Propionic Acid-Treated Hay: Is it Safe?

By Lynda M. Vanden Elzen

IMG_1463As horse owners, we always want to find the highest quality hay available for our horses.  Opinions vary regarding which type of hay is ideal, and a hay that is ideal for one horse may be detrimental to another.  One consistent requirement, however, is that hay is free of detectable mould and dust.  What horse owners may not be aware of is how narrow the window is for farmers to bale hay successfully, not only at the optimal plant maturity level, but also at safe moisture levels, while contending with unpredictable weather at the same time.  Losses due to rain and insufficient drying time can cost a farmer a lot of money, and cause heating, loss of nutritional value, and even spontaneous combustion.

Ideally, farmers tend to want to bale when the plants have dried to moisture levels from 13-17%, so that the hay is dry enough to avoid heating and significant loss of nutritional value and bale weight, but not so dry that it shatters.  Ideally, this hay will then cure to contain 10-12% moisture or less while maintaining nutritional value and palatability.  Mike Rankin of the University of Wisconsin Hay Extension points out:

“Moist hay that is put into storage can suffer extensive dry matter loss because of increased plant respiration and microbial activity. There is typically a 1% loss of dry matter for each percent moisture loss during storage to reach a stable equilibrium. These losses are from the non-fiber components of the plants.”

As a result of all of these factors, the risk of significant losses is high for hay farmers, and if wet hay moulds in storage, it will not be suitable horse feed.  In an effort to increase the window of acceptable moisture levels at baling, some farmers are now using drying agents, the most widely-used of which is called buffered propionic acid.

What is Priopionic Acid and Why Use It?

The Ontario Ministry of Agriculture, Food, and Rural Affairs defines propionic acid as follows:

“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.”

Propionic acid is a short chain fatty acid produced by microbial fermentation in the hindgut.  It is touted as safe because it is a natural acid that is already present in the horse’s digestive system.

In practical terms, treating hay with propionic acid during baling increases the baling window substantially, from the usual 13-17% moisture to up to 25-30% moisture.  Propionic acid application decreases the drying time needed significantly, once the hay has been cut, and also decreases the chance of heating and decreases nutrient losses once the hay is baled.  If a storm is coming when a farmer’s hay crop is drying before baling, the use of propionic acid can mean the difference between that farmer losing his entire crop and being able to bale and sell his hay.

According to Agriculture Victoria, the benefits of propionic acid use are as follows:

  • allow the safe baling of hay from slightly above target moisture levels up to 25% (or 30%) moisture depending on preservative type
  • allow baling after a shorter curing period which reduces risk of rain damage and sun bleaching. It may also allow baling earlier in a season in certain areas, when fodder is less mature and nutritive value higher reduce dry matter and nutrient loss caused by leaf loss and shatter, microbial activity and moulds enable baling over a longer period each day, resulting in more effective machinery and efficiency of labour usage
  • maintain hay colour (due to increased leaf retention) and often smells better
  • prevent dry matter and quality loss in storage due to bacterial, yeast and mould activity
  • reduce risk of spontaneous combustion
  • may increase animal intake
  • Animal and human health not affected due to lack of mould spores.

Many horse owners wish to avoid feeding hay that has been treated with propionic acid, but we also want pretty green hay that smells good, that hasn’t been bleached by the sun, is free of detectable mould and dust, and has optimal nutrient levels.  Perhaps propionic acid treated hay is something we should consider?

Disadvantages of Propionic Acid

Many sources suggest that propionic acid-treated hay is 100% safe to feed to livestock.  We have not found this to be true, necessarily, but due to a lack of scientific study, we can only hypothesize about potential harmful effects at this time, based on our anecdotal observations and those of equine nutritionists.

Handling Safety: In the early days when propionic acid was first introduced, it was found to be corrosive to baling equipment, so ammonium propionate (buffered propionic acid) was introduced instead, and is now recommended as an alternative.  Buffered propionic acid is much less corrosive and is safer to handle, but it is still corrosive and caustic.

