Most hay producers are quite familiar with the problems associated with baling moist hay. Normally, these problems include spontaneous heating, increased evidence of mold, loss of dry matter (DM) during storage, poorer nutritive value and (in extreme cases) spontaneous combustion.

Normally, spontaneous heating is initiated soon after baling via respiration by active plant cells or micro-organisms associated with the hay.

These processes consume plant sugars in the presence of oxygen to yield carbon dioxide, water and heat. Moisture thresholds for safe hay storage vary with bale type and size but generally are 18 to 20 percent for small (less than 100-pound) bales and 3 to 5 percentage units drier for larger bales.

Experimentally, spontaneous heating often is measured using a heating-degree day (HDD) concept, which is calculated as the daily difference between the internal bale temperature and 86ºF, summed over all the days in storage.

For discussion purposes, this can be viewed as a single number that represents both the magnitude and duration of heating within a bale.

Numerous factors influence spontaneous heating: bale moisture, bale type or size, bale density, environmental factors such as air movement or humidity, storage site and use of preservatives.

Within a given bale type and size, the moisture concentration of the forage at baling is the most important factor affecting heating.

However, large hay packages are much more likely to achieve greater internal bale temperatures than traditional small (less than 100-pound) rectangular bales. These concepts are summarized from recent work conducted in Wisconsin (Figure 1).

Large round bales of alfalfa-orchardgrass hay were made in 3-foot, 4-foot and 5-foot diameter bales over a wide range of bale moistures, ranging from about 10 to 45 percent.

Several important points can be gleaned from the data: Spontaneous heating is greater in wetter hays; greater HDD were measured in larger-diameter bales; for 3-foot and 4-foot diameter bales, the relationship between HDD and bale moisture was linear, but linearity was lost and HDD were accumulated at a more rapid (quadratic) rate within 5-foot diameter bales; and there was little scatter or variability of data points around the regression lines, indicating bale moisture is the primary factor driving heating within any specific bale diameter.

Use of preservatives
Generally, hay preservatives can be grouped into three general categories that include organic acids or their salts, ammonia-based products and microbial additives.

Of these, the most commonly used are various formulations of organic acids; normally, these formulations contain mostly propionic acid.

Theoretically, these products work by inhibiting growth of aerobic microbes within the hay, thereby reducing microbial respiration, accumulation of heat and limiting losses of DM and reductions in nutritive value.

Propionic acid-based preservatives are applied as liquids and are usually buffered to reduce corrosive damage to farm equipment. Historically, the performance of propionic acid-based preservatives has been evaluated in published research for nearly 40 years.

Although results have been somewhat inconsistent, most studies have demonstrated some effectiveness in limiting spontaneous heating relative to untreated control hays.

Researchers in 1991 summarized 10 experiments in which a propionic acid-based preservative was applied to small rectangular bales of alfalfa hay.

Each experiment contained a positive (dry) hay control that was baled between 10 and 20 percent moisture, hays treated with the preservative at elevated bale moistures (20 to 37 percent), and untreated hays baled at moisture concentrations comparable to treated hays (19 to 40 percent).

Acid application rates ranged from 1 to 2.3 percent of bale weight, but the product was diluted by 50 percent prior to application. Results of these studies are summarized in Figure 2 and illustrate several key points.

•   Results varied across experiments.
  
  
• Regardless of baling treatment, all hays exhibited a positive relationship between HDD accumulated during storage and initial bale moisture.
  
  
• Spontaneous heating was reduced, but not eliminated, within bales made at elevated moisture concentrations and treated with the preservative.
  
  
• Bales treated with the propionic acid-based preservative accumulated fewer HDD (27 HDD per percentage unit of initial bale moisture) than untreated control hays (59 HDD per percentage unit of initial bale moisture).

Recent studies with large hay packages
Recently, large rectangular 3×3× 6-foot bales (613 to 644 pounds) of alfalfa-orchardgrass hay were made at 27.4, 23.8 or 19.6 percent moisture and treated with the preservative at 0, 0.6 or 1 percent of wet bale weight. In this experiment, the preservative was applied directly into the bale chamber.

For high-moisture (27.4 percent) hays, there was a clear application-rate effect in which bales treated with the 1 percent application rate accumulated only 30 percent of the HDD exhibited by untreated hays, with the 0.6 percent application rate providing an intermediate response.

Hays baled without the preservative at 23.8 and 19.6 percent moisture accumulated far greater HDD than observed for treated hays, regardless of application rate.

Within these drier hays, both application rates performed similarly. However, it is important to note that hay bales evaluated in this study were monitored for heating on an individual basis (no stacking).

Caution should be applied in extrapolating these data to stacks of large rectangular bales, in which there is less surface area per unit of DM and fewer routes for heat dissipation out of the stack.

Unlike the studies with large rectangular bales, other recent experiments with 5-foot diameter round bales of alfalfa hay have been disappointing.

In these studies, acid preservative was applied at a common rate (0.5 percent of wet bale weight) across 42 alfalfa hay bales ranging in moisture concentration from 10.2 to 40.4 percent.

Acid-preserved bales were paired with untreated controls made from the same forage. All bales were stored outdoors on individual wooden pallets until internal bale temperatures became ambient.

Figure 3 summarizes the heating responses within these hays; unfortunately, there was only very marginal benefit to acid application.

The vastly different responses between large round and large rectangular bale types are difficult to explain within the space limitations of this article.

One potential explanation involves the configuration of balers. For large round balers, acid preservatives often are applied through after-market distribution systems mounted directly above the pick-up attachment on the baler.

Preservative is sprayed over the entire pick-up area, which results in potential for waste, especially when windrows are thin. If acid preservatives are to be used with large round bale packages, a few suggestions for improved performance could be considered.

•  Limit bale diameter – A 4-foot diameter round bale will have greater surface area per unit of DM and greater potential to dissipate heat compared to a 5-foot diameter bale.
  
  
• Satisfactory results with large round bales are most likely to be obtained when hays are less than 25 percent moisture at the time they are baled (drier is better).
  
  This management option (acid preservation) appears to be most viable with hays that are only marginally too wet to bale. Wetter hays are less likely to respond to preservative treatment.
  
  
• When using after-market distribution systems on large round balers, use raking or merging equipment to ensure that a windrow the full width of the baler pickup is presented to the baler.
  
  This will maximize the percentage of the preservative actually applied to the forage and limit losses discharged onto the ground.
  
  
•  Never (ever) assume that acid-treated hays are “automatically safe” from fire and use extreme caution with respect to confined storage, such as in barns or hay sheds, which will likely deter dissipation of heat and moisture.

Hays baled in large hay packages have an increased likelihood of heating, reaching very high internal bale temperatures and combusting spontaneously. Be cautious and monitor hay moisture in the field closely before baling.

References omitted due to space but are available upon request. Click here to email an editor.