Innovations since 1920 have changed not only the nature of agriculture but also the nature of rural society. The introduction of tractors to Nebraska farms began before World War I, but widespread acceptance was not immediate. During the 1940s, a period of high farm profits that coincided with widespread availability of machinery, tractors almost completely supplanted horses. The mechanization of agriculture in Nebraska and the United States was made possible not simply by the development of the internal combustion engine but also by the consumption of inanimate energy used to power tractors and other machinery. One of the most important agricultural innovations of the twentieth century has been the development of improved seed varieties. Hybrid corn, widely adopted in Nebraska during the 1940s, is produced by seed companies within the state and throughout the Midwest. The most dramatic impact of technology on Nebraska’s agriculture is the rapid growth of irrigation.