Hay is generally ready to bale when its moisture content falls below about 18 to 20 percent for small square bales and below roughly 18 percent for large round bales, though the exact threshold depends on bale size, forage species, and storage conditions. Getting this number wrong by even a few percentage points can mean the difference between high-quality feed and a barn full of moldy, overheated bales that could catch fire. The tricky part is that moisture varies across a field and throughout the windrow, so a single grab-and-twist test rarely tells the whole story.
Target Moisture Levels by Bale Type
Small rectangular bales are the most forgiving format because their shape allows heat to escape more easily. Most producers aim for moisture content between 15 and 20 percent at baling, with 18 percent or lower being a widely accepted safe ceiling. Large round bales and large square bales hold heat much more effectively, so they need to be drier before wrapping. Research on alfalfa-orchardgrass hay found that baling at 23 percent moisture led to dramatically higher internal temperatures compared to baling at 18 percent. In large round bales stored at that 23 percent level, internal temperatures spiked to 90°C at the bale center, causing measurable damage to the protein in the forage.1Journal of Dairy Science. Effect of Moisture Content at Baling on Nutritive Value of Alfalfa Orchardgrass Hay in Conventional and Large Round Bales At 18 percent moisture, heating was far less severe. The general rule: the bigger the bale, the drier it needs to be, because a larger mass of plant material traps more metabolic and chemical heat.
Hands-On Field Tests
Before moisture meters existed, farmers relied on physical cues, and those cues are still useful as a first pass. The classic approach is to grab a handful of hay from inside the windrow, not from the sun-bleached top, and twist it tightly. If the stems snap cleanly and feel brittle, the hay is likely below 15 percent moisture and may actually be too dry, meaning you will lose leaf material during baling. If the stems bend without breaking and your hand feels cool or damp, the hay is still too wet. The sweet spot is when stems resist twisting and snap with moderate force, and your palm stays dry.
Another quick check is to take a small wad of hay and hold it against your cheek or the inside of your wrist, where skin is more sensitive to moisture. If it feels noticeably cool, there is still significant water in the stems. You can also scrape a stem with your thumbnail. If you can easily press a groove into it, the stem interior still holds moisture. These methods are imprecise, and experienced producers often disagree with each other by several percentage points when estimating by feel alone. But they are fast, free, and can keep you from making an obvious mistake like baling hay that is clearly still at 25 percent or above.
Electronic Moisture Meters
For anyone making hay as a business, a handheld moisture meter is close to essential. The most common type works by measuring electrical resistance: you push a probe into the windrow or a freshly made bale, and the meter reads out a moisture percentage based on how easily current flows through the forage. Wetter hay conducts electricity more readily. These meters are reasonably accurate in the moisture range that matters most for baling decisions, roughly 12 to 25 percent, but they have a well-documented weakness. Temperature inside the hay affects the electrical reading. Research using resistance-based probes found that when air temperature within the hay was included in the prediction model, accuracy improved substantially, with correlation values reaching as high as 0.95 at lower moisture levels around 20 percent.2ASABE Technical Library. Continuous monitoring of moisture content during hay bale drying At higher initial moisture levels, accuracy dropped. In practical terms, this means a probe reading taken in cool morning hay can give a different number than one taken in the same hay at midday, even if the actual moisture has not changed much.
Some newer meters use near-infrared sensors rather than electrical resistance, and these tend to be less affected by temperature. They are also more expensive. Regardless of the technology, the best practice is to take multiple readings across the field, both in thick windrows and thin ones, and pay special attention to the lowest-lying areas where dew lingers longest. A single probe reading from one spot can be misleading. Ten readings from different parts of the field give you a much better sense of whether the crop is uniformly ready.
For reference-quality accuracy, laboratories determine moisture by oven drying a sample and recording the weight lost, or by using a chemical method called Karl Fischer titration that directly measures water content.3Oxford Academic (Journal of Animal Science). Challenges in measuring moisture content of feeds These methods are not field-practical, but they illustrate an important point: all field meters are estimates. The oven-dry method itself is empirical and can vary depending on temperature and duration, so even “exact” moisture values carry some measurement uncertainty. Treat your probe as a guide, not gospel, and build in a margin of safety by targeting a few points below your threshold rather than right at it.
