Hay Preservatives

Rainfall on mown hay significantly lowers forage yield and nutrient content. Shortening the field curing time reduces the risk of precipitation damage to the hay. Mechanical conditioning of freshly cut forage, tedding, and laying out wide windrows are excellent methods to hasten field drying of hay. Another tool for shortening the time from mowing to baling is to use a hay preservative, which can allow hay to be baled at a higher moisture content, up to 30%.

Generally, small square hay bales with a moisture content of 20% or less will not spoil during storage. Round bales should not be made until the hay is approximately 18% moisture. Large square bales should be 15% moisture or less. If hay is baled at higher moisture contents than recommended above, it often becomes moldy and the feeding value is reduced. Hay with a moisture content of about 25% provides an excellent environment for growth of aerobic microorganisms. These microbes rapidly break down the nutrients in the forage, resulting in a large increase in microbial populations and the generation of heat. The heat build-up causes a reaction to occur between proteins and carbohydrates, which renders both less digestible. Protein digestibility can be reduced to almost zero with severe heating. Heating also reduces the amount of dry matter that is ultimately stored.

The amount of heat necessary to produce a reaction between proteins and carbohydrates depends on several factors. Generally, bale temperatures less than 100 degrees Fahrenheit cause no problems, and it is not uncommon for bale temperatures to reach 110 F to 120 F for a few days. Bale temperatures above 130 F and especially 150 F almost always severely reduce protein and carbohydrate digestibility. When bale temperatures remain between 100 F and 150 F, the length of elevated temperature determines the amount of nutrient loss. Damage occurs more rapidly at higher temperatures. As temperatures increase above 150 F, so does the risk of barn fires.

Hay preservatives allow higher-moisture hay to be harvested without as many temperature risks, and can be used on legumes, grasses, and grass-legume mixtures. Preservatives work by altering the pH in the bale to inhibit or reduce the growth of aerobic microbes in the moist hay. Most hay preservatives do not improve the nutritional quality of the forage, but they prevent excessive microbial growth. With lower microbial growth, heating and the subsequent reduction in digestibility does not occur. Also, baling hay at a higher moisture content reduces leaf shatter, resulting in fewer dry matter and nutrient losses from hay harvesting. Ammonia-based hay preservative products are the exception. These products can improve the feed value of low-quality forages by improving nutrient availability for microbes in the rumen, but caution must be used to avoid toxicity risk.

Chemicals and Application Procedures

Hay preservatives can be grouped into three categories:

1. Organic acids
2. Ammonia-based
3. Microbial additives

These preservatives are designed to protect hay from harmful bacterial growth, reduce dry matter losses, and reduce the risk of heating. No preservatives have proven to be consistently effective on moisture levels above 30%, and their suppression of microbial growth on hay between 25% and 30% moisture can be limited.

Tanks and a spray application system must be attached to the baler for most preservative applications. Spray nozzles must be spaced to distribute the chemicals evenly over the forage as the forage enters the baling chamber. Systems need to be calibrated to the bale formation speed and applicator spray volume to ensure proper coverage of the forage. More detail can be found in “Value of Baler Preservative Applicators” (extension.psu.edu/value-of-baler-preservative-applicators) (Frankenfield, 2024). 

Organic Acid Preservatives

While a variety of organic acids, such as propionic, acetic, and lactic, have been used in trials and provided some level of effectiveness, propionic acid or a propionic/acetic mixture has proven most effective. This preservative is often available as ammonium propionate, which is buffered to reductive corrosiveness. Commercially available propionic acid-based preservatives are liquid formulations.

The amount of active ingredient that must be applied depends upon the moisture content of the hay. See Table 1 for an outline of application rates

Most propionic acid-based preservatives are effective at inhibiting bacterial growth in hay up to 25% moisture, with variable effectiveness when hay is between 25% and 30% moisture. Follow manufacturer directions for specific application rates depending on the product concentration. The product can be diluted to 50% solution with water to alleviate concerns about adequate coverage. Apply twice as much of the diluted acid to ensure the correct amount of active ingredient is applied.  Any unused diluted acid mix has the potential of freezing in cold temperatures.  Use precautions to prevent damage to application equipment.

The main disadvantages of propionic acid are its corrosiveness and the cost of the equipment necessary to apply the liquid to hay. Newer buffered formulations have greatly reduced corrosion problems. The estimated cost of adding spraying equipment to a baler starts at $1,000 or higher (2025 prices) based on options such as on-board moisture sensors and automatic adjustment of application rates based on current moisture readings.

Research at Cornell University and the University of Illinois has shown propionic acid-treated hay is safe for horses. While horses may prefer to eat untreated hay, horses that consumed treated hay have not shown differences in weight gain or health.

Ammonia-Based Preservatives

Ammonia is toxic to many microbes and can be a very effective preservative for moist hay (up to 30% moisture) when applied at 1% (dry-matter basis). Lower-quality straw, mature grass hay, and corn stover can be ammoniated at 2%–4% of the dry matter weight and be fed safely to ruminants. Higher quality forages such as alfalfa, immature grasses, and cereal forages should only be treated with ammonia at the rate needed for preservation (1% of dry matter) because of the risk of toxicity to the animals consuming it.

The chemical form of an ammonia-based preservative is usually anhydrous ammonia, and the major disadvantages of using it as a preservative are user safety and its difficulty of application. Devices are available to inject anhydrous ammonia into large round bales. The recommended means of treating moist hay with anhydrous ammonia is to cover the bales with plastic and then inject the appropriate amount of ammonia. However, the ammonia may not be distributed uniformly throughout the moist hay when anhydrous ammonia is injected into a bale. This can result in spoiling through portions of the hay.

