Bran mashes have traditionally been touted as being good for older horses to help them maintain weight, which is the result of tooth wear and aging. Today, it seems that giving bran mash could be dangerous for your horse's hooves. In summary, don't brand your horse if he has hoof problems.
Oats or bran from other cereals contains phytate, which is high in phosphorus and blocks calcium absorption in the small intestine, leading to systemic calcium deficiency. Calcium is necessary for the strong binding of keratinized cells to produce a strong hoof.
But, what about the fact that you were trying to regulate stool consistency? Try sugar beet pulp instead. If you're using bran to prevent sand colic, use psyllium. In fact, to be honest, you're unlikely to give your horse so much bran mash that his hooves will have problems. Bran mashes are usually not the only feed given, they are used in combination with other fodder and pellets.
If you have hoof problems, why not use biotin, a popular hoof growth supplement? Research shows that if your horse has a normally functioning digestive tract, a biotin supplement is not necessary. Feeding above the physiological amount of biotin does not improve the growth and resistance of the hooves. Ok, fine, so what's the point? Instead of biotin or mash, give bulk salt, vitamin C, copper, and zinc.
Make sure your horse has access to loose salt and not salt blocks. On the other hand, horses fed a premixed feed often receive adequate amounts of salt, so you won't need bulk salt. Copper and Vitamin C are catalysts for the formation of healthy, connective tissue for hooves. Ensure that copper and vitamins are provided in sufficient quantities. DL-methionine, praline, glycine, and glutamic acid are the main building blocks of healthy connective tissues.
Zinc is necessary for hoof health. Without it, the hoof suffers from parakeratosis, a keratin maturation defect, which makes the hoof soft and structurally weak. By the way, zinc and copper must be in balance.
In conclusion, don't give supplements for the sake of it. Determine if you really need it in the first place by talking to your veterinarian. And if so, only start with one at a time to see if it's really effective.