The phasing out of pack and draft animals in favor of mechanization was actually a fairly long, multi-stage process, particularly in North America. Donkeys lost popularity after the Gold Rush as they were no longer needed to work machinery or haul packs into remote areas. Most of these donkeys were simply turned loose; their descendants can still be found in the form of feral donkey populations. Oxen had been a dominant draft animal since ancient times, since cattle had been extensively domesticated in Africa and Eurasia, then spread by colonialism to Australia and the Americas. They were less expensive than horses and less costly to feed; however, they worked more slowly than draft horses[1,2,3]. New farming equipment invented in the 1820s required draft horses to operate, so by the mid-19th century, horses had replaced oxen as the draft animal of choice in Europe and North America[5,1]. In the UK, railroads were also replacing ox carts, pushing the transition away from oxen as key draft animals. Oxen which were no longer used on the farm were often eaten[1,2]. This meant draft horses were the species being actively displaced by modern tractors. According to statistics compiled by Bruce L. Gardner, the population of working horses in the USA peaked right after WWI, with about 25 million draft horses[1]. Herd books from the era indicate about 40,000 active brood mares for Percherons in the USA alone[4].
The Great Depression, however, reduced the market for draft horses, and during WWII, the government pushed the manufacture and adoption of tractors to increase food production[1]. As a result, breeding of draft horses slowed dramatically, and the population of working horses in the USA dropped to about 10 million by 1945. In the UK, a similar drop in the population of Clydesdale horses occurred during the same time period-- farmers simply stopped breeding them[1,4,5].