The main thing a horse needs if frequently excercising is electrolytes these are vitamins and minerals that are needed because the horse is losing vitamins and minerals his/her body needs to be healthy.
Frequently exercised horses need extra minerals and grain.
calories and salt
Salt
horses that are frequently exercised need feed with higher fiber and less suger,
Extra food for energy.
they need lots jumps and they need lots of troting polls
if they are walking on a rough terrain then they will need horseshoes. these can be put on by a farrier.
on howrse the answer is increased protein and carb.
increased fat
Oats and grains are high energy foods, great for horses that get frequent exercize.
A dose of some oats and oftenly fresh corn grain
electrolytes, (vitamins and minerals lost when the horse sweats).