Question:  Last year, my veterinarian diagnosed my 18yo QH gelding has having inflammation in his right dorsal colon. Ultrasound showed no thickening of colon, occult blood fecal was negative, ACTH was within normal limits (I'll have ACTH rechecked this year). Basically the diagnosis was based on gelding's gassy-watery manure squirts, cow pie type manure, sensitive belly/flank. Vet said feed him timothy cubes soaked in water; the gassy-squirts stopped immediately. When I introduced 1/2 flake of hay 6 months later he showed signs of manure dribble but not as bad as before. Question: other than hay cube diet, will you recommend anything else to help him heal his colon?

Dr. Kellon: This probably isn't so much an issue of the colon having a problem (colonic wall would be thickened if it did), but rather an age-related inability to handle high fiber fraction hays.  The combination of less efficient chewing with age, even if teeth look OK, and reduced variability and numbers of organisms in the large bowel make many horses of this age prone to the situation you describe with high fiber fraction hays.  Hays chosen for pellet or cube formation often have lower levels of fiber fractions because this makes them stick together easier. 
As a start, you should look for hays with NDF fiber below 60 and ADF below 40 (the lower the better).  Supplementing with Absorb-All or Quadrafac will also support the fiber fermentation.