cereal and feed-grain cultivation without fallowing (Mazoyer and Roudart, 2006). That revolution led to important increases in the food supply and thus ultimately permitted increased population growth.

Another important change in agriculture resulted from the application of an increasingly scientific approach to plant breeding, which developed from the recognition of the cell as the primary unit of all living organisms in the 1830s (Vasil, 2008) and the work of Mendel (Kloppenburg, 2004). With the rediscovery of Mendel’s principles of genetics in the early 1900s, progress in plant and animal breeding was accelerated. The continuous growth in crop yields and agricultural productivity during the 20th century owes much to those biological discoveries and to a series of mechanical and chemical innovations driven by agricultural research and development.

One of the more significant innovations in plant breeding during the 20th century was the development of hybrid crops, particularly corn, in the United States. Hybrid corn varieties, which are developed from crossing different inbred lines, out-yield pure inbred lines, though the seeds produced by hybrid varieties yield poorly. When corn hybrids were first developed, they had no discernible yield advantage over the existing open-pollinated corn varieties of the time (Lewontin, 1990). However, seed companies were motivated to develop high-yielding hybrid varieties; saving and planting the seeds of hybrid corn did not produce equal yields, so seed companies had a financial incentive to invest in these varieties. The research and development efforts devoted to hybrid corn produced tremendous yield improvements over the last 70 years. It is unclear if the same amount of investment could have resulted in similar yield increases for open-pollinated varieties; regardless, because of their limited potential for return on financial investment, efforts to develop high-yielding open-pollinated varieties were not made. Modern hybrids, which have been bred to allocate more of their energy to producing grain rather than stover (leaves and stalks), also demonstrate an ability to maintain high grain production in densely planted fields (Liu and Tollenaar, 2009), and they can exhibit increased tolerance to environmental stresses (such as drought, cold, and light availability).

Plant breeders in the 20th century also identified varieties of wheat and rice with shorter stalks and larger seed heads. They were crossed with relatives to create semidwarf wheat and rice varieties, which produced greater yields in part because they responded well to applications of nitrogen and did not lodge despite having heavier seed heads. The development of semidwarf wheat and rice spurred the Green Revolution of the 1960s and 1970s in developing countries (Conway, 1998). Such improvements in plant breeding increased global crop yields in rice and wheat substantially in countries with suitable growing conditions and markets.

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