What is healthier for a horse to eat oats or hay?

Hay is the basis of a diet for horses. Grain is technically supplemental for working horses, mares in foal, stallions, etc. A horse should be able to get all of its nutrients from hay, and if its not, that's when you supplement. But like they said, oats can cause founder so it should really be fed in moderation.

alfafa hay....for the average horse, grains are very hot and can founder a horse if not used properly and in propotion....