Ask the Expert: Horses and the use of preservatives when baling hay

Krishona Martinson
University of Minnesota Extension

Question: I’m searching for a 2021 hay supplier and a few of them I’m talking with use preservative (e.g. propionic acid) if needed when baling. What is that and is it safe for horses?

Answer: Preservatives are commonly used during times of frequent rainfall or poor drying conditions (e.g. high humidity or heavy dew). Propionic and acetic acids are commonly used hay preservative that are applied to hay as it is baled to allow baling of slightly wetter than normal hay with a reduced chance of mold formation during storage. Moisture at the time of baling is directly related to mold formation. Generally, hay baled at ≤15% moisture is unlikely to mold; however, this is impacted by bale-type and mass. For example, small square bales can be baled up to 17% moisture with limited risk of mold formation; however, large round bales must be baled at ≤15% moisture to reduce the risk of mold formation. Preservatives are most effective at inhibiting mold growth, and most economical, when they are applied to hay baled between 17 to 22% moisture.

In separate studies, researchers from Cornell University and the University of Illinois found that when given a choice, horses preferred hay that was not treated with a preservative over hay that had been treated with a preservative. However, horses readily consumed the treated hay when a choice was not given. Yearlings fed hay treated with a preservative had similar intakes and weight gains during the trial period compared to yearlings consuming untreated hay. Clinical measures of the yearling’s well-being were not affected by consumption of preservative-treated hay, indicating the preservative had no negative effects on the horses.

Interestingly, a horse’s hindgut bacteria actually makes propionic acid as a result of microbial fermentation. Therefore, researchers agree that feeding horses hay treated with preservatives is a safe and common practice, especially when poor weather conditions exist for making dry hay.

Baled hay treated with preservatives didn’t appear to have negatives effects on horses that ate the hay.