The cover of nutritional needs by hay only depends on two factors : the nature of needs and the quatilty of hays. These two factors may vary enormously, this is why the answer of this interrogation is not just « yes » or « no », but « it depends »
Regarding the hay, harvesting practives and storage practices they impact a lot the nutritional quality of this one. Actually, as a function of plants species present in the pasture from vegetative stade of cut, from time and drying time and storage, the nutritional composition af a hay may vary by twice as muchin terms of energy intake, proteins, fat and micronutrients (vitamins, oligos and minerals).
Only an analysis of the complete hay will make it possible to know exactly these intakes.
Regarding needs, they are also extremly variables according to the activity of the horse.
In this way of a very high quality of hay and a a moderate work, the only delivery of hay, the horse will suffer of deficiency, especially in minerals, oligos and vitamins. The problem with defiency is contrary to defiency in proteins and energy, who quickly affect the health condition of the horse, they will impact the organism over the long term, penalizing immune defenses, tissues regenerations... Their consequences will be visible later on and unfortunately incurable … That's why we are advising under all conditions, a food intake in micronutrients in the food ration.