Yep, we have the least efficient food production system ever devised in the entire history of civilisation, by several orders of magnitude.

Personally, I see that as a cause for hope - it means that we have ample opportunity to improve it. We've gone down the brute force route simply because that was cheapest and easiest (well, that and a few historical accidents). When brute force solutions are no longer cheap or easy, then perhaps we can start trying more elegant approaches (I'm thinking Permaculture here).

As for slaves, I doubt they'll be needed. If the choice is work the land or starve, I know which one I'll be choosing. I've lived the life of an agricultural labourer before, and it really wasn't all that bad.

There is land that is adequate for pasture that is not adequate for much else. Grass fed beef can be raised without foregoing other crops, so what needs to be changed is the foolish feeding of grains to beef, not the elimination of the high quality protein that meat is.

I couldn't tease a relation between oil price and crop yield out of the data. Last year's wheat crop was abysmal because of weather, not because of price. I touched on that in the note. There are too many variables for yield other than fertilizer use.

http://home.entouch.net/dmd/Oilcrisis.htm