Hay is simply grass or other forage plants that have been cut and dried to a moisture content low enough to prevent spoilage, typically around 15 to 20 percent. Getting there requires managing a surprisingly complex set of variables, from sunlight and humidity to the speed of the conditioning rollers on a mower. Once the grass is dry and baled, it serves as the backbone of livestock feeding worldwide, but it also plays roles in erosion control, pet nutrition, and even meadow ecology that most people never think about.
How Grass Dries in the Field
The simplest and oldest method of making hay is field curing: cut the grass, spread it out, and let the sun and air do the work. But the physics behind that simple picture are worth understanding, because they explain most of the decisions a hay producer makes.
During daylight hours, solar radiation is the single biggest driver of how fast a swath of cut grass loses moisture. Research on switchgrass drying found that radiation correlated more strongly with drying rate than any other weather variable during the day. At night, the picture flips. With no sun, the vapor pressure deficit of the air takes over as the dominant factor, essentially how dry the surrounding air is and how aggressively it can pull moisture out of the plant tissue.1Agricultural and Forest Meteorology. Drying models to estimate moisture change in switchgrass and corn stover based on weather conditions and swath density
Wind speed behaves in a counterintuitive way. During the day, stronger wind can actually slow drying, because it strips away the heat that solar radiation builds up on the swath surface. At night, however, wind helps by moving drier air across the cut grass. The same study also showed that spreading grass in wider, thinner swaths speeds drying compared to narrow, dense windrows, with swath density negatively correlated with drying rate.1Agricultural and Forest Meteorology. Drying models to estimate moisture change in switchgrass and corn stover based on weather conditions and swath density
This is why farmers obsess over the weather forecast before cutting a field. A stretch of sunny, low-humidity days with moderate breezes is ideal. Cut grass that sits through an unexpected rain event picks up moisture, extends field time, and loses nutritional value with every extra hour it spends on the ground. Simulation models used in farm-management research incorporate sub-models for pasture growth, drying rates, expected losses, and weather forecasts, all to help producers decide whether the risk of cutting today is worth taking.2Agricultural Systems. Simulation of hay-making systems: Part 1— development of the model
Mechanical Conditioning at Cutting
A freshly cut grass stem is surrounded by a waxy cuticle that resists moisture loss. If you simply mow and leave the stems intact, drying takes considerably longer than it needs to. Mechanical conditioning cracks, crimps, or abrades that cuticle, giving moisture an easier exit path.
There are two main conditioning designs. Roll conditioners pass the cut forage between a pair of interlocking fluted steel or rubber rollers, crimping and cracking the stems. Impeller conditioners fling the forage against a metal housing at high speed, which scuffs and bruises the plant tissue. A field trial comparing these approaches found that grass conditioned at a faster impeller speed dried about 13 percent faster on the first day than grass conditioned at a slower impeller speed.3Applied Engineering in Agriculture. A COMPARISON OF FOUR MOWER CONDITIONERS ON DRYING RATE AND LEAF LOSS IN ALFALFA AND GRASS
The tradeoff is leaf loss. Aggressive conditioning removes more of the waxy barrier, but it also knocks leaves off, and leaves are where most of the protein and digestible energy live. Producers have to balance faster drying against lost nutritional value, especially with legumes like alfalfa, where the leaves are fragile and far more nutritious than the stems.
Tedding, Raking, and Moving Cut Forage
After the initial cut and conditioning, many producers use a tedder to fluff and spread the swath, exposing more surface area to sun and air. Later, a rake gathers the drying forage into windrows for baling. You might assume that tedding provides a significant drying boost, but research on hay harvesting systems found that tedding had little measurable effect on drying rate, and that different raking methods produced similar drying outcomes.4Transactions of the ASAE. Hay Harvesting System Losses and Drying Rates
That result surprises many farmers, but it makes sense when you consider the physics. If the swath is already spread wide by the mower-conditioner, tedding does not add much additional exposure. Where tedding does help is when the initial swath is narrow or when rain has matted the forage down. In those cases, physically lifting and spreading the material resets its exposure to air flow. Raking, meanwhile, is mainly a logistical step: it arranges the forage into a shape the baler can pick up efficiently, rather than contributing much to the drying process itself.