Why Baling Too Wet Is Dangerous
The immediate risk of baling high-moisture hay is spontaneous heating. When hay goes into a bale above 20 percent moisture, microbial activity kicks in. Bacteria and fungi feed on sugars and plant material, producing metabolic heat that raises the bale’s internal temperature. In a well-packed bale, that heat cannot escape easily, so temperatures climb. But microbial activity is not the only driver. Chemical oxidation reactions in the plant material continue even after the microbes die off, and these reactions depend on moisture being present. Research on spontaneous combustion found that these chemical reactions can push temperatures significantly higher than microbes alone would, and they do not require prior microbial activity to begin.4Journal of Applied Chemistry. Spontaneous combustion of hay At around 170°C, the hay has dried out internally, and the oxidation of that now-dry material can ignite it.
Barn fires from spontaneously combusting hay are not folklore. They happen regularly, particularly in seasons when rain interrupts curing and producers feel pressure to bale before the next storm. The progression from warm bale to fire can take days to weeks, which is part of what makes it so dangerous. A bale may feel warm on the outside but be approaching critical temperatures deep inside, where you cannot easily check.
Monitoring Bales After Storage
Even with careful moisture testing before baling, monitoring bale temperatures after you put hay in storage is a smart hedge. The standard approach is to push a long-stemmed thermometer or thermocouple probe into the center of several bales daily for the first two to three weeks. Researchers studying alfalfa-orchardgrass large round bales used thermocouples positioned at the geometric center of each bale and recorded temperatures daily, then tracked cumulative “heating degree days” above 30°C as a measure of both how hot and how long the heating lasted.5Journal of Dairy Science. Effects of bale moisture and bale diameter on spontaneous heating, dry matter recovery, in vitro true digestibility, and in situ disappearance kinetics of alfalfa-orchardgrass hays
For on-farm purposes, a few temperature benchmarks are useful. Internal bale temperatures up to about 50°C (120°F) during the first week are common and not alarming; the bale is “sweating” as residual moisture migrates and microbes do their initial work. If temperatures climb above 60°C (140°F), the hay is heating too aggressively and you should increase airflow around those bales, separate them from the rest of the stack, and watch closely. Above 70°C (160°F), you are approaching a fire risk and should not attempt to move bales without having water available. At that point you may need to call the fire department for guidance rather than rearranging a stack that could be smoldering inside.
Grasses Versus Legumes
Not all forages dry at the same rate, and this matters for deciding when hay is ready. Legumes like alfalfa and clover have thicker stems than most grasses, and those stems hold moisture much longer than the leaves. By the time a pure alfalfa stem is dry enough to bale safely, the leaves may be so brittle that they shatter during handling, taking a large share of the plant’s nutritional value with them. Grass hay, by contrast, tends to dry more uniformly because the stems are thinner and the difference between leaf and stem moisture is smaller.
One practical strategy is to grow grasses and legumes together in the same field. Research on field drying found that mixed stands reduce the impact of the slower-drying legume component, essentially letting the grass act as a spacer that improves airflow through the windrow.6ScienceDirect. Water and Quality Loss During Field Drying of Hay If you are making pure alfalfa hay, you have to be more patient or more aggressive with conditioning, because those thick stems will hold moisture long after the leaves are overdried.
Conditioning and Tedding
Mechanical conditioning at cutting is one of the most effective ways to speed field drying and narrow the window between mowing and baling. Conditioners work by crimping, crushing, or scraping the stems to break their waxy outer layer, allowing moisture to escape faster. A study comparing different harvesting systems found that a cutterbar mower paired with a roll conditioner and a rotary mower paired with a flail conditioner both dried hay faster than either mower type used alone.7ASABE Technical Library. Hay Harvesting System Losses and Drying Rates Late conditioning with a separate crimper after the initial cut also sped drying, though it came with higher field losses from leaf shatter and mechanical damage.
Tedding, the practice of fluffing and spreading the windrow to expose more surface area to sun and wind, is widely believed to speed drying. The same research found that tedding had surprisingly little measurable effect on drying rate compared to conditioning at cutting. That finding might seem counterintuitive, but the explanation is that tedding works best when the hay is still very wet, in the first few hours after cutting. Once the surface layer dries and the remaining moisture is locked in stems, fluffing the windrow does not help much. If you are going to ted, do it early while the hay is still lush, not the next morning when the outer layer has already crusted over.
