Moldy feed: Risks and prevention

Horse eating hay

All of us have battled moldy feed. In fact, it’s unavoidable. We commonly encounter it in our refrigerators, whether it’s the spotty bread, or the expired Jello with a green layer of fuzz on the surface.

Reactions to mold in our own food stores will vary from “ick!” to “oh, just scrape it off.”

Much like we should minimize moldy foods in our own diets, we should minimize it in livestock diets as well.

We had a wet start to summer this year, and finding time to make hay was challenging for many producers during May and June.

August brought about somewhat drier weather, so mold may not be on your mind at the moment. But mold does not necessarily need wet weather to develop in hay. Dry hay with moisture levels as low as 15% can develop mold under some storage conditions.


Not all mold in feed is dangerous. Likewise, mycotoxins — toxins produced by fungi — can be present in feeds that appear to be mold-free.

It is hard to tell if moldy feed contains harmful mycotoxins or is free of toxins. If animals exhibit behavioral or health issues that cannot be explained by other causes, it may be time to take a closer look at the food source.

While feed processing and ensiling conditions may kill the microbial culprits, these conditions will not remove the toxins from the feed. Corn silage and haylage are especially prone to mycotoxin production, although many other types of feed, from wet brewers grains to protein concentrates, can also contain toxins.

In cereal products, such as distillers grains, mycotoxins are often more concentrated. Many mycotoxins can remain in feed for several years and can only be detected by having feed tested.

Equine and other non-ruminants tend to be more prone to the effects of the toxins in feed. Ruminants can tolerate some mycotoxin contamination because the microbial composition in the digestive system is able to break down some of the toxins.

Toxin symptoms

Symptoms of toxicity include, but are not limited to: circling, loss of muscular control, drop in milk production, infertility, gastro-enteric hemorrhage, skin lesions, vomiting, kidney and liver damage, blindness and excess slobbering.

Symptoms will depend on livestock species as well as type of mycotoxin and will often affect more than one animal eating the same feed. The effects of mold in the feed supply go beyond just acute symptoms. Microbes in feed can reduce the digestibility of the feed and, subsequently, energy and nutrients animals get.

In hay, total digestible nutrients and dry matter can be broken down by mold, leading to a reduction in feed quality. Mold will reduce palatability of hay, resulting in reduced intake of nutrients and energy. Combined, this can cause up to a 10% drop in performance if enough mold is present.

Furthermore, stressed animals may also have suppressed immune systems and poor hair-coats due to the poor quality of feed. Many problems associated with mold come from inhaling the spores. In horses, inhaling spores can lead to a condition called heaves, which is similar to asthma in humans.

Of course, farmers working with moldy hay can also inhale too many spores, which results in farmer’s lung. Once in the lung, spores can germinate and grow into the lung tissue. Pay your doctor a visit if a chronic cough or exhaustion occurs after handling moldy feeds.


Mold growth can contribute to the total heat produced by microbial activity in wet hay. Hay can lose quality if temperatures become warm enough, and if heat-loving bacteria are present, temperatures can reach up to 170 degrees. These conditions drastically increase the chances of spontaneous combustion.


So how can we prevent mold from occurring in our stored feed? Many of the fungi are already on the plant before it is mowed.

Once cut, fungi will begin to feed on sugars and exudates as the plant dries. If the plant material dries quickly in a windrow, these fungi will not have as much time to feed on the plant. Of course, drying quickly is weather-dependent and easier said than done.

Baled hay provides a more hospitable environment for a different group of fungi.

These fungi develop quickly when moisture levels are between 20% and 30% and temperatures begin to rise. This is when we get fungi that are capable of producing the mycotoxins, although toxin production should remain low if moisture levels do not increase beyond 30%.

Haylage and silage are more prone to mycotoxin contamination because of the higher percentage of moisture. The bottom line is controlling moisture.

Potassium or sodium carbonate can speed up the process and dry plant material quicker if you run into a period of wet conditions. Hay preservatives such as propionate can be utilized to limit microbial development and to bale in a timely manner if moisture cannot be easily controlled.

Barn design also plays a role. Areas open to the elements, spaces with poor drainage and poor ventilation in general should be minimized.

Minimizing risk

Sometimes we don’t have a choice. Moldy feed may be the only feed available. In these cases, minimize the amount of moldy feed given to horses and young and lactating animals, which are more vulnerable to exposure.

Get your hay tested to determine rations. If mycotoxins are suspected, do not give feed to animals until it has been tested and cleared. While animals may readily eat hay containing some mold and show no symptoms, it’s important to keep in mind that it just takes the right conditions for hay quality to decline and for mycotoxins to accumulate.

Sure, you may survive eating Jello after scraping the mold off of the surface. You just may end up with food poisoning. Likewise, livestock can eat some poor quality hay, but it might end up increasing vet costs and limiting production and performance.


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Erika Lyon is the OSU Extension educator for agriculture and natural resources in Jefferson and Harrison counties. She can be reached at or at 740-264-2212, x.203.



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