The 1950 census showed only 7.6 million draft animals, and a decade later the USDA stopped counting them. The epochal transition from horses, animate prime movers that provided draft power for millennia, was thus completed in the United States in about 80 years, and most of it was accomplished in less than half of that time, between 1917 and 1957. This rapid shift looks even more impressive because it had to be accompanied (or preceded) by the development of necessary infrastructures to extract, transport, and process liquid fuels.
