Propionic acid-based preservatives have been evaluated in published research for nearly 40 years. Most studies have demonstrated some effectiveness in limiting spontaneous heating relative to untreated control hays. However, these responses often have been inconsistent. Work compiled by Rotz et al. (1991) summarized 10 experiments in which a propionic-acid based preservative was applied to alfalfa hay. Each experiment contained: i) a positive (dry) hay control that was baled between 10 and 20% moisture; ii) hays treated with the preservative at elevated bale moistures (20 to 37%); iii) untreated hays baled at moisture concentrations comparable to treated hays (19 to 40%). Acid application rates ranged from 1.0 to 2.3% of bale weight, but the product was diluted by 50% prior to application. Some experiments included more than one treated vs. untreated comparison. Results of these studies are summarized in Figures 3 and 4.

These results illustrate several key points:

• Results were inconsistent across experiments (Figure 3).
• Spontaneous heating was reduced, but not eliminated, within bales made at elevated moisture concentrations and treated with the preservative (Figure 3).
• Regardless of baling treatment, all hays exhibited a positive relationship between HDD accumulated during storage and initial bale moisture (Figure 3).
• Bales treated with the propionic acid-based preservative accumulated fewer HDD (27.2 HDD/percentage unit of initial ball moisture) than untreated control hays (58.9 HDD/percentage unit of initial bale moisture; Figure 3).
• Losses of DM were directly related to HDD accumulated during the first 30 to 45 days storage; therefore, applications of acid preservatives reduced DM losses primarily by limiting temperature (Figure 4).  In these studies, there were no clear reductions in DM loss after 6 months of bale storage that could be attributed to acid treatment.

Have propionic acid-based preservatives been effective in limiting spontaneous heating within large rectangular bales?

Previously, studies conducted with large-rectangular bales in Wisconsin (Shinners, 2000) have shown that losses of DM and forage quality are positively associated with initial bale moisture, and that the most effective management approach for maintaining quality and DM recovery is to bale hay at < 16% moisture. Unfortunately, this is not always feasible in humid environments, such as Wisconsin. Another observation obtained from studies with both small- and large- rectangular bales was that hays treated with propionic acid-based preservatives maintained greater concentrations of moisture throughout the storage period, and potentially up to 6 months after baling, which may be related to the hygroscopic nature of propionic acid (Rotz et al., 1991; Shinners, 2000). As a result, it can be speculated that biological activity may continue for longer periods of time in acid-treated hays compared to untreated hays.

Recently, propionic acid-based preservatives were evaluated in a study conducted at the Marshfield Agricultural Research Station (Coblentz et al., 2013). Large-rectangular (3 × 3 × 6-foot) bales of alfalfa-orchardgrass hay were made at 27.4, 23.8, or 19.6% moisture and treated with the preservative at 0, 0.6, or 1.0% of wet bale weight (613 to 644 lbs). The preservative was applied directly into the bale chamber of the baler, and HDD accumulated during storage are summarized for each baling treatment in Figure 5. For high-moisture (27.4%) hays, there was a clear application-rate effect in which bales treated with the 1.0% application rate accumulated only 30% of the HDD exhibited by untreated hays. Generally, the response with the 0.6% application rate was intermediate, exhibiting some reduction of HDD relative to untreated hays, but heating more severely than hays treated with the 1.0% application rate. Hays baled without the preservative at 23.8 and 19.6% moisture accumulated 549 and 463 HDD, respectively, which was far greater than observed for treated hays, regardless of application rate. Within these drier hays, both application rates performed similarly. 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 dissipation heat out of the stack.