Chemical Drying Agents
When weather windows are tight, some producers spray chemical drying agents onto the forage at the time of cutting. These are not preservatives in the usual sense. They work by disrupting the waxy cuticle on the plant surface, letting moisture escape faster than it otherwise would.
The most studied agents are based on alkali metal carbonates, particularly potassium carbonate. Research on lucerne (alfalfa) found that potassium carbonate solutions cut drying time to a target moisture by about 65 percent, while sodium carbonate and lithium carbonate reduced it by roughly 55 and 45 percent, respectively. The potassium ion appears to have a specific role in increasing water loss through the cuticle, and the carbonate provides the high pH needed for this effect to work.5Grass and Forage Science. Accelerated drying of cut lucerne (Medicago sativa L.) by chemical treatments based on inorganic potassium salts or alkali metal carbonates
A combination approach using potassium carbonate plus potassium sorbate has also been tested. In one trial, the mixture not only shortened field drying time for alfalfa hay but also improved preservation during storage, with treated bales showing less heating and smaller dry matter losses than untreated bales.6Animal Feed Science and Technology. Hay desiccation and preservation with potassium sorbate, potassium carbonate, sorbic acid and propionic acid A separate study measured drying rates directly: alfalfa treated with a carbonate-citric acid blend dried at about 0.48 percent moisture per hour compared to 0.40 percent per hour for untreated forage.7PubMed. Effect of chemical drying agents on alfalfa hay and milk production response when fed to dairy cows in early lactation
Chemical drying agents remain a niche tool rather than standard practice. They add cost, they require a spray rig on the mower, and many producers simply prefer to wait for better weather. But in climates with short dry windows, or for high-value crops where every hour of field time risks rain damage, they can make the difference between baling good hay and baling compost.
What Happens Inside the Plant During Drying
A cut grass plant does not simply sit there passively losing water. It is still metabolically active for hours after cutting, and the conditions you dry it under determine what happens to its sugars and other nutrients. At low temperatures, below about 50°C, slow drying gives the plant’s own enzymes time to break down stored carbohydrates, and respiration continues burning through sugars. At very high temperatures, above about 80°C, thermochemical degradation kicks in and destroys some of those same compounds through heat.8Grass and Forage Science. INFLUENCE OF DRYING AND STORAGE CONDITIONS ON NONSTRUCTURAL CARBOHYDRATE ANALYSIS OF HERBAGE TISSUE—A REVIEW
The practical takeaway is that moderate, fairly rapid drying is the sweet spot for preserving nutritional quality. Field curing at typical summer temperatures achieves this for most producers. Barn drying with forced air can do it more reliably, but the equipment cost is significant. The worst scenario is slow drying in humid conditions, because every extra hour allows enzymatic degradation and continued respiration losses, meaning you are losing feed value you will never get back.
Preservatives and the Risk of Spontaneous Heating
Even after baling, the job is not quite done. Hay baled at a moisture content above roughly 20 percent is at risk of microbial heating, where bacteria and fungi begin growing inside the bale, generating heat as they consume plant sugars. In mild cases this degrades the hay’s nutritional value and palatability. In extreme cases the internal temperature can climb high enough for chemical oxidation reactions to take over, and the bale can spontaneously ignite. The phenomenon has been documented in agricultural science going back over a century, with researchers describing a progression from microbial fermentation to chemical self-heating to combustion.9Science. THE SPONTANEOUS HEATING AND IGNITION OF HAY AND OTHER AGRICULTURAL PRODUCTS
Propionic acid-based preservatives are the main tool for managing this risk. Applied at baling, they suppress mold and bacterial growth, keeping internal temperatures lower during storage. A study on large rectangular bales of alfalfa-orchardgrass hay found that propionic acid treatment provided clear protection against spontaneous heating, with a noticeable dose-response effect in the wettest bales, around 27 percent moisture. Both tested application rates effectively reduced heating in drier bales, and the modest improvements in nutritive value the researchers observed during storage were largely tied to the reduction in heating.10PubMed. Storage characteristics, nutritive value, energy content, and in vivo digestibility of moist, large rectangular bales of alfalfa-orchardgrass hay treated with a propionic acid-based preservative
Producers often use propionic acid as insurance. You cannot always predict whether conditions at baling were perfectly dry, and a stack of large bales represents a serious investment. Applying preservative to bales that end up slightly wetter than ideal is cheaper than losing a barn to a hay fire.
