Great topic Steve, I’m looking forward to more.
As for figures for draft animals, I’ve read roughly 1/3 of crop land had to be devoted to pasture, hay, and grain feed.
One horsepower is equivalent to 750 watts, but as you point out, that’s only the tip of the iceberg in figuring real muscle power output.
For a sustainable future, I feel that syngas, biomass fueled shaft power is the only possible salvation. So the crop waste, or woodlot powers the working of the land, and other necessary power applications.
A syngas powered tractor with a pto generator could run a welder, and any heavy duty power application. Hard to get oxen to weld steel, no matter how determined…
Syngas, and mechanization should actually turn out to be more energy efficient, and sustainable than horse farming ever was.
But to get there, we should be building small tractors like locomotives used to be designed, with a century design life. Fortunately the old tractors weren’t far off that design mark.
I see this as a road to permaculture, where we are going to have to go.