Propionic acid is effective in inhibiting growth of molds, yeast and bacteria in hay, and buffered products are as effective as non-buffered, according to the University of Kentucky Forage News. In one study, a buffered propionic acid product significantly improved nutrient yield in high-moisture hay. But the improvements weren’t large enough to pay for the preservative, the newsletter reports.

In order for propionic acid to be effective, correct levels must be used. Commercial solutions vary from 10% to 100% propionic acid. The use of very dilute products isn’t recommended because more water is applied to the crop. Because only parts of bales may be high in moisture, it's important to distribute the acid uniformly. Depending on the product and application rate, it will cost $5-20/ton of hay to treat with a propionic acid-based product. Suggested application rates are 10 lbs of 100% propionic acid equivalent per ton of 20-25% moisture hay; 20 lbs/ton of 26-30% moisture hay, according to the newsletter.