In this case, it may tell you whether or not your horse is consuming enough roughage.
Any changes in manure consistency, color, smell, or amounts should be discussed with your horse's veterinarian. If no obvious reason for the change is found, you may want to look at roughage consumption as a cause, especially if your horse is dentally challenged.
Even though most senior feeds are high in "fiber," that fiber may not be a suitable substitute for roughage. Think of it as the difference between Metamucil powder and green beans. Both provide bulk, but of different consistencies. The "long fibers" in roughage provide mechanical benefit in the gut, keeping things moving along at an appropriate pace - not too slow, not too fast - and can help improve digestive efficiency and reduce risk of colic.
Our first Manurology photo shows a normal pile. Nice firm fecal balls, well separated, not too dry, but not overly wet or sticky.
And here, subtle suggestions of individual segments, but mostly "doughy" feces. This horse is definitely not getting enough long fiber.
And finally......"cow pies." These piles came from horses unable to consume any grass or hay at all. This is what we see when functionally toothless horses are fed exclusively senior mash.
To combat the inability to chew grass or hay, these horses may be offered a substitute such as well soaked forage cubes, in moderate portions, several times a day. Many do very well on these "processed" roughages and can return to normal or near normal manure production, a sign of a more normally functioning digestive system.
If you give soaked forages a try, remember to soak and serve only what the horse can easily consume in one meal. Soaked forages can spoil quickly during warm weather and will freeze solid in winter. At TREES, we soak alfalfa or timothy/alfalfa cubes in hot water for 30-60 minutes, then "squish" them with our hands to make sure no hard portions remain.