No, just no. Most horses are not fed grains of any type. They eat grass and hay. Grains aren't really great for them anyways. Performance horses and elderly horses will get supplemental style grains, but a horse needs to basically eat 24/7 to keep their guts properly working.
Look guys, not looking for a fight here, but maybe just don't tell the people who are already in somewhat of a war with vegans to go and eat literal grass 😅