It’s very interesting that they made bread for horses even! It is pre digested, so that does help!
>Now the small native ponies of the Steppe can subsist entirely off of grass, but the sort of horses available in the agrarian world are bred too big and strong to eat entirely grass. Their nutrition requirements are too high and so they require feed, at least some 4.5kg of it per day assuming local grass is available along with time to let the horses graze it (during which the wagon is, of course, stopped).