I want to share my experience with this same problem. I have a 17yr Morgan gelding(Mr.Ben) that I have owned for 15 1/2years that displayed excessive fluid passed with a formed stool and the passing of gas, when on a hay diet. If even a small amount of grass was in his diet it was greatly alleviated. Being an older Morgan and needing to be on an IR diet, grass is a problem with its sugar content and only seasonally available. My vet and I tried everything to help this horse through the years. Last summer Mr. Ben developed a case of hives to something that was in his vitamin mix. When we put him on an antihistamine with in 24 hours his stool problem that we had been dealing with for years was gone! The hives weren’t gone but the age old problem was. My Vet diagnosed Mr. Ben with a food allergy. He is allergic to something that is in grass. When the grass has been processes into hay (dried) it is intolerable, in fresh grass because of the water content it does not cause the problem. I switched Mr.Ben from the pharmaceutical antihistamine to an allergy herbal blend and it completely controls it. He is on a total forage only diet (hay) and the bathing of the butt days are gone!