I also give her a pound of Enrich 32, which is a ration balancer, to make sure she gets the vitamins and minerals she needs.
It's not a lot which is really nice. I've found that getting alfalfa is the cheapest option for me since I'd have to feed her SO much grass hay to make up for what a single flake of alfalfa does. One 140lb bale of alfalfa lasts me for over 3 weeks.
Of course, you'd need to figure out what's appropriate for your pony but one easy keeper is a pretty great thing! :)
If your going to do this, I really hope that farmer lets you lease his field. Otherwise, your backyard is going to turn into a mud pit.
I thought that having a horse on 3 acres was going to be pretty mud-less but that is not the case. There's mud everywhere. Not as bad as it'd be if I had more than one horse but it's pretty bad.
ETA- that's an excellent idea to cover your sacrifice area with something to keep the mud down. I bet if you used some sort of outdoor arena footing (maybe that's your plan), you could be in business.