Steaming or Soaking Hay?

Written by U. Wyss & N. Pradervand Agroscope, Instiute of Livestock Sciences INT, Posieux
Thursday, October 27, 2016 — No Comments Yet


Sensitive horses are already allergic to a small amount of dust particles and mould in the hay (Meyer and Coenen 2014). Special hay steaming devices or watering of the hay are the remedies. By soaking or dampening hay, the volatile dust particles are wetted and the inhalation of dust is reduced. To what extent the microbiological quality can be improved with steaming or soaking and the ingredients can be influenced, was examined in two different hay samples.



Two different hay samples were used for this experiment. For steaming, the apparatus Haygain HG 1000 (Propress Equine Ltd, Hungerford, UK) was used. After heating, the feed was steamed for 50 minutes. From the starting material, immediately after steaming, and after the steamed hay was stored for three days, samples were taken and analyses carried out. In addition, hay was soaked from the same raw material. The hay was soaked for 5 minutes, 1 hour, 6 hours and 24 hours. In addition, a further sample was taken in the hay soaked for five minutes three days later. The microbiological quality (aerobic mesophilic bacteria, mould, fungi, yeasts) as well as the dry substance (TS) contents and the ingredients were investigated in the samples using the near infrared light (NIRS) method.



By steaming, the TS content of the hay dropped from an average of 89 to 80%. The TS content of the hay decreased much more strongly by watering. Depending on the duration of the irrigation, the TS values ​​were still between 32 and 17%. The crude nutrient contents of the two yeast samples prior to the treatments are shown in Table 1.

By steaming, the sedimentation of aerobic mesophilic bacteria, moulds and yeasts decreased (Figs. 1 to 3). The values ​​did not increase even during the sampling period three days after steaming. It looked different in soaking. Here the germ contents did not decrease significantly with a watering of 5 minutes. The germ contents, especially the yeasts, increased sharply with increasing duration of soaking. In the watered hay samples, in which the germ contents were determined only after three days, the feed was warm during sampling. These samples showed very high germ contents and were considered to be highly spoiled.

Steaming had little effect on the ingredients, especially the sugar content (Figure 4). On the other hand, the sugar content decreased as a result of the soaking. On the one hand, this is due to the washout and, on the other hand, to the activity of the yeasts, which have reduced a portion of the sugar.

The results of this study coincide with the results obtained from Moore-Colyer et al. (2016).

Regarding the two mycotoxins zearalenone and deoxynivalenol, the two feeds had very low values ​​before the treatment. They were clearly below the tolerated limit concentrations (DLG 2000). It should be noted that these mycotoxins are heat-stable and are not degraded by steaming, which has been confirmed by additional analyses.



By steaming, the bacteria content can be reduced. As a result of soaking, the bacteria content increases on the one hand and the sugar content decreases on the other. Long soaked hay is no longer suitable for feeding.



DLG 2000. Avoid mycotoxins instead of fighting them. DLG Communications 8/2000.

Meyer H. and Coenen M., 2014. Horse feeding. Enke Verlag, Stuttgart, 332 pages.

Moore-Colyer M.J.S, Tayler J.L.E. And James R., 2016. The Effect of Steaming and Soaking on the Respirable Particle, Bacteria, Mold, and Nutrient Content in Hay for Horses. Journal of Equine Veterinary Science, 39, 62-68.


TABLE 1 Dry Substance (TS) and content of the two hay samples before the treatments

Sample 1 2 TS% 89.9 88.2 Crude ash g / kg TS 68 71 Crude protein g / kg TS 63 112 Crude fiber g / kg TS 357 295 Sugar g / kg TS 56 98 VEP MJ / kg TS 8.1 9.3

VEP: Digestible energy horse


Description: Aerobe mesophile Bakterien © Agroscope

Aerobic Mesophile Bacteria in hay samples  (colony-forming units per gramme) 
© Agroscope

Steaming is red after one hour and after 72 hours; Soaking is blue after 5 mins, 1 hour, 6 hours, 24 and 72 hours


Description: Schimmelpilze © Agroscope

Fungi in hay samples  (colony-forming units per gramme) 
© Agroscope

Steaming is red after one hour and after 72 hours; Soaking is blue after 5 mins, 1 hour, 6 hours, 24 and 72 hours



Description: Hefen in den Heuproben © Agroscope

Yeast in hay samples  (colony-forming units per gramme) 
© Agroscope

Steaming is red after one hour and after 72 hours; Soaking is blue after 5 mins, 1 hour, 6 hours, 24 and 72 hours



Description: Zuckergehalte © Agroscope

Water Soluble Carbohydrates in hay samples (g/kg dry matter) 
© Agroscope


If the aim of the exercise is to reduces sugars in the hay, then soaking is the most effective method. However, it is important to not lose sight of the microbial pollution this causes. The drastic increase in fungi and yeast over increasing soaking time cancels out the positive aspect and poses a health risk which should not be underestimated. If the aim is primarily to reduce respirable particles and improve hay quality, then horse owners who want to be on the safe side should steam their hay.


Statistics translated (Notes by JF/BJ)

Here are some figures (provisional interpretations from graphs):

  • Soaking: Bacterial load up 100x (from just under 100 million to over 10 billion)
  • Steaming: Bacterial load reduced by over 99% (from just under 100 million to as low as 50 thousand)
  • Soaking: Fungi increased by around 50%
  • Steaming: reduced by over 99.9%
  • Soaking: yeast 10,000 times higher (1 million % more)
  • Steaming: yeast down 99%
  • Soaking: Water Soluble Carbohydrate retention—leaches out over time to almost zero
  • Steaming: 10-20% reduction
  • Soaking: dry matter reduction from 89% down to between 32 and 17%
  • Steaming: from 89% to 80%

All of the above are VERY compelling arguments from independent scientists to promote steaming over soaking and we believe this will prove absolutely invaluable in the future.


This study was carried out by Agroscope, Institute of Livestock Sciences INT, Posieux, Switzerland (an arm of the Swiss government).

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