the song was referred to as 'Open Country Song' in the early days of SMiLE, so it's pretty clear what the words were... at least at that point.
I've always heard the words "open country" on the demo, since first hearing it on the Endless Harmony CD. Brian's voice gets low there, but he's not mumbling.
I can kind of understand them changing the lyric to "agriculture" but I honestly don't see how anyone can actually think that's what is being sung on the demo.
Upper Country makes no sense (unless you're British apparently). The song is about enjoying fresh air and wide open spaces. And grits!