is the surrounding atmosphere. Hay is hygroscopic, meaning it absorbs and releases water vapor depending on the temperature and humidity of the air around it. Every forage material has an equilibrium moisture content: the point at which it stops absorbing or releasing moisture because it has reached a balance with the air. Research on grass and legume forages has shown that this equilibrium is extremely sensitive to relative humidity, particularly once the air climbs above about 80%.

2Journal of Agricultural Engineering Research. The Relationship Between the Equilibrium Moisture Content of Grass Mixtures and the Temperature and Humidity of the Air

What this means in practice is straightforward. Hay stored in a barn with poor ventilation during a humid summer, or bales left on the ground where morning dew and fog are common, will pick up moisture from the air. Even hay that was properly cured to 12 or 14% at baling can creep upward if the storage environment stays damp. In the southeastern United States or other humid climates, this is a persistent challenge, and it is one reason covered, well-ventilated storage matters as much as good curing before baling.

The relationship also works in reverse: on a dry, warm day with low humidity, the outer layers of a bale will lose moisture to the air. But the core of a tightly packed bale exchanges air very slowly, so the drying process is much slower than the wetting process. Moisture tends to get in easier than it gets out.

The Heating Cascade

When moisture in a bale stays elevated, a chain of events begins that can range from mildly annoying to genuinely dangerous. The first stage is microbial. Bacteria and fungi that are always present on forage surfaces become active when moisture and temperature allow them to grow. Their metabolic activity generates heat, which raises the temperature inside the bale. A modest rise to around 100–120°F is common in freshly baled hay with slightly elevated moisture and usually stabilizes without causing major damage.

Problems escalate when microbial heating pushes temperatures higher and moisture remains available. Chemical oxidation reactions begin to take over from biological ones, and these reactions do not depend on the microbes continuing to grow. Research on spontaneous combustion in hay has found that the chemical reactions capable of driving temperatures toward ignition require moisture, involve direct oxidation of cellulose, and are not dependent on previous microbial activity. At roughly 170°F the hay has dried out internally, and continued oxidation of the now-dry material can lead to combustion.

3Journal of Applied Chemistry. Spontaneous combustion of hay

Barn fires caused by spontaneous combustion are not common in absolute terms, but they are well documented and devastating when they occur. The risk is highest in the first two to six weeks after baling, which is exactly the window when internal moisture redistribution and ambient absorption are most active. Monitoring bale temperature with a probe during this period is standard advice, and any bale reading above 150°F should be treated as an emergency.

What Excess Moisture Does to Feed Quality

Even when heating stops short of combustion, the damage to hay’s nutritional value can be substantial. Spontaneous heating binds protein into forms that are less digestible by ruminants, increases what lab tests report as lignin and acid detergent fiber, and degrades carbohydrates through oxidation and browning reactions.

4Transactions of the ASAE. Mechanism of Spontaneous Heating of Hay Part 2 — Chemical Changes in Spontaneously Heated Hay

The browning reactions are the same type of chemistry that turns bread crusts dark in an oven: sugars react with amino acids under heat, producing compounds that are largely indigestible. Livestock owners sometimes notice heat-damaged hay by its tobacco-like smell and brown or caramelized appearance. Animals may eat it readily because of the sweet smell, but the protein they are consuming is partly locked up and unavailable. A forage analysis after heating will often show that crude protein looks normal or even high on paper, but the “bound” fraction has increased sharply. For anyone buying hay, knowing whether it went through a heating event is almost as important as knowing the species and maturity at cutting.

The spoilage picture extends beyond heating. Microbial activity at elevated moisture also causes straightforward dry matter loss, which is a polite way of saying the organisms ate some of your hay. The extent of spoilage during storage depends on bale moisture, bale size, density, shape, wrapping, the type of forage, and the quality of the storage facility.

