I didn't ask my vet either before my horse's surgery a couple months ago. I went with a large evening before hay feed, a light morning of hay feed and no grain the morning of.
The reason people and other animals aren't supposed to have food for several hours before a surgery is so we don't puke after they administer anethesia, horses can't puke, so that part of it isn't an issue. But, you also don't want a big heavy meal sitting in their gut while they are under, as the anethesia also slows down digestion a little. Also, don't rush to let them eat after the surgery, make sure they are fully awake from the anethesia, so they don't choke on the feed. [And, if I'm wrong on this, I'm sure someone will let me know quickly.]