Heterosis Decreasing in Hybrids--Yield Test Inbreds

U.S. corn production was 53 million Mg annually and corn yield averaged 1518 kg ha in the 1930s, when corn hybrids were first commercially grown. Corn production grew to 76 million Mg annually in the 1950s, to 150 million Mg annually in the 1970s, and to 219 million Mg in the 1990s (USDA/NASS 2007). In 2001 corn became the highest tonnage cereal crop worldwide: 557.6 million Mg of corn, 542.4 million Mg of paddy rice (Oryza sativa L.), and 535.6 million Mg of wheat (Triticum aestivum L.) (UN/FAO 2002). In 2004 the U.S. record corn yield was 10 059 kg ha, and in 2007 the U.S. record corn production was 332.7 million Mg (USDA/NASS 2007).