You didn't mention the breed of horse or the age. Some horses need more hard feed but I wouldn't up the grain if she isn't exercised. Have you tried soaked beet pupl, this can put weight on a horse in a very short time, make sure it's soaked overnight and during the day for night feeding. It also won't make your horse antsy like a heavy load of grain, plus it won't founder the horse. Your horse needs free choice hay, not rationed for meals.