horses will eat hay but we can tell grass is what they like better. and be careful about too much grass when the growing season first starts. you'll need a few good books and a good vet on call if you're new at this
♥ Horse Stuff ♥ Considerations on feeding. I believe we used the book "Nutrient Requirements of Horses" in college, which provides charts breaking down intake amount compared to age, work, and reproductive state.