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Thirteen trials were conducted to evaluate propionic acid as a preservative for high-moisture alfalfa hay using spray equipment and procedures available to a farmer. An experimental device was also evaluated which injected the preservative into hay in the baling chamber of a rectangular baler. Propionic acid treatment provided a reduction in heating and dry matter loss during the first month of storage with less visible mold in the hay following storage when compared to untreated high-moisture hay. After six months of storage, dry matter loss in high-moisture hay was similar with or without the acid treatment. When compared to untreated dry hay, the treated high-moisture hay had much greater heating and dry matter loss during storage as well as more visible mold, poorer color and greater fiber and fiber bound protein concentrations following storage. Propionic acid injected into bales did not perform as well as spray application because the acid was not dispersed uniformly throughout the bale.