However, I believe that officers below the rank of Major were responcible for the upkeep of their own mounts and did not have the authority to draw upon Government stores for the upkeep of those horse. Junior grade officers (Captains and below) would not have used their horses in combat. Actually one of the factors, especially the first couple of years for Junior Officers to have a horse, was so the body servants could use them to travel and gather food for the officers mess, since officers did not draw rations from the Commissary.
I may be wrong, but I believe that only Colonels, Lt. Colonels, and Majors, because of their need for the use of a horse for command and control of the entire regiment, were allowed horses at Government expense. As the war progressed more and more of the other officers sent their horses and body servants home, or sold them.