For an underweight horse, the best nourishment is good-quality hay or pasture grass. Give him free choice hay unless there is a medical reason, such as metabolic syndrome, laminitis, or Cushing's illness. You can also gradually introduce horses to grass to avoid the danger of founder, colic, or diarrhea. The majority of horses thrive on grass or hay on their own. It may only need more hay in its feeder or more grazing time to gain weight. However, grass hay and pasture grasses are generally sufficient for weight gain. Alfalfa and other legumes aid in increasing the hay and pasture richness.