If the horse is receiving the majority of its ration from good quality forage, then supplementing with grain (which is a high-energy feed that may also balance other nutrients with the forage) once a day is fine. The concern is each meal must be limited in size (maximum meal size is 5-6 lbs).
The horse's body condition (an indication of whether energy is adequate or not) indicates what nutrients need to be supplemented above forage, and the quality of the forage will dictate the effectiveness of once-a-day feeding more than anything else.
We recommend reviewing information on feeding horses from our resources.
