Wheat bran really isn't as good for horses as was once thought. Why exactly are you giving your horses bran? Should You Feed Bran Mash to Horses? Feeding Bran - HorseChannel.com
If you're trying to get more water into them, just soak hay pellets or beet pulp (the kind without molasses). Add a few peppermints and a handful of sweet feed if they won't eat beet pulp. If you use alfalfa or alfalfa mixed hay pellets, the horses should eat it right up!
If you're going for a laxitive effect, buy some psyllium supplement (sand clear) or ground psyllium husk.