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You hear it increasingly: hay disinfection. But what should we make of the advertising claims regarding preservatives and disinfectants? Super solution for problematic hay quality or potential disaster for the digestion?

Why is the availability of high-quality hay diminishing?

In recent years, summer weather has become increasingly unpredictable. Longer dry periods are becoming less common, while heavy rainfall events are becoming more frequent, making it increasingly challenging to produce high-quality and, importantly, dry hay. How wonderful it would be if there were a method to preserve hay that hasn’t been dried optimally, or to neutralize mould spores present in the hay.

The residual moisture content of good hay should ideally be below 14%, a target that becomes virtually unattainable in some summers. However, damp and warm winters also worsen the issue of hay spoilage. During cold and dry winter days, the proliferation of microorganisms present in hay, a natural product, is limited. However, in winters as wet and warm as those we’ve experienced in recent years, the microorganisms present in even the finest hay at harvest time will find optimal conditions to multiply during storage. For instance, hay that has been harvested at its peak and stored in optimal dry conditions can still end up moldy by the end of winter.

There are now numerous remedies available to address issues with moldy hay. These primarily consist of preservatives. Potassium sorbate, sodium benzoate, and sodium propionate actually originate from silage production. However, they are increasingly being advertised as suitable for hay with a residual moisture content of 75% at harvest (in comparison: good hay should ideally have 10-14%) if applied directly during harvest. However, there are also products that are only added after the harvest or shortly before feeding, but by the time they are added, the hay is typically already spoiled.

Mouldy pile of hay
Haylage is not optimal for the horse’s stomach and intestines. © Sanoanimal / Fritz

The same principle applies to hay treated with preservatives from silage production (also known as “preserved” or “injected” hay) as to the feeding of haylage. These agents strongly acidify the hay, and the lower pH value prevents the growth of microorganisms. However, this is not optimal for the horse’s stomach and intestines.

The products applied to the hay shortly before feeding are known as hay disinfectants, which essentially consist of water.

Sodium hypochlorite is used in swimming pools and also in drinking water to disinfect it in case of contamination or for general disinfection purposes. Anyone who has ever consumed chlorinated tap water while on holiday can understand that it’s not a culinary highlight.

According to the manufacturer, sodium hypochlorite should be entirely safe for horses, even if the hay is treated with it daily. Unfortunately, there are no studies to substantiate this claim. Anyone who spends a lot of time in the swimming pool knows the impact of chlorine vapours on the mucous membranes. Horses have much more sensitive mucous membranes than humans, and it’s challenging to imagine that “chlorinated hay” would be palatable to horses in the long term.

But taste is only one aspect of the issue.

Much more important is the question: What effects do these substances have on the mucous membranes in the mouth and gastrointestinal tract?

If hypochlorous acid is used in large quantities or in enclosed spaces, it can irritate the mucous membranes. Coughing, conjunctivitis, and respiratory issues can result from exposure to hypochlorous acid. Chlorine vapors are also suspected of contributing to the development of asthma in very young children. To date, there have been no studies on the effects of horses consuming hay that has been disinfected with chlorine. It is quite conceivable that it may not be well tolerated by the mucous membranes in the stomach or intestines, not to mention the potential effects of the disinfectant on the essential microbiome of the large intestine.

Other disinfectants for hay contain 1,2-propanediol, which is cautioned against for consumption, particularly for allergy sufferers, the very group that should avoid dusty hay. Under no circumstances should it be used to vaporize the hay, as aldehydes such as toxic formaldehyde can be produced.

But what is the alternative?

If the hay is truly so dusty that it poses a health hazard for the horses and you must feed it because no other option is available, then steaming is currently the only method that effectively renders the molds and other spoilage microorganisms harmless without harming the horse. Temperatures of around 90 degrees are reached during steaming, ensuring that the microorganisms present are reliably killed.

Hay that has been steamed in this manner should also be fed immediately and should not be stored for too long, as otherwise, unwanted microorganisms can recolonize the damp hay. Moreover, steaming hay for an entire stable can be quite expensive. But it is a sensible and healthy alternative, especially for bales that are no longer of perfect quality. Disinfecting the hay with sodium hypochlorite can also inadvertently disinfect the horse from the inside, leading to extremely problematic side effects.

If the goal is simply to reduce dust in the hay to prevent coughing, another option is to lightly moisten the hay with finely atomized water. A pressurized pump sprayer, commonly used in horticulture, is particularly suitable for this purpose.

Of course, it is best to ensure good quality at the time of harvest and to store the bales with adequate ventilation to prevent excessive infestation with microorganisms from occurring in the first place. If the hay must be harvested with a high residual moisture content, warm air drying is also a very effective alternative that produces high-quality hay.


Despite all advertising promises to the contrary, neither preservatives nor disinfectants are suitable for feeding to horses.

Sodium hypochlorite is simply a useful surface disinfectant to thoroughly clean the hay store after a moldy batch, ensuring that proper hay can be stored again safely. That’s what it’s actually designed for.