Taking into account the annual feeds for work horses (1,300 kg of corn grain, 1,600 kg of alfalfa and 500 kg of harvested roughage) and the national yields for these crops during the past decade, they conclude that the 23 million horses would require 9 million hectare of agricultural land for food, or 6 percent of US cropland. To “feed” the tractors with crops, 7.4 million hectares of agricultural land is needed, or 5 percent of cropland, which makes tractors slightly more efficient than horses.
To make a fair comparison, however, it should also be taken into account that horses make their own fertilizer without any extra energy input and that they reproduce themselves, while tractors need artificial fertilizers and have to be manufactured (and replaced). The researchers express these energy needs in terms of cropland requirements, to be able to compare them with the other results (they take the view that the fertilizers and tractors are produced with energy delivered by energy crops). They also included the energy needed to turn crops into fodder.
The cropland needed to feed the horses then rises to 16 million hectare or 11 percent of US cropland (because of the energy needed to produce fodder from crops), while the cropland needed to “feed” and manufacture the tractors rises to 38 million hectares or 26 percent of American cropland. Conclusion: when everything is taken into account, powering agriculture with tractors requires almost 2.5 times more energy than powering agriculture with horses.
Challenging Assumptions: Tractor vs Horse