Q. This year was difficult for harvesting hay, so the only hay I could get for my herd of horses has had preservative sprayed on it to prevent mold growth. Should I be concerned about this? Is it safe for them to consume?

A. Harvesting high-quality hay can be challenging for horse owners. To cut and bale premium hay, it must be cut at the right time, dried quickly, and baled at the correct moisture, then moved out of the elements and into storage. Although this sounds straightforward, it can be challenging because it relies heavily on weather conditions. In seasons that have a lot of precipitation, baling adequately dried hay becomes an issue. However, for horse hay, it is crucial that the bales not contain mold and are adequately dried.

Farmers monitor moisture content of forage throughout the harvesting process—when the hay is too wet mold is a concern and those bales could even become a fire hazard, and if the hay is baled too dry, there is significant nutrient loss due to leaf brittleness. Therefore, a happy medium is key.

Farmers might invest in inoculants or preservative sprays to reduce the likelihood of losing a crop because they allow the farmer to bale a hay at slightly higher moisture levels. For example, if the hay is not quite dry enough, but there is rain approaching in the forecast, the farmer might invest in a preservative spray for that field to ensure it can be baled prior to the rainfall. These products are most commonly used in wet and cool climates.

Manufacturers offer two main types of preservatives: bacterial inoculants and organic acids. Bacterial inoculants are typically characterized by the addition of lactic-acid-forming bacteria such as Lactobacillus, Streptococcus, and Bacillus. These organisms compete with mold-forming organisms in order to maintain forage quality. These products allow farmers to bale the hay at 3 to 5 percent higher moisture.

The other common preservative farmers spray on hay is organic acids (e.g., propionic acid). These products produce an acidic environment that is not supportive of mold growth. Again, this allows the hay to be baled slightly wetter so the grower can reduce the amount of time that it is in the field drying with a risk of it being rained on.

Farmers only use these products when absolutely required because they are an investment; however, without them, more hay would be lost in wet years, which would create shortages and cause hay prices to skyrocket.

Regarding horse health, researchers have shown that horses preferentially eat nontreated hay when given the choice. However, it is safe for them to consume treated hay. In one study looking at feeding hay treated with commercial preservatives to yearlings, scientists found there was no difference in feed consumption or weight gain between the treated and untreated hay groups. When we investigate hay treated with propionic acid, keep in mind that the product is buffered; despite it having a very low pH on its own, it is buffered to be closer to neutral (pH of 7) and will not cause harm to the horse when consumed. Additionally, the horse naturally produces propionic acid in the hindgut when hay is fermented.

Although farmers prefer harvesting conditions that allow for the baling of high-quality horse hay without the use of preservatives such as organic acids and bacterial inoculants, these products are a safe and effective way to reduce the risk of mold development in hay. So, from a nutritional standpoint, hay that has been treated with a preservative such as propionic acid is safe for horses.

Do you have a horse nutrition question?