Yes, hay is essentially grass that has been cut and dried, a process known as curing. This preservation transforms a highly perishable plant into a stable, storable feed source by arresting the biological processes that would otherwise lead to rapid decay. The goal of haymaking is not simply to dry the grass, but to preserve the maximum amount of its nutritive value for later use, especially during seasons when fresh forage is unavailable.
Defining Hay and Fresh Grass
Fresh, standing grass is a living plant, typically containing a high moisture content, often ranging from 70% to 85% water. This active biological state means the plant is constantly undergoing respiration and photosynthesis. When the grass is cut for hay, respiration continues, consuming the plant’s valuable stored sugars and starches. The fundamental difference between fresh grass and hay is the dramatic reduction in moisture, which halts the internal biological activity and prevents microbial spoilage.
Hay is the resulting product, where the plant material has been stabilized to a much lower moisture level. This arrested state allows the forage to be stored for extended periods without significant loss of quality. Curing is specifically designed to transition the plant from a highly perishable, high-moisture state to a storable, low-moisture state, defining hay as a preserved feed.
The Curing Process: How Grass is Transformed
The transformation of grass into hay involves a multi-step process engineered to rapidly remove water from the plant tissue. After the grass is mowed, it is spread out in a wide layer, known as a swath, to maximize its exposure to sunlight and air. This exposure helps keep the plant’s stomata, the pores on the leaves, open longer to allow for initial, rapid moisture loss.
As the grass dries, it is often mechanically turned or “tedded” to expose the underside of the swath and promote uniform drying. To accelerate the drying of the thickest parts of the plant, mower-conditioners are used to crimp or crush the stems. This mechanical action breaks the waxy outer layer, allowing water to escape at a rate closer to that of the thinner leaves.
The final stage is baling, which must occur only when the moisture content has been reduced to a safe level, typically below 20%. For dense large round or square bales, the moisture often needs to be even lower, sometimes as low as 15%, to prevent spoilage. Baling above this threshold creates an environment where aerobic microbes and fungi can thrive, leading to heating and the risk of spontaneous combustion.
Nutritional Differences and Preservation Goals
The primary goal of the curing process is to preserve the plant’s nutrient density, even though some nutritional trade-offs are unavoidable. After cutting, the plant continues to respire until its moisture level falls below approximately 40%. During this time, simple sugars (nonstructural carbohydrates) are metabolized and lost, which is a main consequence of a prolonged drying period.
Exposure to intense sunlight during curing causes the degradation of certain nutrients, most notably the precursors to Vitamin A. Despite these losses, the curing process successfully preserves the bulk of the structural carbohydrates, such as cellulose and hemicellulose. Preserving the protein content is also a major objective for livestock health. The quality of the final hay product is determined by how quickly the curing process can be completed to minimize nutrient-consuming respiration.
Hay Versus Straw and Silage
The term hay is often confused with other stored forages, but it is distinct from both straw and silage. Hay is defined by being cut specifically for its nutritional value, meaning it is harvested at a relatively immature stage of growth before the plant has fully developed its seed head. Its primary purpose is to serve as a calorie and protein-rich feed source for animals.
In contrast, straw is the dried stalk and stem material remaining after a grain crop, such as wheat or oats, has been harvested for its seed. It has a much lower nutritional value than hay because the plant was mature and most energy and protein were concentrated in the harvested grain. While hay is a feed, straw is mainly used for animal bedding or as a source of roughage fiber. Silage represents an entirely different preservation method, as it is forage cut at a higher moisture content (40% to 60%) and preserved through fermentation in anaerobic, oxygen-free conditions.