Hay Versus Silage and Haylage
Hay is not the only way to preserve cut forage. Silage (forage stored wet, at high moisture, in an anaerobic environment) and haylage (a middle ground, wilted to around 40 to 60 percent moisture before sealing) are common alternatives, especially in regions where reliable drying weather is scarce. Each method has distinct effects on what the animal actually gets to eat.
Early research comparing all three systems found that barn-dried hay generally produced the highest animal acceptance, milk production, and live weight gains, while high-moisture direct-cut silage ranked lowest on those measures. Haylage fell in between, outperforming direct-cut silage in palatability but producing similar milk yields. Storage losses told a different story: haylage stored in gas-tight silos lost as little as 4 percent of its dry matter, while direct-cut silage lost 22 to 24 percent.11Journal of Dairy Science. Preservation and Feeding Value of Alfalfa Stored as Hay, Haylage, and Direct-Cut Silage
More recent work has confirmed the intake advantage of hay. A trial comparing grass forages grown under identical conditions and conserved either as silage or as barn-dried hay found that cows fed hay ate about 2.4 kg more dry matter per day and produced significantly more energy-corrected milk, roughly 28 kg per day versus 25 kg per day for the silage group. Hay also preserved much higher levels of water-soluble carbohydrates, a readily fermentable energy source that gets consumed by microbes during ensiling.12PubMed. Grass forages grown under identical conditions and conserved as silage or barn-dried hay: Effects on feed intake, performance, apparent total-tract digestibility, and fecal microbiota in dairy cows
A separate comparison of hay and silage from similar parent grasses confirmed these patterns: hay-fed cows tended to eat more forage, ingested considerably more water-soluble carbohydrates, and produced higher milk fat yields. Interestingly, total chewing times were the same between groups, suggesting the physical fiber effectiveness was equivalent, but cows on hay spent less time ruminating per unit of feed consumed, indicating the hay broke down more easily during eating.13Animal Feed Science and Technology. Comparing the effects of silage and hay from similar parent grass forages on organic dairy cows’ feeding behavior, feed intake and performance
None of this means hay is always superior. Silage has real advantages in rainy climates where field-drying hay is a gamble, and it handles high-moisture crops like corn well. Haylage offers a middle path with low storage losses if the seal is maintained. The choice often comes down to equipment, weather, and what the producer is feeding.
Hay in Dairy and Beef Cattle Diets
For ruminants, hay is not just a source of calories and protein. Its physical structure matters. Long-stemmed hay forces the animal to chew more, and chewing drives saliva production, which buffers the rumen against excess acidity. Dairy nutritionists talk about physically effective fiber, which is essentially the fraction of fiber in the diet that is long enough to stimulate chewing. Studies have shown that increasing the amount of physically effective fiber in a dairy cow’s diet linearly increases both chewing time and ruminating time.14PubMed. Physically effective fiber: method of determination and effects on chewing, ruminal acidosis, and digestion by dairy cows
There is a wrinkle, though. While more effective fiber means more chewing, the relationship between chewing and actual rumen pH is less straightforward than you might expect. Research has found that increasing dietary particle size reliably predicts chewing activity, but increased chewing does not always translate into reduced ruminal acidosis.15PubMed. Effects of physically effective fiber on intake, chewing activity, and ruminal acidosis for dairy cows fed diets based on corn silage Rumen pH depends on a complex interplay of factors including how fast fermentable carbohydrates are being delivered, how quickly volatile fatty acids are absorbed, and the cow’s individual physiology. Still, hay remains a central component of most dairy diets precisely because its fiber provides a reliable baseline of chewing stimulation and gut function.
