My original study on soaking hay done in 2003 has been repeated with good results, for the most part.  Some exceptions and concerns have circulated, so perhaps it’s time for a look at the new literature and a more detailed discussion on how to soak hay to remove the most sugars.  

It has been gratifying to see my study repeated by other groups.  Generally, the newer studies corroborate my findings; soaking hay removes significant amounts of WSC even though some hays that are extremely high in WSC are not salvageable.   Soaking hay reduces WSC (1) and has been proven to be useful in decreasing insulin response in metabolically normal horses (3) and in ponies with Insulin Dysregulation (2).

Some studies done in the UK raised concerns about the increase in mold and bacteria after soaking for 9-16 hours. (5)   This is not surprising.   Hays made in humid climates have more mold and bacteria to begin with compared to hay made in dry climates.   I have been shocked to see the brown, dusty hay that people feed horses in some places.  They say that green hay is just not available in their area.  Anyone from the western half of the USA would consider it unfit to feed horses.  In climates with rainy, humid summers, they sometimes feed silage or haylage to horses because making good hay is just too difficult to even try.  The studies focusing on the unhygienic properties of hay soaked overnight are often cited by companies marketing hay steamers or bagged forage.  While steaming hay does reduce mold, yeast, and bacteria from hay that has hygienic issues, it does not reduce WSC significantly. (5)  The increase in microbes is not due to soaking per se; it is due to soaking hay inherently high in microbes to begin with and soaking for too long. 

Study treatments have focused on the length of time the hay is soaked, but I don’t think this the most important factor.  When looking at the methods used in various studies, greater amounts of WSC were removed with larger amounts of water.    One study showed lower results than others, but upon questioning the researcher in a Q & A session at a conference, it was disclosed that it followed ‘common practice’ of barely covering the hay with water as people do to prevent inhalation of mold spores. (4)  My original study, conducted at Dairy One Forage lab by their staff, soaked 50 grams of hay in 4 liters of water.   That’s a lot of water for a handful of hay, but a significant amount of sugar was leached out in just 30 minutes.  

Rather than focusing on time, a more useful strategy would be to create a steeper diffusion gradient.  From Wikipedia:  “A diffusion gradient is a gradient in the rates of diffusion of multiple groups of molecules through a medium or substrate. “ When we soak hay, we are trying to get the most sugar out of the hay through the medium, which is water.  It’s a passive dissipation of the sugar through the pores in the plant cell walls until the amount of sugar in the hay equals the amount of sugar in the water.  Then the gradient is equalized, and diffusion stops.  When the ratio of water to hay is higher, the rate (speed) and the amount of diffusion is higher.  You get more sugar out when the volume of water is greater in relation to the volume of hay.  If you have high sugar hay, or you are in a hurry, you might need a whole muck bucket of water for one flake (biscuit, armful, 3-4 lbs.) of hay, or use hot water when available.  Or, you could soak it twice with fresh water, making a new steeper diffusion gradient to get out more sugar the second time.  One preliminary study got significant amounts of sugar out when hay was chopped before soaking.(3) This makes sense, as stems can act as a storage organ for sugars, and cutting them open should facilitate more rapid dispersion.  

The amount of time for soaking doesn’t make much difference after 1-2 hours IF YOU USE ENOUGH WATER.  In a study using 25 liters of water for one flake of hay, WSC, ESC loss maximized after 1 hour. (1) There is no reason for soaking hay overnight.  Some people have devised clever setups with irrigation timer/valve control units that will drain a soaking tub after a set time.  In my climate with 10-15% humidity, I could spread soaked hay on chicken wire to dry and feed later in the day. Get inventive, and deal with the advantages and limitations in your situation.

You may notice that those selling bagged forage often cite the studies showing hay soaking is not reliable (when you don’t use enough water). (4) If you can afford to buy bagged forage that is guaranteed low enough in sugar and you are too rushed to soak hay, or board your horse somewhere were the staff won’t do it, that may be your best option.  However, those of you with horses at high risk for laminitis have enough to worry about and spend your money on.   Just use more water when you soak your hay to do the best job of it. 

 Is the soaking water an environmental pollutant? (4,5) Again these studies soaked hay up to 16 hours, which is too long, and does create risks for the breakdown of nutrients due to the action of yeasts, fungi, and bacteria.  I concede that excess organic nutrients can create imbalances in the environment and should be disposed of with some care; the same caution we use disposing of horse manure and urine.  We’re just throwing stuff away before it goes through the horse instead of after.

I always thought soaking would be just a first-aid measure until people could find lower sugar hay, but a lot of people have told me that they can only buy enough hay to last a few weeks at a time.  Others say that finding low sugar hay in their area is just too difficult. They have to feed what they can get.  They say that soaking hay long term is the most economical and practical forage solution in their situation.  Some are so dedicated to soaking hay that they use their guest bathtub in winter and haul wet hay to the barn with a wheelbarrow, or soak in a large insulated cooler so it doesn’t freeze.  

 If circumstances force you to soak hay on a long term basis, make sure you are feeding a broad spectrum vitamin /mineral supplement to replace those leached out of the hay. 

1.     The Effect of Soaking on Carbohydrate Removal and Dry Matter Loss in Orchardgrass and Alfalfa Hays. K Martinson, H Jung, M Hathaway , C Sheaffer. JEVS 32 (2012) 332-338

2.     Insulinaemic and glycaemic responses to three forages in ponies. Carslake HB, Argo CM, Pinchbeck GL, Dugdale AHA, McGowan CM Vet J. 2018;235:83-89.

3.      Cottrell, E., Watts, K., Ralston, S. Soluble sugar content and glucose/insulin responses can be reduced by soaking hay in water. In proceedings of the ESS 2005; 293-298

4.     Effects of soaking on the water-soluble carbohydrate and crude protein content of hay. Longland AC1, Barfoot C, Harris PA. Vet Rec. 2011 Jun 11;168 (23):618

5.     The Loss of Water-Soluble Carbohydrate and Soluble Protein from Nine Different Hays Soaked in Water for up to 16 Hours A.C. Longland, C. Barfoot, P.A. Harris, JEVS (2009) 29:5, p. 383-384

6.     The Effect of Five Different Wetting Treatments on the Nutrient Content and Microbial Concentration in Hay for Horses M J Scott Moore-Colyer, K Lumbis,  A Longland,  and Patricia Harris PLoS One. 2014; 9(11): e114079