Bran

Bran is high in fiber and absorbs water well. It may be fed as a bran mash, which acts as a laxative.

Add boiling water to 1—2 lb (0.5-1 kg) of bran in a bucket to form a crumbly consistency, cover, and leave to ' cool and steam.

You can add a spoon of salt, a few oats, or some epsom salts as well. Feed while still just warm.

Bran should not be fed in large quantities as it is has  a low calcium/high phosphorus ratio, which is not desirable for horses.