Everything depends of the horse's needs and the the nutritional quality of hay and grass.
In the case of a horse without activity and also hay or grass very nutritive (in terms of energy, proteins, vitamins, traces elements, minerals), the only hay annd grass it may be enough.
As soon as horse's needs increase (activity even slight, growth, sexual reproduction).
Nutrient intakes of hay principally but also of the grass become limited in terms of energy, proteins, vitamins and copper. If also the hay have bad nutritional quality, this amplifies the risk of nutritional dificiency. The nutritionals defiency in energy and ans proteins will be results in a loss of conditions or action potential for horses in activity. Nutritional defiency in micronutrients will be more sneaky because effects will be perceptible to medium term and wont be seen with the naked eye.
To eliminate any doubt, we have to do regulary anaysis of the fodder to know precise composition. From our experience, in most cases, the quality of hay and grass don't securise the food needs so it's necesary to give a food supplement to secure recovery balanced nutrition.