Urea can be converted to ammonia by bacteria naturally found on hay. Application of urea is much simpler than using anhydrous ammonia gas. Researchers have found that relatively large amounts of pelleted urea (5%–7%, as baled weight basis) applied during baling can be an effective preservative for hay containing up to 30% moisture. Urea is only effective if the hay is covered tightly with plastic sheeting shortly after baling. If 1-mil plastic is used, multiple layers are necessary. The use of bale wrapping should be sufficient for retaining ammonia within bales, but this may create an anaerobic environment which could counteract the need for a preservative.

Applying the proper amount of ammonia is extremely important. Application rates below about 0.8% (dry-matter basis) are much less effective than the 1% rate. High-quality hay must not be treated with more than 1% ammonia (dry matter basis). Applying more than 1% ammonia to high-quality forages can result in the formation of an unknown toxic compound. Animals consuming ammoniated high-quality forage often exhibit hyperexcitability followed by death. The toxin is transferred into milk, so nursing calves and lambs are also susceptible to the toxin.

Microbial Preservatives

Many types of microbial products have been promoted for use as hay preservatives, but very little positive or consistent value has been demonstrated with these products in independent research trials. Microbial products count on proliferation of the introduced microbes and production of metabolites that prevent the growth of mold and yeast. These microbes often work through production of lactic acid or other compounds. They can reduce the growth of mold under certain conditions but as living organisms their effectiveness can vary greatly with environmental conditions. In general, the microbial products used as hay preservatives do no harm but have shown few consistent nutritional benefits.

Storage Considerations for Preservative Treated Hay

Preservative treatment of hay does not reduce moisture content of the hay. Thus, baled preservative-treated hay that contains high moisture levels should not be mixed with field-cured hay that is lower in moisture, as the moisture will migrate from the wet, preservative-treated bales to the drier bales, causing them to spoil. Do not allow wetter bales to touch drier bales and be sure to have ample airflow in storage areas.

Propionic acid-based products have been found to be the most effective, reliable, and practical for hay preservation. However, their preservative effect does not last indefinitely. A study of long-term hay storage demonstrated that propionic acid-treated hay bales limit dray-matter losses during the first 4 weeks of storage (Rotz, et al., 1991). The propionic acid content in the hay will dissipate over time. Thus, it is important to use preservative-treated hay as feed in a timely manner.

Advantages of Using Hay Preservatives

1. Preservatives allow hay to be baled at a higher moisture content, which reduces the length of time hay lays in the field and lowers the risk of rain damage.
2. Baling at a higher moisture content reduces dry matter and nutrient losses caused by leaf shatter during baling.
3. Preservatives lengthen the potential baling period. Hay can be baled during early-morning and late-evening hours if dew does not raise the moisture level above 25%–30%.

Disadvantages of Using Hay Preservatives

1. Unbuffered propionic acid is corrosive and can damage machines and injure workers.
2. Anhydrous ammonia is difficult to apply and is a hazardous chemical.
3. Some hay preservatives currently available may not work under many conditions. This is particularly true of microbial inoculants.

Summary

1. Propionic acid and anhydrous ammonia are effective at a moisture content of 20%–25%. Both can also be used at a rate of 1% of wet forage weight on hay with 25%–30% moisture, although the efficacy of propionic acid and anhydrous ammonia is less consistent when moisture content is above 25%.
2. Other preservatives may be effective on hay containing 20%–25% moisture (follow manufacturer directions), but many microbial inoculants have not been scientifically tested.
3. Preservatives other than ammonia and urea do not improve feeding value but can reduce storage losses.
4. It is essential that the moisture content of the hay be known. Application rates of preservatives should be modified according to the moisture content of the hay. Hay containing more than 30% moisture should not be baled as dry hay even with a preservative. It should be baled as baleage.
5. The cost of adding a preservative applicator to a baler and the cost of the preservative needs to be compared against the increased value of treated hay versus untreated hay.  Hay baled with too much moisture and no preservative will heat and lose quality.  Hay that is rained on will also decrease in quality.
6. It is important to feed preservative-treated hay in a timely manner, since the preservative effect begins to dissipate after one month of storage.

Additional Resources

• Acceptability of alfalfa hay treated with an organic acid preservative for horses
  (cdnsciencepub.com/doi/10.4141/cjas87-026)
• Forages, Ohio State University Extension
  (forages.osu.edu)
• Hay preservation effects on yield and quality
  (doi.org/10.2135/cssaspecpub22.c4)
• Meta-analysis of the effects of chemical and microbial preservatives on hay spoilage during storage
  (doi.org/10.1093/jas/skac023)

References

Frankenfield, A. (2024). Value of baler preservative applicators. Penn State Extension.

Rotz, C. A., Davis, R. J., Buckmaster, D. R., & Allen, M. S. (1991). Preservation of alfalfa hay with propionic acid. Applied Engineering in Agriculture, 7(1), 33–40.
elibrary.asabe.org/abstract.asp?aid=26187

This fact sheet was previously written by R. Mark Sulc, Professor Emeritus and Extension Forage Specialist, Ohio State University Extension; Bill Weiss, Professor Emeritus, Department of Dairy Science, Ohio State University Extension; and John Underwood, Professor Emeritus and District Agronomy Specialist, Ohio State University Extension.