In the same experiment, hays were offered to growing lambs to assess total-tract digestibility. Generally, the digestibility of forage nutritional components was related directly to the spontaneous heating that occurred within each baling treatment (Figures 6 and 7). Digestibility of organic matter (OM) from these hays declined at a rate of approximately 0.5 percentage units per 100 HDD; therefore, digestibility of OM was depressed by about 4 percentage units over the range of HDD exhibited by this selection of baling treatments. Conversely, digestibility of heat-damaged protein (ADICP) increased with spontaneous heating (Figure 7). There is some native ADICP in all forages, which is assumed to be completely indigestible. In practice, the Marshfield research data supports this premise, exhibiting a y-intercept of 16% of ADICP, which is an estimate of ADICP digestibility when HDD = 0 (no heating). However, concentrations of ADICP increase as hays incur spontaneous heating during bale storage, and some of this additional (heat-damaged) ADICP retains some (limited) bioavailability (Figure 7). It is important to emphasize that the positive association between ADICP digestibility and HDD is not desirable from a management perspective, and that heating substantial enough to cause these responses is accompanied by losses of energy from the forage, primarily through respiration of highly digestible plant sugars. This concept is illustrated for these same hays in Figure 8, where TDN declined in a close linear relationship with HDD, losing about 0.5 percentage units of TDN for every 100 HDD incurred during hay storage. Generally, losses of energy (TDN) are often overlooked as an essential component of hay production and management. A recent survey study reported that 50% of 3612 hay or haylage samples sent to commercial laboratories lost at least 4 percentage units of TDN via spontaneous heating during storage (Yan et al., 2010). Among these damaged hays, 586 (16% of all samples) lost at least 8 percentage units of TDN; these losses of energy constitute a significant economic loss for many livestock producers.

Have these propionic acid-based preservatives been studied for use on large-round bales?

Unlike the studies with large-rectangular bales, recent experiments at the Marshfield Agricultural Research Station with 5-foot diameter round bales of alfalfa hay have been disappointing (Coblentz and Bertram, 2012). In these studies, acid preservative was applied at a common rate (0.5 ± 0.14% of wet bale weight) across 42 alfalfa hay bales ranging in moisture concentration from 10.2 to 40.4%. 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 9 depicts the heating responses within these hays as affected by initial bale moisture during the first 28 days of storage. Acid treatment reduced heating during this time period in all hays baled at < 38% moisture relative to untreated controls; however, there was considerable heating in all hays baled at > 20% moisture, and the differential between treated and untreated hays was limited (≤ 241 HDD). When the evaluation period was extended to include the entire time interval that bales accumulated HDD, there was very marginal benefit to acid application. Accumulations of HDD were reduced with acid treatment in all hays baled at < 28% moisture (Figure 10), but the maximum difference between treated hays and untreated controls was only 121 HDD in hays baled at about 20% moisture. Hays baled at > 28% moisture actually accumulated more HDD when treated with a preservative than the corresponding untreated control hays.

The vastly different responses between large-round and large-rectangular bale types also are difficult to explain. Acid-treated hays in the large-round bale study retained more moisture for months after baling compared to untreated control hays, much as described previously for small- and large-rectangular bales (Rotz et al., 1991; Shinners, 2000). Furthermore, observations made during the trial suggest that this retention of moisture permitted extended microbiological activity. In effect, treated large-round bales exhibited what can be described as a low-grade fever for an extended period of time (often weeks) relative to untreated control hays, and simultaneously accumulated HDD at a slow, but consistent, daily rate long after untreated hays ceased heating.

For large-round balers, acid-preservatives often are applied through after-market distribution systems in which application nozzles are 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. Normally, the preservative is applied at constant pressure; therefore, the application rate is determined by the size of the spray nozzles, and the time needed to make a bale. Even the most diligent hay producer would not be expected to change spray nozzles more than once or twice per day in response to hay condition; therefore, application rate per bale is primarily a function of baling time. This often works against the operator because the forage in thin windrows also is often drier, yet more time is required to form a bale, resulting in over-application. Conversely, heavy windrows often are wetter, but the bale also is made faster, resulting in under-application. Some of these application problems can be overcome by altering baler ground speed, but this represents very coarse rate control. In addition, it becomes difficult to increase percentage-based application rates in wetter hays (for example, 1% of wet weight) because attempts to increase the amount of applied preservative are offset by concurrent bale-weight increases resulting from trapping additional water in the bale.

If acid-preservatives are to be used with large-round bale packages, a few suggestions for improved performance could be considered:

1. 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.
2. Satisfactory results with large-round bales are most likely to be obtained when hays are < 25% moisture at the time they are baled (drier is better). This management option (acid preservative) 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.
3. When using after-market distribution systems on large-round balers, use raking and/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 that is actually applied to the forage, and limit losses discharged onto the ground.
4. Never (ever) assume that acid-treated hays are ‘automatically safe’ from fire or significant losses of DM, protein availability, and energy. Use extreme caution with respect to confined storage, such as in barns or hay sheds.

Summary and Suggestions