Mold and Mycotoxins in Horse Hay
- Intake reduction or feed refusal.
- Reduced nutrient absorption and impaired metabolism, including altered digestion and microbial growth, diarrhea, intestinal irritation, reduced production, lower fertility, abortions, lethargy, and increased morbidity.
- Alterations in the endocrine and exocrine systems.
- Suppression of the immune system, which predisposes horses to many diseases.
- Cellular death causing organ damage.
What is Propionic Acid and Why is it Used on Hay?
Some hay growers apply preservatives (organic acids, yeast cultures, enzymes, etc.) to prevent the growth of the bacteria and fungi that sometimes cause heat, musty odor and mold in inadequately dried hay. Most preservatives applied to horse hay contain organic acids that are the same as those found in the horse's gastrointestinal tract. Propionic and acetic acid, the most common organic acids in hay preservatives, are produced naturally in the cecum and colon of the horse because of microbial digestion of fibrous feeds. These organic acids can be used as mold inhibitors and applied when hay is not yet dry enough to bale safely, but rain is coming, and the crop may be lost if not baled early. Studies have shown a decrease in the heating and molding of hay during storage with the use of preservatives.
A study conducted at the University of Illinois found that yearlings receiving hay treated with a mixture of propionic and acetic acids consumed just as much hay and gained just as much weight over a one-month feeding trial as yearling's consuming untreated hay. Clinical measures of the horses' wellbeing were not affected by consumption of preservative-treated hay, indicating that the hay had no negative effects on the horses. A study conducted at Cornell University showed that when given a choice, horses preferred untreated alfalfa to alfalfa that was treated with a mixture of propionic and acetic acid. However, when only given the choice of acid treated hay, the horses readily consumed it. Thus, after a short conditioning period, horses will consume acid-treated hay.
However, caution should be used when feeding hay that was baled at very high moisture levels, using higher levels of propionic acid. It is important to let that hay cure for several weeks so that the acid has time to dissipate and the hay has a chance to cure. This is especially true when feeding large round and square bales. There have been suspected cases of colic when horses were fed hay baled at very high moisture levels (29%) containing high levels of acid. Hay that is baled at high moisture levels should not be stored beneath or next to hay that was baled at appropriate moisture levels without the addition of an acid preservative. The moisture dissipating from the acid treated hay can move into the dry hay and cause it to mold.