Fiber digestibility also varies between hay batches. Research on neutral detergent fiber digestibility has found significant differences between forage samples, with a positive link between forage fragility and how digestible the fiber actually is.16PubMed Central. Indigestible neutral detergent fibers: Relationship between forage fragility and neutral detergent fibers digestibility in total mixed ration and some feedstuffs in dairy cattle In practical terms, this means two bales of hay that look identical can have meaningfully different feeding values, which is why many operations test their hay rather than relying on visual assessment alone.
Hay for Horses
Horses are hindgut fermenters rather than ruminants, and their relationship with hay is different from a cow’s in some important ways. Hay is typically the foundation of a horse’s diet, often making up 1.5 to 2 percent of body weight daily, and the choice between grass hay and legume hay (usually alfalfa or clover) affects energy and protein intake considerably.
One area where hay quality matters enormously for horses is respiratory health. Dusty or moldy hay is a well-known trigger for inflammatory airway disease and recurrent airway obstruction, conditions that can end a performance horse’s career. A systematic review of harvested forages for horses found that silage and haylage contained fewer respirable particles than hay, which is relevant for horses with compromised airways.17Grass and Forage Science. Harvested Forages for Horses From a Feeding Perspective—A Systematic Literature Review Soaking or steaming hay before feeding reduces dust and mold spore counts and is a common management practice for horses with respiratory sensitivity. Wrapping hay at a slightly higher moisture, essentially making haylage, is another approach used in equine operations where respiratory issues are a concern.
Hay as a Staple for Rabbits and Small Herbivores
Hay plays a role that goes well beyond livestock. For pet rabbits, guinea pigs, and chinchillas, it is the single most important dietary component, and for reasons that have as much to do with dental health as with nutrition. These animals have continuously growing teeth, and the abrasive chewing required to process long-stemmed hay is what keeps those teeth worn to a healthy length.
Veterinary guidance on feeding rabbits and herbivorous rodents identifies hay as the recommended staple diet item, based on both practical availability and the health problems that arise when animals eat too much fruit, starchy vegetables, or grain-based pellets instead. Diets low in long-stemmed fiber are associated with dental malocclusion, gastrointestinal stasis, and obesity in these species.18Elsevier / Journal of Exotic Pet Medicine. Clinical Technique: Feeding Hay to Rabbits and Rodents Timothy hay is the most common recommendation for adult rabbits and guinea pigs due to its moderate protein and calcium content, while alfalfa hay is reserved for young or breeding animals that need more energy and minerals.
Haying and Meadow Ecology
An often-overlooked dimension of haymaking is its effect on the land itself. In managed grasslands and meadows, the annual cycle of cutting and removing hay has consequences for plant diversity and soil chemistry that accumulate over decades.
A long-running study on a small Danish freshwater meadow tracked the effects of annual cutting and hay removal over 21 years. The results were striking: vascular plant species richness tripled, climbing from 48 species to 150 over 19 years of continuous haying. Vegetation height and shoot mass declined markedly, and the proportion of tall, competitive grasses dropped. Soil organic matter and total and exchangeable phosphorus both fell by about half over the study period.19Elsevier. Long-term influence of hay-cutting on plant species richness, biodiversity and soil fertility in a Danish fen
The mechanism is straightforward: every bale of hay removed from a meadow takes nutrients with it. Over time, this nutrient depletion favors smaller, slower-growing species that are adapted to low-fertility conditions but get outcompeted when nutrients are abundant. The result is a more diverse plant community, which in turn supports a wider range of insects and other wildlife. Conservation managers across Europe deliberately use hay cutting as a tool to maintain species-rich grasslands, and the practice is built into many agri-environment schemes. The irony is worth noting: an agricultural activity often associated with intensive land use turns out to be one of the most effective tools for maintaining botanical diversity in grassland habitats.