Hay high in carbohydrates should not be fed to horses with insulin resistance unless the hay is soaked in water.  Soaking hay to remove sugar has become commonplace in these cases, but how much sugar is actually removed? Soaking it reduces the water-soluble carbohydrates, and the amount reduced depends on how long it has been soaked.  High levels of carbohydrates cause an increase in blood insulin levels, which is detrimental to horses with laminitis and founder or can cause laminitis to develop.  Dr. Annette Longland from Wales tested nine different hays to determine the amount of carbohydrates and protein decreased by soaking.  The study involved soaking 4 pounds of hay in 6 gallons of tap water that was at a temperature of 46°F  (7.7°C). Different batches were soaked for 20 minutes, 40 minutes, 3 hours, and 16 hours.  Results indicated soaking did decrease carbohydrates but the amount depended on the length of time the hay was soaked.  Soaking hay for 20 minutes resulted in the lowest decrease in carbohydrates of only 5%, while soaking hay for up to 16 hours showed the greatest reduction at 27%. 

The differences in soaking times did not significantly affect the loss of protein.  So, although soaking 16 hours was effective, the decrease in carbohydrates even after soaking may not be enough to be safe for some horses.  A lot depends on the level of carbohydrates in the hay to start with as the lower level of carbs the hay contains, the less you have to soak out.  Lots of folks soak hay for 30-60 minutes and that is likely not enough time, depending on the hay.  Ideally, you would find hay that has less than 10% non-structural carbohydrate content so that soaking would not be required.  Talk to your equine veterinarian.