Soaking hay causes dramatic rise in bacteria – research


Soaking hay is often used by horse owners wanting to reduce its carbohydrate content, but are there risks?

British researchers who carried out a study found that soaking hay caused a worrying rise in bacterial levels when compared to dry hay.

The losses in water-soluble carbohydrates from soaking hay – which is why most horse owners soak hay in the first place – varied considerably, they found.

“While soaking hay, or steaming followed by soaking, can be an effective way of reducing water-soluble carbohydrate content, the losses were highly variable,” reported Meriel Jean Scott Moore-Colyer and her colleagues, who found a range of 0–53 percent in their study.

They also found that soaking significantly increased the bacterial content, compared to the original, dry, untreated hay, which “reduced the hygienic quality of the hay”.

The researchers, whose findings have been published in the open-access journal, PLoS ONE, found that steaming was effective in reducing microbial levels in hay, provided it wasn’t then soaked afterwards, which caused an increase in bacteria.

The researchers used five different hays to determine the effect of five different soaking and steaming treatments on the water-soluble carbohydrate and microbial content.

The methods used were:

  1. Dry hay (no treatment at all);
  2. Steamed for 40 minutes in a Haygain-600 steamer;
  3. Soaked in water at 16 degrees Celsius for 9 hours;
  4. Steamed for 40 minutes then soaked for 9 hours;
  5. Soaked for 9 hours then steamed for 40 minutes.

After treatment, each of the hays underwent testing for water-soluble carbohydrates and microbial content (with bacteria and mould levels assessed seperately).

The researchers found that protein and ash proportions were unaltered in any of the treatments, the latter being an indication that mineral content was largely unchanged.

Hay shortages are starting to bite in Victoria, Australia.They found that all three treatments that used a soaking element (3.4, and 5) were equally effective at reducing water-soluble carbohydrate levels.

Hay that was steamed had significantly less bacteria compared with the hay that had been soaked without any other treatment. Hay that had been soaked and then steamed also had significantly less bacteria than soaked hay.

Hay shortages are starting to bite in Victoria, Australia.Mould contents were significantly reduced by steaming alone, including in the hay that had been soaked and then steamed, but no difference was seen between dry, soaked, or steamed then soaked.

The researchers concluded that soaking for 9 hours followed by steaming for 50 minutes in the Haygain steamer was the most effective method for reducing water-soluble carbohydrates and microbial contamination.

“Soaking or steaming-plus-soaking lowered water-soluble carbohydrates, but significantly reduced the hygienic quality of the hay which could potentially compromise the health of the horse,” Moore-Colyer and her colleagues reported.

Steaming, and soaking then steaming, reduced microbial counts by 93 percent and 99 percent respectively, while soaking alone increased the levels to four-times the amount of bacteria found in the dry hays.

The average loss of water-soluble carbohydrates with steaming was only 3 percent across the hays, with a range of 1.4 percent to 6.9 percent.

The use of soaking caused an average loss of water-soluble carbohydrates across all the hays tested in the study of 34 percent.

The average bacterial contamination across all five hays was increased five-fold when the hays were soaked for 9 hours in water.

They noted that the hays that underwent being steamed then soaked for the nine hours did not have a reduction in bacterial contamination.

“Steaming alone has been reported to kill between 60 percent and 99 percent of bacteria present in hay.

“The bacteria remaining in the post-steamed hay are likely to be thermo-stable spore-formers that rapidly proliferate once they become hydrated during soaking,” the researchers suggested.

They said that soaking then steaming caused the greatest reduction of water-soluble carbohydrates and the largest reduction in bacterial and mould content. “Thus, this treatment produced clean and comparatively low sugar hay suitable for feeding to obese and laminitis-prone horses and ponies.

“Steaming alone produced hygienically clean hay but did not reduce the water-soluble carbohydrate content to below the suggested threshold of 100 grams per kilogram for feeding to horses with metabolic disorders.

“The commonly used practice of soaking hay does result in losses of water-soluble carbohydrate although these are variable and prolonged soaking also results in a substantial increase in bacterial contamination.

“Therefore, soaking hay for prolonged periods without further treatment compromises the hygienic quality of the hay, which could have clinical consequences, but more work is needed to confirm this,” they said.

Moore-Colyer, joined in the research by Kimberly Lumbis, Annettee Longland and Patricia Harris, said grass hay was still the most common fodder fed to stabled horses in Britain and globally.

Hay was classified into two categories – seed hay, which is generally composed of one or two specially sown grass species; and meadow hay, which is a mixture of different grasses and other herbage in permanent pasture.

The nutrient quality of both is primarily influenced by the stage of growth at harvest as well as the mixture of different grasses and other herbage in permanent pasture.

Nutrient quality, particularly the content of water-soluble carbohydrates, is strongly influenced by weather conditions at harvest, while the protein content is more influenced by physiological age and soil nutrient status.

However, they noted that even mature hay taken from a stressed pasture that was subjected to drought could have high levels of water-soluble carbohydrates.

Such hays would be unsuitable for horses with the likes of equine metabolic syndrome, polysaccharide storage myopathy, and laminitis, they said.

Thus, in an attempt to reduce levels of water-soluble carbohydrates, owners sometimes soaked hay for long periods, often for 12 hours or more.

The researchers said that while soaking did decrease the carbohydrate levels, other research had already shown that its effectiveness varied considerably.

In that research, losses of water-soluble carbohydrates ranged from 9 percent to 54 percent after a 16-hour soak in water at 16 degrees Celsius. Furthermore, the extent of these losses did not correlate with grass species, or carbohydrate content, so it was not possible to predict how soaking would affect water-soluble carbohydrate loss from any individual hay.

Soaking hay for extended periods left liquid reported to have a biological oxygen demand nine times that of raw sewage. As such, it should be disposed of carefully and not poured down stormwater drains.

Moore-Colyer MJS, Lumbis K, Longland A, Harris P (2014) The Effect of Five Different Wetting Treatments on the Nutrient Content and Microbial Concentration in Hay for Horses. PLoS ONE 9(11): e114079. doi:10.1371/journal.pone.0114079

The full study can be read here

One thought on “Soaking hay causes dramatic rise in bacteria – research

  • April 28, 2017 at 1:27 am

    I am sorry, but in this article they soaked the hay for 9 hours…..why? Most vets recommend you soak it for 60 minutes in cold water or even less time in warm water so there is really much less time for bacterial build up….why would anyone soak hay for 9 hours? Of course your going to generate more bacteria!


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