Four big families of nutriments are essentiels for the functioning to cover all needs on maintenance (tissues regeneration) and activity : proteins, lipids, carbohydrate and micronutrient (minerals, oligos and vitamins). The healthy diet is the one that will provide all those nutriments in good quantity to organism's needs.
In addition to to teh water, two elements of the food ration alow to cover those needs :
Firstly feeds : grass or hay (minimum 1,5 Kg of hay per 100k of weight of your horse, so 7,5kg per day for a horse of 500kg).
Then, a supplement of this feed,to choose according to the activity, for adults horses. More intensive is the activity with your horse, more his needs will be important so more the supplement will be concentred and enriched in nutriments to protect muscles. Generaly, brands of food segmented offers on this criteria, show on documentation. The specifics foods for gorwing young horses or brood mares will suppose to cover all specifics needs are linked to these physiological stades.