Many people believe that soaking hay removes water-soluble carbohydrates, making the hay safe to feed to laminitis-prone horses. This may not be the case.
In this may 2009 study,:
“The results showed a highly variable leaching of WSC and substantially less leaching than reported previously for chopped hay soaked for 30 minutes” explained Clare Barfoot. “Very few samples reached below 10% WSC, despite prolonged soaking. The concern is that this strongly suggests that soaking may not be sufficient to render some hays safe to feed to horses and ponies prone to laminitis.”
In addition, there is concern that soaking for a long time may remove so many other nutrients that deficiencies in protein, vitamins and minerals may result.
For more information, please read the Equine Science Update article, Laminitis: value of soaking hay?