Funny how things change, back in the day the feed shed at the stables I used to work was a fascinating place of endless metal dustbins each containing a separate feed.
In the corner was a huge dome topped bin that held the bran, a scoop was added to every bucket to start with, and then every horse had an individual ration made up from rolled oats, brusied barley, flaked maize. Then there were bins of shredded beet, some fancy boarders had the latest thing a pre mix food, silly them fancy paying all that money for what we were already mixing at home, pony nuts, and sometimes steamed squashed peas.
The ponies were never given oats as it was considered to make them to hot, that was reserved for the bigger guys, although we did up the oat ration for everyone prior to hunting days. Barley and or maize were the standard weight gain staples, and the bran was used mainly as a mixer, everything was fed damp and the bran bound it, or held anything that was added to the ration, I can't remember what was fed as supplements, apart from making linseed (flax) jelly. Bran mashes were always set up on hunting mornings and then fed for supper when the horses got home.
I now feed whole oats during the winter, mixed with a little soaked beet and their loose minerals.