The 20th century introduced Americans to total war. World Wars I and II demanded the total mobilization of all resources by all contenders. In both conflicts America became the food reservoir of the Allies. From a technological view, the wars engendered a level of prosperity which both allowed and encouraged farmers to adopt new methods and devices. The principal technological change in farms was the widespread adoption of the internal combustion tractor, first used in 1892. Inventors and manufacturers gradually but constantly improved tractors along with the various devices attached to them. Most notable were the corn picker, in 1909, and the cotton picker, in 1942. (Dates are for commercial production in each instance.) Farmers found both machines impracticable until a power source independent of the ground wheel had been developed. More than anything else the tractor and its related equipment finally set men free from the worst drudgery of farming. It also set many farmers free from the need to farm at all. The tractor and its equipment accomplished several other remarkable things, some obvious and some not so obvious. First, it allowed the farmer to get rid of horses and mules, and these animals steadily declined--to such an extent that in the 1960s the census did not even bother to count them. As a result of this decline, land that farmers had used to raise feed for animals could grow food for people or fodder for dairy animals. The amount of land thus released for other needs finally amounted to perhaps 60 million acres, and maybe even more. The change took place with increasing rapidity into the 20th century. Also, the tractor sharply reduced labor needs for the major crops of the United States. Even dairying, least susceptible to this sort of improvement, felt the impact of the tractor in such things as harvesting fodder and storing silage by running loaders off the tractor power-take-off. Since the very founding of agriculture men had discovered only one way to prosper in farming. The farmer had to exploit somebody or something. Animals, serfs, slaves, tenants, sharecroppers, or whatever, including the farmer's family and farm, had at various times been exploited on the farmer's way to success. After the age of machinery, however, the farmer tended to exploit the machine instead of other people or things. People had to leave farming, but in the long run they benefited from their removal. The machine had set them free. Chief of the machines was the gasoline tractor.