Why do you feed horses bran mash?

Bran mashes were fed long ago in the belief that they worked on horses as a laxative in the same way they work on people. However modern science has proven the amount of fiber in a bran MASH is not high enough to work as a laxative for a horse. It was also proven that bran has an inverted Phosphorus to calcium ratio and if fed daily or in high amounts can cause the body to become calcium deficient as it cannot absorb calcium when too much phosphorous is present.

Instead of bran mashes you can instead feed warm mashes of hay pellets or another more nutritious feed, or even a small amount of cooked oatmeal. There are many recipes online for non-bran mashes.