The aroma of freshly cured hay should be sweet and pleasant, a sign of high-quality forage ready for storage. When the drying process stalls, this desirable scent is replaced by an abnormal, often musty or sour odor, indicating spoilage and reduced feed quality. This unwelcome change signifies microbial activity consuming the plant’s valuable nutrients, which compromises livestock health and leads to substantial dry matter loss. Eliminating this foul odor requires accelerating the drying rate to halt biological deterioration.
Identifying the Source of Undesirable Odors
The presence of a musty or sour smell is a direct result of uncontrolled microbial growth within the cut forage. Hay that remains above a safe moisture level, greater than 14 to 15 percent, provides an ideal environment for spoilage flora, including various molds and bacteria. These microorganisms actively respire, consuming the highly digestible carbohydrates—the sugars—within the plant material. This respiration generates heat, carbon dioxide, and water, which further accelerates the spoilage cycle.
The specific odor can help identify the dominant problem. A musty smell indicates mold and fungal contamination, while a distinct sour or fermented odor suggests anaerobic bacterial activity, common in tightly packed, high-moisture material. If hay is baled too wet, uncontrolled microbial consumption causes internal bale temperatures to rise, leading to heat-damaged protein and reduced nutritional value. Rapid moisture removal is required to stop this destructive biological activity.
Pre-Harvest Techniques to Ensure Rapid Drying
The most effective strategy against undesirable hay odors is prevention, beginning the moment the forage is cut by maximizing water loss. Timing the harvest to coincide with favorable weather forecasts, specifically cutting after the morning dew has evaporated, ensures the maximum number of sun hours are available for drying. This adjustment allows the drying process to start immediately, minimizing the prolonged plant respiration that consumes valuable stored energy.
A mechanical process called conditioning helps the thick, waxy stems dry at a rate similar to the thin, easily dried leaves. Roller conditioners crush or crimp the stems, while impeller conditioners abrade the stem surfaces, both creating channels for internal moisture to escape. Without conditioning, the leaves will dry too quickly, become brittle, and shatter during subsequent handling, leaving only the slow-drying, high-moisture stems in the field.
Managing the cut material’s exposure to the sun and air through proper swath management is the most important physical factor. Forage should be spread into a wide swath, covering at least 60 to 70 percent of the mowed area, rather than being left in a narrow windrow. This maximizes the surface area exposed to solar radiation, the primary driver of initial, rapid moisture loss. Spreading the hay wide significantly reduces field-curing time, helping the crop drop its moisture content from 85 percent to 60 percent in just a few hours.
A final pre-harvest consideration is the height of the cut, which should be around three to four inches for better air circulation. This higher stubble allows air to move freely beneath the swath, preventing the cut material from wicking moisture from the damp ground. This technique, combined with a wide swath, creates an open structure that uses solar energy and air movement to rapidly drive down moisture content and inhibit microbial colonization.
Remedial Actions When Odors Begin
If the distinctive musty or sour smell has already begun to emerge, immediate remedial actions are necessary to save the crop from extensive spoilage. The first step involves aggressive mechanical turning of the material using a tedder or a rotary rake to lift and invert the hay. This action exposes the wetter, lower layers of the swath to the sun and air, distributing the moisture more evenly for final drying.
Tedding is most beneficial when the hay is still relatively wet, ideally above 40 percent moisture content, to prevent leaf shatter, though it is often less suitable for fragile crops like alfalfa. Raking, which gathers the material into a windrow for baling, should be performed when the hay is between 30 and 40 percent moisture to minimize the loss of valuable leaves. This aggressive aeration stops the anaerobic conditions that promote fermentation, allowing the sun and wind to finish the drying process before baling.
When weather conditions are unavoidable or moisture content remains higher than desired, chemical preservatives offer a method to control microbial activity post-baling. Organic acids, particularly propionic acid, are the most effective and widely tested preservatives, acting as a fungicide to inhibit mold and bacterial growth. These acids are applied as a liquid spray directly into the baler as the hay is compressed, providing an acidic environment that suppresses the spoilage organisms.
The required application rate for propionic acid depends on the hay’s moisture content, increasing from about 0.5 percent of the wet weight for hay at 20 to 25 percent moisture, up to 1.0 percent for hay nearing 30 percent moisture. Although concentrated propionic acid can be corrosive to equipment, modern buffered formulations are available that reduce this risk without compromising effectiveness. While less effective than organic acids, some bacterial inoculants that produce lactic acid are also available, though they have shown inconsistent results in controlling spoilage in high-moisture hay.