Do horses have to eat feed or can they just eat hay?

A horses diet is affected by many things, including, but not limited to: Age, health, weight, level of work/ performance, genetics. Many horse on light to no work can happily live on hay and a vitamin and mineral supplement. However when a horses workload increases to moderate and higher they generally need more calories than just hay can provide.

Broodmares also generally need some kind of supplemental feed as they need the calories to produce adequate milk for the foal.