5Grass and Forage Science. Factors affecting nutrient losses in hay production

Mold and Mycotoxin Thresholds

Mold growth is the most visible consequence of baling or storing hay too wet, and it is also the one that raises the most concern for animal health. Not all mold is equally dangerous, but certain species produce mycotoxins that can cause respiratory problems, reproductive failure, or organ damage in horses and cattle.

Research on moulded hay has found that mycotoxin production becomes a real concern when forage is stored above about 35% moisture, while there is little evidence that mycotoxins develop in hay stored below 30% moisture.

6Animal Feed Science and Technology. Establishing a feed value for moulded hay

The gap between those two numbers matters. Hay baled at 25% might seem comfortably below the mycotoxin threshold, but if moisture redistributes internally or rises from humid storage, specific zones of that bale can easily cross 35%. This is especially true for large round bales and large square bales, where the core stays wet much longer than the exterior. For horse owners in particular, who tend to be more cautious about respiratory irritants, even visible mold without confirmed mycotoxin presence is often reason enough to reject a bale.

Measuring Moisture Accurately Is Harder Than It Looks

One reason post-baling moisture surprises are so common is that field measurement of hay moisture is imprecise. The gold standard for determining moisture content is oven drying a sample at 103°C for 24 hours and comparing the weight before and after. That method has a coefficient of variation of about 6%, which is respectable but not perfect. The handheld electronic meters that farmers actually use in the field are less precise. Testing of four commercial meters found that the best ones had a coefficient of variation around 8.4%, while the less accurate ones were closer to 10.7%.

7ASABE Technical Library. Electronic estimation of hay moisture content: precision and accuracy

In practical terms, this means that a meter reading of 18% could easily represent true moisture anywhere from about 16% to 20%, depending on where you stuck the probe and which meter you used. Conductance-based meters, the most common type, measure the electrical resistance between two metal probes pushed into the bale. They are sensitive to the density of the hay around the probes and to whether the probes happen to land in a wet or dry zone. One meter in the study showed an error of estimate that varied by almost 8% across the 15–25% moisture range, meaning its readings needed large corrections to approximate true moisture.

The takeaway for producers is not that meters are useless, but that treating a single meter reading as gospel is risky. Taking multiple readings from different locations and depths in a bale gives a much better picture. And when conditions are borderline, erring on the side of assuming the hay is wetter than the meter says is a form of cheap insurance.

Propionic Acid and Other Chemical Preservatives

Sometimes weather forces your hand. Rain is forecast, the hay is only partially cured, and the choice is between baling at higher moisture or leaving the windrow to be rained on. In those situations, chemical preservatives offer a middle path. Propionic acid is the most widely used and studied option. It works by suppressing the microbial activity that drives heating and mold growth, buying time for the hay to dry down in storage.

Field trials on alfalfa baled at about 32% moisture found that applying propionic acid at a rate of 1% of hay weight inhibited both molding and heating, cutting dry matter loss during storage from roughly 15% down to about 8%.

8Agronomy Journal. Propionic Acid as a Hay Preservative

At very high moisture levels around 40%, higher application rates in the range of 3 to 5% were needed to meaningfully reduce storage temperatures and maintain forage quality.

9Agronomy Journal. Effects of Organic Preservatives on the Quality of Aerobically Stored High Moisture Baled Hay

Other trials using spray equipment that would be available on a typical farm also confirmed that treated hay showed less visible mold and lower dry matter loss during the first month of storage compared to untreated high-moisture hay.

10Applied Engineering in Agriculture. Preservation of Alfalfa Hay with Propionic Acid

Propionic acid is not a magic fix, though. It adds cost per ton, it is corrosive to equipment, and it works best when applied uniformly, which is not always easy at baling speed. An economic analysis found that a preservative treatment with effectiveness similar to propionic acid needs to cost less than about $8 per ton of dry matter to be profitable with limited use, and less than about $4 per ton with heavier use. If a hypothetical ideal treatment could eliminate all storage loss entirely, it would be worth up to roughly $21 per ton of dry matter.

