Following hard work your horse's muscle will go through a period of breakdown, called catabolism. Providing a meal with 30 minutes of work should have the effect of halting catabolic processes and promoting muscle recovery and growth. Also any meals that contains carbohydrates and proteins will achieve this to some extent.
Most importantly though, you need to have quality protein!
The essential 'branch chain amino acids' seem to play an important role in muscle recovery, and the most limiting amino acids lysine, threonine and methonine are crucial for muscle building.