Preventing Mouldy Hay Using Propionic Acid

Table of Contents

  1. Introduction
  2. Mould Damage
  3. Propionic Acid Preservatives
  4. The Hay Drying Curve
  5. Buffered Acid Products
  6. Follow Label Directions
  7. Recommended Moisture Levels
  8. Using Electronic Moisture Testers
  9. Applicator Capital Costs
  10. When Is Using Acid Most Economical?
  11. Cautions
  12. Conclusions


Trying to make dry hay between rainstorms can be frustrating. When haymaking periods without rain are short, we frequently get into a no-win situation. Either the hay isn't dry enough to bale before the next rain and it gets rain-damaged, or it gets baled "tough" before it is quite dry enough and becomes mouldy, poor quality, dusty hay. Propionic acid can be used as a dry hay preservative to prevent mould when baling hay at moistures that would otherwise be too high.

Mould Damage

Moulds greatly reduce the value of dry hay, particularly when targeting the "quality" horse hay or dairy hay markets. Moulds consume hay nutrients and cause dry matter losses, as well as produce toxins that are detrimental to animal health. Mouldy, dusty hay contains spores that can cause respiratory problems, particularly with horses. Mould growth can even result in hay fires from spontaneous combustion.

Propionic Acid Preservatives

Propionic acid is an organic acid that acts as a fungicide, inhibiting the growth of aerobic micro-organisms that can cause heating and moulding. Other organic acids, such as acetic and citric acids are sometimes also included, but propionic acid is the most effective as a mould inhibitor.

The propionic acid inhibits mould growth while the bales "sweat' and "cure" down to safe moisture levels by dissipation and evaporation. Do not confuse organic acid hay preservatives with enzyme, bacterial inoculant or nutritive additive products, which differ in modes-of-action and effectiveness. Propionic acid is sprayed onto hay as it enters the baler. Equipment includes a baler-mounted applicator with a pump, nozzles, and tank.

Hay treated with buffered propionic and other organic acid products is safe to feed to livestock. Propionic and acetic acids are organic acids that are produced by microbes in the rumen (and the cecum and colin of horses) and then used by the animal as part of the digestion process.

The Hay Drying Curve

A standing crop of forage is about 70 - 80% moisture. Initially the drying rate is quite rapid, but slows considerably when it gets to the low 20s. Getting the moisture down that last few percent before baling can take a lot of drying time.

Inevitably, there will be situations when the storm clouds are moving in, but the hay isn't quite ready to bale. Rain on almost-dry raked hay is much more damaging than rain on hay that has just been cut. Using propionic acid enables us to bale considerably earlier. This is especially true with poor, slow drying conditions, such as high relative humidity and low wind speed. With large square balers, the use of propionic acid is almost a necessity, because the moisture must be very low to avoid spoilage.

Buffered Acid Products

The original propionic acid products were unbuffered, which meant they were highly corrosive, very volatile, and difficult to work with. Products now marketed are buffered to a pH of 5.8 - 6.0 with ammonium hydroxide. Buffered products are much less volatile and corrosive, making them much easier to use. Other ingredients sometimes included are surfactants and green colouring. Products differ in concentration of propionic acid, so purchase decisions should be based on the price per kg of active ingredient.

Follow Label Directions

Read and follow label directions. Enough acid must be applied using the correct rate of active ingredient at various moisture levels for it to work properly. Different products have different concentrations of active ingredient. Using very dilute products provides greater coverage, but requires more water to be applied on the hay you are trying to dry.

Recommended Moisture Levels

Optimum moisture levels for safe storage vary according to bale type and density. Dry hay storage moisture guidelines without propionic acid for various bale types are:

Small square
15 - 18%
Large round (soft core)
13 - 16%
Large square & large round (hard core)
12 - 15%

Specific acid application rates at various moisture levels are detailed on the product labels. At lower moisture levels, product costs are typically in the $7 - 14 per tonne range. If targeting quality hay, these costs are easily recoverable. While some product labels indicate acid can be added to hay up to 35% moisture, this would be at a much higher risk of heating and spoilage, as well as significantly increasing the amount and cost of the product per tonne of hay, making this less practical. When using propionic acid, most hay producers seldom exceed 25% moisture.

