If the horse doesn't have constant access to grass then in my experience they get grain twice a day. (Along with their hay) as for HOW MUCH to give the horse, that will depend on the horses metabolism, work schedule, and type of grain. I would recommend to consult your vet to determine a healthy diet for your horse. The diet can sometimes be determined by trial and error. But it's better to follow the recommended amount by the grain company and then change the diet accordingly.