11ASABE Technical Library. Economic Potential of Preserving High-Moisture Hay

For many operations, the math works out favorably when the alternative is losing a cutting to rain or accepting severe quality losses. For others, particularly those making premium horse hay where any chemical additive raises buyer skepticism, the better strategy is simply waiting for proper curing conditions.

Storage Practices That Limit Moisture Gain

Beyond what happens at baling, how you store hay has a large influence on whether moisture increases afterward. Bales left on the ground outdoors will wick moisture from the soil and absorb rain and dew through the outer layers. Even tarped bales can trap condensation underneath if there is no air movement. For round bales stored outside, elevating them on pallets, gravel pads, or old tires reduces ground contact. Arranging rows with space between them and oriented so prevailing wind can move through helps surface moisture evaporate rather than soak in.

Indoor storage in a well-ventilated barn is the gold standard, but even barns can cause trouble if they are sealed too tightly. A barn full of freshly baled hay releases a lot of moisture vapor as the bales equilibrate. Without adequate ventilation, that humidity hangs in the air and gets reabsorbed by the very hay that released it. Ridge vents, sidewall openings, and fans all help break this cycle. In humid climates, some producers use dehumidifiers in smaller storage buildings, though the energy cost is only practical for high-value hay.

Stacking arrangement also matters. Tight stacks with many layers generate more internal heat and trap more moisture than open, loosely stacked arrangements. Leaving air channels between rows and not stacking freshly baled hay directly against older dry hay reduces the risk of moisture migration from new bales into previously stable ones.

When Wrapping Changes the Rules

Baleage, or hay that is baled wet and sealed in plastic wrap, is a completely different system. Here the goal is not to prevent moisture gain but to create an anaerobic environment where fermentation preserves the forage, much like silage in a bunker. Baleage is typically wrapped at 40–60% moisture, far above what would be acceptable for dry hay. The plastic prevents oxygen from reaching the forage, so instead of the aerobic microbial activity that causes heating and mold in dry hay, anaerobic bacteria ferment the sugars and lower the pH, stabilizing the feed.

The relevance to the moisture question is that wrapping effectively removes the bale from the atmospheric moisture exchange that drives post-baling changes in dry hay. A wrapped bale does not absorb ambient humidity because it is sealed. But if the plastic is punctured by rodents, equipment, or rough handling, oxygen enters, aerobic organisms activate, and the high moisture content that was an asset becomes a liability almost immediately. A single hole in a baleage wrap can lead to a visible mold bloom within days.

For producers who frequently face wet harvest windows, baleage offers an alternative to gambling on marginal drying conditions. The tradeoff is higher wrapping cost, the need for specialized equipment, and a product that must be fed relatively quickly once the wrap is opened.

How Hay Type and Maturity Affect Post-Baling Moisture Behavior

Not all hay behaves the same way after baling. Legumes like alfalfa have thick stems that hold moisture much longer than their leaves, creating a wider gap between stem and leaf moisture at baling time. This means more internal redistribution after the bale is tied. Grass hays tend to dry more uniformly, but coarse-stemmed grasses like orchardgrass can behave more like legumes in this regard.

Maturity at cutting also plays a role. Younger, leafier forage has a higher ratio of surface area to mass, which allows it to release moisture faster in the windrow but also pick it up faster from humid air after baling. Mature, stemmy forage dries more slowly in the field but is somewhat less hygroscopic once cured because of its higher fiber content. The equilibrium moisture content models developed for forages apply across a range from ryegrass mixtures to alfalfa, but the specific equilibrium point at a given humidity and temperature shifts with forage composition.

2Journal of Agricultural Engineering Research. The Relationship Between the Equilibrium Moisture Content of Grass Mixtures and the Temperature and Humidity of the Air

For anyone managing a mixed-hay operation, this means that a single moisture target at baling may not be equally safe for all forage types. Alfalfa baled at 18% has more internal moisture variation than timothy baled at 18%, and the alfalfa bale is more likely to develop hot spots in storage. Adjusting your target downward for legume-heavy hay, or using a preservative when conditions are marginal, accounts for this difference.