My six year old Thoroughbred has a crack in the toe of his right front hoof. Several shoeings have not taken care of this. His hoofs tend to flair as they grow and growth is very slow.
Most cracks are the result of shock and concussion or direct trauma.
They can also be caused by improper shoeing or trimming or by poor hoof quality. I will assume that your horse's toe crack originates at the bottom of the hoof and that the horse is not lame since more exact information is not given. Different types and severity of cracks require much different methods of treatment. You stated that your horse's hoofs tend to flare as they grow out and that growth is very slow. There are several points that we need to consider.