The most important part of the Centauride is not to achieve my goal, but rather to keep Apollo healthy throughout the ride. We'll certainly be getting him enough exercise! In order to walk so far and carry our gear and a rider (me), I have to plan for him to eat right.
Horses are hind-gut fermenters, which means that like cows they depend on microorganisms in their digestive system to do the work of breaking down their food. But unlike cows, the microbes live in the intestine. This normally efficient system is subject to disturbances if the microbes don't get the same food that they're adapted to eat. Abruptly hanging from grass to alfalfa, oats to corn, or even brome grass to blue grass can lead to colic or death.
Luckily, the slow pace of a long ride is helpful in dealing with this risk. Following the U.S. Calvary method, we will be stopping once an hour for Apollo to graze, in addition to morning and evening grazing. This will keep his microbes better adapted to the native grasses as we move from biome to biome.
There are some advantages of feeding a small amount of supplemental feed even when grazing is plentiful, including regularity of feedstuff, ensuring caloric intake is sufficient, and -- best of all -- enticement if Apollo gets loose and would not otherwise want to be caught. The sound of a feed bag can save lots of time looking for a horse that wandered off!
In the spirit of ultralight packing, we would carry only a couple days worth of supplemental feed. This will be possible for most of the trip, where grazing is available. However, in parts of the West, we will need to pack hay cubes or complete feed; when this is the case, I will be walking a lot more so that Apollo does not have to carry me in addition to the extra feed weight.