The short answer is never if your horses are not diagnosed with laminitis, equine metabolic syndrome (EMS), polysaccharide storage myopathy (PSSM) or hypokalemic and periodic paralysis (HYPP), according to Krishona Martinson, PhD, with the University Of Minnesota Equine Center.
If you are experiencing these problems, soaking hay in water is a recognized strategy to reduce the level of nonstructural carbohydrates (NSC) and potassium in the diet, according to Martinson.
In excess, NSCs and potassium can be a problem for horses suffering from these conditions. Reducing levels of these nutrients in diets for horses dealing with laminitis, equine metabolic syndrome (EMS), polysaccharide storage myopathy (PSSM) and hypokalemic and periodic paralysis (HYPP) will help alleviate these problems, she adds.
Soaking removes some nonstructural carbohydrates (NSC) from hay. NSCs are water soluble. Horses diagnosed with laminitis, PSSM and EMS should have no more than 10 percent NSC in the overall diet. Horses with EMS or laminitis should be limited to 12 percent NSC. Grain and other dietary components also contain NSCs.
Not all forages contain the same amount of NSCs. That’s a good reason to have hay tested. Legumes (alfalfa and other clovers) tend to be lower in NSCs compared to cool season grasses (timothy, orchard, and others).
Soaking most grass hays for 15 to 30 minutes will remove enough NSC for horses with PSSM, EMS or laminitis. Soaking for more than 60 minutes is usually not necessary and may be harmful due to excessive leaching of essential nutrients and loss of dry matter.
Legumes and cool-season grasses tend to be very high in potassium and often exceed the recommended 1.1 percent maximum level for horses with HYPP. Soaking hay for 60 minutes is often necessary for horses with HYPP.
Wetting hay is sometimes recommended for horses with respiratory disease including heaves. This not to be confused with soaking hay as covered here, which involves much more water and potential for nutrient leaching.
For more information, go to: http://www.extension.umn.edu/agriculture/horse/newsletter/docs/june-2014.pdf.