Genetic Yield Improvement of U.S. Maize Cultivars under Varying Fertility and Climatic Environments1

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Abstract
Yield of decade groups of maize (Zea mays L.) cultivars typical of those used by U.S. Corn Belt farmers in each of the decades since 1930 was evaluated across a range of soil fertility and climatic environments. This study gave estimates of the rate of genetic improvement in yield for each environment. In addition, a measure of the advisability of continued development and deployment of high yielding, management responsive single cross maize hybrids in light of potential reductions in fertilizer inputs and potentially less favorable future climatic conditions was provided. These cultivars were grown in replicated field trails in a total of 11 environments from Tennessee to Colorado in 1980 and 1981. All plots were grown according to accepted modern agronomic practices except that a low soil fertility treatment was included in both years and two limited irrigation stress treatments were included in 1981. Mean yield of cultivars from more recent decades was consistently higher than those from older decades across or within environments. Regression analysis of mean decade group yield on decade of use within each environment and on an environmental index (treatment/location/year mean yield) across environments gave significant correlations and b values which were always positive, regardless of environment. These results are consistent with the conclusion that U.S. maize production will be best served by the continued development and deployment of improved single cross maize hybrids even if less favorable soil fertility or climatic conditions should Occur.
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