The horse’s digestive system is specially equipped to continually process high-fiber, low-energy food called forage.
It's good to know that horses commercial horse grain mixes from large animal feed stores will be balanced and contain all that a horse needs to stay healthy. While everything else it may need it can just get from grass. I'll have to always stock up on grain and hay then. My cousin is working abroad for a year so she left me in charge of taking care of her house and kid. Those two I can manage, but it's her horse that I have no experience dealing with. I'm glad to know that feeding it doesn't seem like it'll be that difficult. If anything goes wrong I'll just take it to a vet like you suggested.