Rainfed and irrigated systems in which corn is grown either in rotation with soybean or as a continuous monocrop are the predominant cropping systems in North America. About 30 million ha of corn are harvested annually for grain in the USA, of which eleven states in the Corn Belt produce more than 210 million tons or 35% of the global corn supply (Dobermann and Cassman, 2002). During the past 35 years, average corn yields have increased linearly at a rate of 1.7 bu/acre per year (109 kg ha-1 per year, Fig. 1). Average corn yields now approach 140 bu/acre (8.8 t ha-1), but progressive farmers routinely harvest 160 to 220 bu/acre (10 to 14 t ha--1).
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