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‘Dusty or mouldy hay is bit of a divisive topic among equestrians. When horse owners and stable operators come across hay that doesn’t meet their expectations in terms of appearance, smell, and texture, their reactions can vary greatly. Some immediately discard the hay because it’s dusty or lacks the fresh, fragrant qualities they desire. On the other extreme, some still put the hay in the rack even when it’s visibly covered in grey-black clumps. As is often the case, the truth is a bit more nuanced.

Mould in hay is unacceptable!

It should be evident to everyone that spoiled hay is unsuitable for horses. It can lead to faulty fermentation processes in the large intestine, causing colic, laminitis, or chronic respiratory problems. In broodmares, it can even result in abortions. This is primarily due to toxins released by spoilage-indicating microorganisms, particularly moulds, present in the bales, including mycotoxins. Furthermore, many moulds produce antibiotic substances that can significantly disrupt the diversity and balance of the microbiome in the large intestine. Regular inhalation of mould spores can also trigger allergic reactions in horses’ respiratory mucous membranes, leading to chronic coughing. Therefore, those who prioritize hay quality are taking proper care of their horse’s health.
But how can you identify mould in hay?

Heufütterung im Offenstall
© acceptfoto / Adobe Stock

The smell inspection

Many equestrians expect their hay to always have the fragrant aroma of herbs, the distinct “hay smell.” What most horse owners associate with “hay” actually comes from the sweet vernal grass (Anthoxanthum odoratum). Its high coumarin content imparts that typical scent. If there is no bluegrass present in the hayfield, even the best hay will not have that aroma.

To train your olfactory senses for detecting hay scents, it can be helpful to sniff various types of hay. You’ll notice that each batch of hay has its own unique smell. It can range from “smells like nothing at all” to “herbal tea,” and occasionally hay can even take on different odours due to storage above a cow or sheep stable. What’s crucial is that it doesn’t have a musty smell, as this indicates the presence of mould. If the hay smells fermented, somewhat akin to fresh tobacco, it was likely packed too tightly, leading to unhealthy fermentation processes involving bacteria or yeasts. Such hay is also unsuitable for feeding. If it smells sour, it may either be (dry) haylage or the hay was treated with preservatives during the baling process, which unfortunately horses tolerate very poorly.

If you bury your nose deep into a large handful of freshly baled hay and detect a musty, fermented, sour, or otherwise unpleasant smell, it’s advisable not to feed that hay as a precaution.Instead, it’s best to have it tested in a laboratory to ensure its safety.

The shaking inspection

Determining whether the hay is dusty or not is typically only possible upon opening the bale and shaking the hay. If the bale has already been placed in the rack with a net over it, it may be too late to notice any issues. However, if you regularly stuff hay nets, fill racks with loose hay, or shake up hay to form little hay mountain in the corner of your horse’s stable, you will quickly discern if it is dustier than usual. It’s important to note that dust doesn’t always indicate the presence of mould.

Without further examination, it’s impossible to distinguish whether the swirling dust consists of mould spores or soil particles. Particularly when obtaining hay from wet meadows, it often appears dusty without being mouldy. This is because extreme dryness is required for machinery to navigate wet meadows; otherwise, the equipment would get stuck and sink in. The soil in such wet meadows usually comprises fine organic material that can easily be stirred up by the machinery due to dry conditions, subsequently ending up in the hay bale. Regardless of the efforts invested in hay production, this hay will always contain dust, but it can still be of excellent quality.

Even if the machines were adjusted properly, the hay can still be dusty afterward because root balls and clumps of soil get pressed into the bale. Dust alone doesn’t indicate whether the hay is mouldy or contains soil. To ascertain this, you need to smell it and possibly have it tested in a laboratory to know for sure.

Pferdeweide mit kurzem Gras
©Residence View / AdobeStock

The visual inspection

Many people assume that hay is good as long as no visible mould is present. Unfortunately, mould is microscopic and not easily detectable by the naked eye. Nonetheless, everyone is familiar with the typical appearance of a mouldy hay bale, characterized by compressed, grey-black plates found at the edges or in the middle of the bale.

This may occur, for instance, when the bale is stored directly on the ground without sufficient air circulation underneath. In such cases, moisture from the ground can infiltrate the bale, leading to the formation of these plates, referred to as “storage mould,” as it results from improper storage conditions. Hay that has been baled with excessive residual moisture often exhibits similar grey plates, typically appearing in the middle of the bale. In such instances, the mould is due to issues during harvesting.

It is a common practice in many stables to discard the sticky, grey areas of hay and feed the remaining portion. You might even hear statements like, “Oh, that’s not a problem, we’ll just remove the outer layers, and the hay will be fine.” This is akin to cutting off the mouldy section of bread and consuming the rest because there is no visible mould. The grey plates in the hay can be considered the mould’s blossoms, much like the greenish-grey fur spots on bread. However, the actual mould mycelium cannot be seen with the naked eye. Typically, it has already permeated the entire bale (similar to bread) by the time we notice these “blossom spots.” Consequently, bales with grey plates definitely belong on the muck heap – there is no need to even perform a smell or shake test.

If you are uncertain about the quality of the hay, whether it is good or possibly mouldy, you can send a sample to agricultural testing laboratories. An examination for “microbiological spoilage” will provide you with assurance about whether you can feed the hay to your animals in good conscience or better not.

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