A warm bran mash is a time-honored recipe to perk up a cold or sick horse. However, any grain can be made into a mash (which is basically porridge) and bran has been found to be harmful to horses if fed in large quantities.
To make a mash, add enough boiling water (or as hot as you can get it from the tap) to make the normal feed ration wet, but not dripping. Start with one part water to two parts grain and experiment for best results; flaked grains will absorb more water than whole grains, while pellets will quickly turn to mush, so the amount of water depends on what grain you're feeding.
Stir the mixture well and cover the bucket for 5 or 10 minutes to let it steam. Feed while it is still warm but not too hot for you to handle with bare hands. After use, make sure to clean the bucket and the feeder well, as wet feed spoils quite quickly.
See Bran; Feeding and Nutrition; Grains