My horse looks super on rice bran as well but I think there is one drawback. Rice bran is high in phosphorus and an excess of that prevents absorption of calcium. I've been told that the calcium intake of the horse should be twice the phosphorus intake to prevent this. Generally I use
this nutrition calculator combined with my book on Australian forage and feed nutrition to determine what the ratio is and how to improve it. Luckily you don't need much rice bran to get awesome results but just something to look out for. I must qualify this by saying I am in no way a nutrition expert (a bit of a chem geek, maybe) but all the books I've read on equine nutrition have stressed the importance of the ratio :)