ABSTRACT

Average U.S. farm maize (Zea mays L.) grain yields have increased from about 1 Mg/ha prior to the introduction of commercial corn hybrids in the early 1930s to about 7 Mg/ha in the late 1990s (Fig. 1), and, similarly, average maize yields in Ontario, Canada, increased from about 2 Mg/ha in the early 1940s, when commercial corn hybrids were introduced, to about 7 Mg/ha in the late 1990s (Tollenaar and Wu, 1999). Yield improvement during the hybrid era has been attributed both to adoption of improved agronomic practices and genetic gains made through plant breeding. Results of studies with Corn Belt varieties and hybrids from the 1920s to the 1980s in side-byside studies (Duvick, 1984, 1992; Russell, 1991) have indicated that 40% to 60% of the yield improvement in Corn Belt hybrids was attributable to genetic improvement, and Cardwell (1982) concluded that 58% of the corn

yield improvement in Minnesota from 1930 to 1980 could be attributed to genetic improvement. When U.S. yield improvement is examined in terms of the amount of applied nitrogen and increased plant population density since 1964, and precipitation and temperatures for the 1950 to 1995 period, 63% of the gain since 1950 was attributable to genetics (Smith, 1998). However, the trend of the genetic gain was quadratic and the results suggested that the rate of gain in 2000 was only 38% of what it was in 1950 (Smith, 1998).