Using Electronic Moisture Testers

An accurate measure of hay moisture is required to determine the proper application rate. Electronic moisture testers estimate percent moisture by measuring the resistance of electricity to move through a hay sample. The wetter the hay, the more electricity flows through. There are 2 basic types - hand-held probes and in-baler sensors. In-baler moisture sensors enable the operator to monitor moisture on-the-go from the tractor seat. Sensors can be located in-chamber on square balers, and on the sidewalls of large round balers. In-baler sensors have the advantage of giving numerous, continuous readings. Application rates can then be adjusted either manually or automatically according to the moisture. In-baler moisture sensors with automatic applicators are virtually standard on large square balers, and are also available for large round and small square balers.

Electronic moisture testers are an excellent tool, but keep in mind that they cannot guarantee there are no errors in application rates. Hay can gain or lose 3-5 percentage points of moisture in an hour, and there can easily be 5 percentage points of variation in a windrow. Accuracy is affected by bale density, whether it is grass or alfalfa, whether it is plant moisture or dew moisture, and whether acid has already been applied. Electronic moisture testers need to be calibrated to the conditions and well maintained. Beware that digital readings do not give you a false sense of accuracy. Moisture testers should be used to supplement personal experience.

Applicator Capital Costs

Basic acid applicators, including a small tank, pump and nozzles, start for about $1000. Probe-type hand-held moisture testers can be purchased for about $300. Of course, adding bigger tanks, in-line moisture sensors and automatic flow regulators can add a few thousand more to the cost.

When Is Using Acid Most Economical?

The main advantages to using propionic acid to preserve hay are less mould, reduced drying time, less potential rain damage and more weather suitable for baling. Using propionic acid provides baling flexibility. You can start earlier, quit later in the day and keep the baler baling when the weather isn't perfect.

There are 3 situations when propionic acid application to dry hay is most economical:

  • used strategically to avoid rain damage on "almost-dry hay" when the weather doesn't co-operate,
  • large dense bales that are difficult to dry to low enough moistures to avoid mould,
  • custom operators and producers baling large volumes that can pass the costs onto customers that demand mould- and dust-free hay.

Baling at higher moisture also reduces mechanical harvest loss from leaf shattering and should increase forage quality. So, does it pay to use propionic acid all the time and bale at higher moistures to prevent leaf loss, or only strategically when the weather doesn't co-operate? This will depend on the expected amount of raking and leaf loss, the final value of the hay product and the nutritional requirements of what it will be fed to. Routine acid application to reduce leaf loss would be more economical on alfalfa hay than on mixed or grass hay, and more beneficial when targeting higher value, well-stored, high quality hay.


TThere is a "learning curve" for a high batting-average when making "no rain, mould-free" hay. Although a useful and successful tool, the use of propionic acid will add to that learning curve. Errors can result in mouldy hay, or even worse, a dangerously heating mow.

Application at the correct and uniform rate is key. Uneven windrows or fields with wet spots will not have uniform moisture. Use a moisture tester to determine application rate, using the highest reading. If you use the average reading, you won't get enough acid on much of the hay to prevent spoilage. Spraying should be as uniform as possible to ensure good coverage.

Hay can still heat and become mouldy and discoloured if inadequate acid is applied. Tightly stacked bales in a confined area don't allow the bales to "sweat" and cure. The acid can dissipate in 4-6 months, which may be before hay moisture is low enough if conditions are unfavourable. Extended periods of high humidity will extend the curing time. Don't store treated and untreated dry hay in direct contact with each other as the moisture will migrate to the dry hay.

Some horse owners aren't comfortable feeding acid treated hay and prefer not to purchase it. There may initially be some propionic odour in the hay until it has dissipated. Be sure to inform hay buyers that propionic acid has been applied.


Propionic acid is most economical when used strategically to avoid rain-damage and mould with poor weather conditions. Propionic acid is very effective with higher density bales, such as large squares, that need to be drier at baling to avoid mould growth.

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Author: Joel Bagg - Forage Specialist/OMAFRA
Creation Date: 12 October 2004
Last Reviewed: 9 February 2010