The moisture content of hay is monitored prior to baling. With too little moisture, hay becomes brittle, loses nutritional value, and can potentially become unpalatable. Too much moisture can cause hay to mold and experience thermal expansion in storage severe enough to cause combustion creating a severe fire hazard.
Optimum moisture content prior to baling is approximately 12-18% depending on the size of the bale. Spray on hay preservatives are used to enable a farmer to begin baling sooner, at moisture levels up to 30%. Various adverse health conditions can occur after long or short term exposure including inhalation and ingestion of “safe” levels depending on the concentrations applied and sensitivity of the livestock.
Some common ingredients used in hay preservatives are:
Ammonium propionate (buffered propionic acid) and propionic acid (more corrosive) are organic acids and the most common main ingredients in hay preservatives. Propionic acid was 1st registered as a pesticide in the early 1970’s. In a chronic feeding study using propionic acid, the high dose rats had hyperplasia, ulcers and other effects in the forestomach. Both can cause skin irritation, serious eye irritation and respiratory irritation. Classified by the U. S. Food and Drug Administration as "generally recognized as safe" (GRAS) but is not authorized as a food additive in Europe.
Propylene Glycol is able to lower the freezing point of water and used as aircraft de-icing fluid but is classified by the U. S. Food and Drug Administration as "generally recognized as safe" (GRAS) for use as a direct food additive. It is prohibited for use in food for cats due to links to Heinz body anemia. Possible adverse reactions are: skin Irritation, allergic reactions, potentially toxic to the kidneys and liver, neurological symptoms, cardiovascular problems, respiratory issues, potentially bio accumulative.
Ammonium Hydroxide is a colorless liquid chemical solution. It forms when ammonia dissolves in water and is found in many industrial products and cleaners. Some of these are flooring strippers, brick cleaners, and cements. Ammonium hydroxide is poisonous. FDA regulations classify ammonium hydroxide as safe (“generally recognized as safe” or GRAS) at certain levels. Potential adverse reactions are skin irritation, serious eye irritation and respiratory irritation.
Food Dyes; green No. 3, yellow No. 5 and blue No. 1 are commonly added to hay preservatives to present a fresh green appearance and improve marketability. These are carcinogenic.
What hays are more likely to contain preservatives?
Cool season grasses such as Timothy, Orchard and Rye because of the wet, cool climates where they are grown, cut, cured and baled in and alfalfa. Larger bales require lower baling moisture percentages; the risk of preservatives present in large square and round bales increases dramatically. Grass hays grown in arid climates (mostly warm season grasses) rarely if ever are sprayed with preservatives; the expense is not necessary. Feeding a combination of warm and cool season grasses is beneficial due to the diverse amino acid profiles and reducing the risk of preservatives in 100% of the hay fed.
Unfortunately hay preservatives are necessary for farmers otherwise too many crops would be destroyed and the price of hay could skyrocket. To be as proactive as possible, ask your grower if they use preservatives. If so, what cuttings/loads/fields received the lowest concentrations and purchase that hay. Inhalation is as much if not more of a health risk as ingestion; your horse's nose is in close proximity of their mouth. Slow feeders may minimize inhalation of acid/chemical dust considering they can't bury their nose in the hay. If you experience skin or respiratory irritation handling hay, preservatives could be the cause. Whether these additives and preservatives are safe is debatable. If given the choice, I'd rather not handle or feed hay with any additives. Being well informed about the forage you are feeding is prudent and enables you to make educated choices.
Our Nitrates In Hay Exposed article may be of interest.
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Propylene Glycol: The Complicated Additive with Potentially Dangerous Side Effects, retrieved from: https://draxe.com/propylene-glycol/
What’s the Right Moisture Content for Your Hay Bales? , retrieved from: http://www.delmhorst.com/blog/bid/354011/What-s-the-Right-Moisture-Content-for-Your-Hay-Bales
Food Dyes Linked to Cancer, retrieved from: https://draxe.com/food-dyes-linked-to-cancer/
Ammonium hydroxide poisoning, retrieved from: https://medlineplus.gov/ency/article/002491.htm
Scientific Opinion on the safety and efficacy of propionic acid, sodium propionate, calcium propionate and ammonium propionate for all animal species, retrieved from: https://www.efsa.europa.eu/en/efsajournal/pub/2446
US EPA Archive Document Propionic Acid, retrieved from: https://archive.epa.gov/pesticides/reregistration/web/pdf/4078fact.pdf