Costs & Rate of Application:  As well, the amount of propionic acid applied to forage as it is being baled depends upon the moisture levels of that forage.  It is not as simple as moisture testing a few areas of the field and then calculating an average required.  Unfortunately, hay fields are not uniform, and so moisture testing must be done in various places and then the highest moisture reading must be used.  This means that the hay from an entire field is most likely to be sprayed with more acid than is required in order to compensate for a few wet areas.  Specialized equipment has been developed that tests moisture levels during baling and adjusts the quantity of propionic acid as it goes, but this is more costly.  Either way, propionic acid adds to the production cost, though if the farmer is in a situation where they may lose some or all of their crop due to rain after cutting and propionic acid treatment would save that hay, it may be cost-effective to treat the hay.

Storage Safety: Hay that has been treated with propionic acid, thus hay with a higher moisture content, should not be stored next to untreated hay, as the moisture from the treated hay can migrate to the dryer hay and cause it to mould.  As well, there is research that shows that over time, the propionic acid will dissipate from treated hay.  If at that time (4-12 months post baling) the moisture content in the hay is still too high, the bales can mould at that time.

Metabolic Effects:  Propionic acid is one of three volatile fatty acids (VFA) that are produced in the hindgut by the hindgut microbiome during forage fermentation.  Dr. Juliet Getty hypothesizes that due to propionic acid’s rule in glucose production, feeding hay treated with propionic acid to sugar-sensitive horses can be potentially harmful:

“Propionate is a major precursor toward glucose production through a process known as gluconeogenesis. And that’s a problem for the IR horse. Once propionate is absorbed and metabolized, it is converted to glucose! So when you feed alfalfa that has been treated with propionic acid, you are essentially increasing your horse’s blood glucose level, just as you would if you had fed a hay with a large amount of sugar and starch. Increased glucose leads to increased insulin. And the rest is… well, you get the picture.”

After reviewing some further studies on the effects of short-chain fatty acid fermentation, it seems that recent evidence suggests that there may be a link between the gut bacteria associated with propionic acid and a host of effects on the digestive system, brain, and behaviour.  Dr. Derrick F. MacFabe, MD, states:

“Propionic acid (PPA) and its related short-chain fatty acids (SCFAs) are fermentation products of ASD [Autism Spectrum Disorder]-associated bacteria (Clostridia, Bacteriodetes, Desulfovibrio). SCFAs represent a group of compounds derived from the host microbiome that are plausibly linked to ASDs and can induce widespread effects on gut, brain, and behavior. Intraventricular administration of PPA and SCFAs in rats induces abnormal motor movements, repetitive interests, electrographic changes, cognitive deficits, perseveration, and impaired social interactions.”

In Conclusion:

At this time, given what we know about the use of propionic acid as a hay preservative, it seems that there may not yet be enough conclusive information available for us to make the call about whether or not to feed treated hay to our horses.  There is certainly reason to be cautious, especially when feeding propionic-treated hay to sugar-sensitive horses.  In contrast, given the growing volatility of our global weather patterns, it may be necessary at times for some farmers to treat their hay in order to ensure that we will have the supply of hay we need to feed our livestock; treated hay is better than no hay at all!  As with most things horse-related, this is not a simple “yes” or “no” answer, and so it seems the most responsible course of action would be to watch for further study, and evaluate the suitability of propionic-treated hay on a horse by horse basis.



“Acid Treated Hay for Horses” – Harvest Tec (

“Preventing Mouldy Hay Using Propionic Acid” – Ontario Ministry of Agriculture, Food, and Rural Affairs (

“Selecting and storing horse hay” – Krishona Martinson, PhD and Paul Peterson, PhD, University of Minnesota Extension (

“Preserving Baled Hay with Organic Acids” – Mike Rankin, University of Wisconsin Extension (

“Hay Preservatives” – Agriculture Victoria (

“Hay Desiccants and Preservatives” – Dan Undersander, University of WIsconsin Extension (

“Role of short-chain fatty acids in the hind gut” – W. von Engelhardt , J. Bartels , S. Kirschberger , H.D. Meyer zu Düttingdorf & R. Busche (

“Alfalfa and the Insulin Resistant Horse – The True Story” – Dr. Juliet Getty, Ph.D. (

“Short-chain fatty acid fermentation products of the gut microbiome: implications in autism spectrum disorders” – Derrick F. MacFabe, MD (

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