Using Preservatives to Bale Wetter Hay
Sometimes weather does not cooperate, and you face a choice between baling hay that is still too wet or leaving it in the field to get rained on again. Acid-based preservatives, particularly propionic acid, offer a middle option. Applied as a spray at the baler, propionic acid inhibits mold growth and reduces the heating that causes dry matter loss in storage. At a 1 percent application rate, one study found that propionic acid cut dry matter losses during storage from about 15 percent down to roughly 8 percent and effectively prevented visible mold.8Agronomy Journal. Propionic Acid as a Hay Preservative Separate research confirmed that propionic acid treatment reduced heating and dry matter loss during the first month of storage compared to untreated high-moisture hay.9Applied Engineering in Agriculture. Preservation of Alfalfa Hay with Propionic Acid
The required application rate depends on how wet the hay is. For hay baled around 31 percent moisture, rates of 1.5 to 2 percent were needed to significantly lower storage temperatures and maintain forage quality. At 40 percent moisture, rates of 3 to 5 percent were necessary.10Agronomy Journal. Effects of Organic Preservatives on the Quality of Aerobically Stored High Moisture Baled Hay That is a lot of acid, and it is not cheap. Economic modeling of preservative use found that a treatment similar to propionic acid needs to cost less than roughly $4 per ton of dry matter for moderate or heavy use to break even, and less than about $8 per ton for occasional use.11Applied Engineering in Agriculture. Economic Potential of Preserving High-Moisture Hay In practice, preservatives make the most financial sense when you are saving a high-value crop like dairy-quality alfalfa from a rainstorm, not when you are trying to salvage low-value grass hay.
Buffered propionic acid products, which are less corrosive to equipment than straight propionic acid, are the most common commercial option today. They work on the same principle but are easier on baler components and safer to handle. Even with preservatives, baling above about 25 to 28 percent moisture is pushing the limits of what the chemistry can manage. Preservatives buy you a window, not a free pass to bale soaking hay.
Time of Day and Weather Judgment
Moisture in a hay windrow is not static. It drops through the afternoon as sun and wind pull water from the stems, then rises again overnight as dew settles. The driest window on a typical curing day falls between mid-afternoon and early evening, roughly 2 p.m. to 6 p.m. in temperate climates. This is the window most producers try to bale in. If your meter reads 17 percent at 3 p.m. on a sunny day, you are likely fine. If it reads 17 percent at 9 a.m. with dew still burning off, you should wait and retest, because the dew has probably only wet the outer layer of the windrow while the stems underneath could still be wetter than you think.
Humidity matters as much as temperature. A hot, humid day with no wind can leave hay sitting at 22 percent moisture for far longer than a breezy 75-degree day with low humidity. Relative humidity below about 50 percent is ideal for curing. Above 60 percent, drying slows dramatically, and above 80 percent, the hay can actually reabsorb moisture from the air. Watching the forecast for two to three consecutive days of dry weather after cutting is the simplest piece of advice and the hardest to follow in climates where rain is unpredictable.
When Hay Is Too Dry
Most of the focus is on avoiding wet hay, but over-drying is a real problem too, particularly with leafy legumes. Below about 12 percent moisture, leaves become extremely brittle. Every pass of the rake, merger, or baler knocks leaves off the stem. Since leaves contain a disproportionate share of the protein and digestible energy in legume hay, losing them to shatter lowers the feeding value of the bale significantly. Grass hay is less affected by over-drying because the leaf structure is different, but even grass hay baled below 10 percent tends to be dusty and less palatable to livestock.
The practical implication is that you are aiming for a window, not just a ceiling. For alfalfa and other legumes, that window is roughly 14 to 18 percent moisture. For grass hay, you can safely go a bit lower, down to around 12 percent, without major leaf loss. If the hay is already too dry by the time you get to it, baling in the early morning when a light dew has added a point or two of surface moisture can actually help reduce shatter. Some producers intentionally bale in a narrow predawn window for this reason, though it requires careful judgment to add just enough moisture without pushing the bale’s average above the safe threshold.
Common Mistakes and the Pressure to Bale
The single most common error is testing moisture in the wrong part of the windrow. The top of the windrow dries first because it faces the sun. The bottom, resting against damp ground, dries last. If you probe only the top or pull your test handful from the surface, you will underestimate the moisture in the bale’s core, because the baler picks up the entire windrow. Always test from the middle or bottom layer. For the same reason, raking two thin windrows into one thick one right before baling can trap moisture in the center of the new, thicker row. If you rake, give the combined windrow at least an hour of sun and wind before baling.
The other persistent mistake is psychological: the urge to bale before rain. When a storm is forecast and the hay is sitting at 22 percent, the temptation to bale and accept the consequences is strong. But the consequences of baling at 22 percent without preservatives can include losing a significant share of dry matter to heating, feeding moldy hay that reduces animal performance, and in extreme cases, losing a barn. Leaving the hay to get rained on is often the lesser evil. Rain-damaged hay loses color and some soluble sugars, but if you can get it dried properly afterward, it is still safe to store. Moldy, overheated hay is a bigger nutritional and safety problem than hay that got rained on once.