Feeding bran mashes to horses is a common tradition dating back a long time, and is often thought to be a help in preventing colic through a laxative effect. Bran is believed to be a laxative in people, but to get that effect in horses, you would actually have to feed it in quantities bigger than your horse could eat. Some horses do produce softer stools the day after eating bran, but this probably reflects bran’s tendency to irritate the lining of equine intestines. If fed daily over a long period of time, bran may actually contribute to the formation of enteroliths.
The bigger danger in feeding bran to horses is the calcium:phosphorus ratio of bran. Calcium and phosphorus work together to build sound bones and assist muscle function. To do so, they must be absorbed in appropriate proportions (preferably 1.2 or more parts calcium to 1 part phosphorus). If, over a long period of time, phosphorus exceeds calcium in the diet, the horse’s body will pull extra calcium from the horses bones to meet it’s needs, and eventually weaken the skeleton.
Wheat bran and rice bran contain approximately 10 times more phosphorus than calcium. Therefore, an occasional bran mash won’t harm the horse, and he will likely relish the treat. However, daily bran regimens in large quantity should be avoided, unless calcium is supplemented in sufficient quantities elsewhere in the diet.