A study conducted at the University of Illinois found that yearlings receiving hay treated with a mixture of acetic and propionic acid consumed just as much and gained just as much over a 1 month feeding period as yearlings consuming untreated hay. Clinical measures of well-being such as serum enzyme levels were not affected by consumption of preservative treated hay, indicating the hay had no negative effects on the horses. A study performed at Cornell University found that when given a choice, horses preferred alfalfa that was not treated with a preservative over alfalfa that had been treated with a mixture of acetic and propionic acid. Beyond any doubt, the most important characteristic of good horse hay is cleanliness. Coughing is abnormal and not insignificant.
Many horses develop permanent lung damage after consuming moldy or dusty hay.
This chronic lung damage, commonly referred to as heaves, affects the horse's ability to breath normally and impairs their ability to exercise. Once a horse has been sensitized to hay dust, mold or pollen, it may react even when clean hay is fed. Moldy hay may also have other negative health effects as well. Therefore moldy hay should never be used for horses. In general, alfalfa is not any more likely to make a horse cough than any other hay, as long as it is dust and mold free.