Genetic Contributions to Yield Gains of U.S. Hybrid Maize, 1930 to 1980

The increase in yield in commercial maize (Zea mays L.) hybrids attributable to genetic improvement averaged 92 kg ha−1 year−1 (linear) from 1950 to 1980 as measured by trials conducted in 1978 to 1980 on a series of 47 commercial hybrids released at intervals from 1934 to 1978 and an open pollinate of 1930 vintage. The genetic gain from 1930 to 1955 was 72 kg ha−1 year−1; the genetic gain from 1955 to 1980 was 112 kg ha−1 year−1. The 50-year average genetic gain of 92 kg ha−1 year−1 is equal to 89% of Iowa's estimated total yield gain over 50 years of 103 kg ha−1year−1. A second estimate of genetic gain from 1930 to 1980, based on sets of single cross diallels, is 73 kg ha−1 year−1, or 71 % of the total yield gain. The genetic yield gains in the commercial hybrids were accompanied by large and consistent improvements in resistance to root lodging, stalk lodging, premature plant death, and barrenness. New hybrids showed the greatest advantage in yield at high plant densities and high soil fertility levels and were consistently superior to the older hybrids in low yield environments. Successive hybrid releases tended to be increasingly tolerant to feeding by second-generation European corn borer and to have a more upright leaf habit. Plant height, ear height, leaf area index, flowering date, and grain moisture at harvest were changed little through the years. Small improvements with large hybrid-to-hybrid variation occurred for resistance to northern corn leaf blight (Helminthosporium turcicum Pass.), to feeding of first-generation European corn borer (Ostrinia nublalis Hübner), and in harvest index. Newer hybrids tended to have greater stover weight per plant, heavier kernels, fewer kernels per plant, and fewer kernel rows per ear. Trials of a series of five single cross diallels, each containing ten single crosses made by intercrossing the five most widely used inbreds per decade for the five decades, 1930 through 1970, showed approximately the same changes over time as the commercial hybrids. Results of the diallel experiment are interpreted as evidence that general combining ability for grain yield, standability, and stress resistance have improved through the years. However, the shape and slope of the yield response curve for commercial hybrids over the years suggests that specific combining ability also may have made an important contribution to yields, especially in recent years. Tests of the five sets of inbred parents showed approximately the same kinds of changes through time as were shown for their diallel single cross progeny. Rates of improvement in yield and standability were less for the inbreds than for their single cross hybrids, but rates of improvement in resistance to barrenness were greater for inbreds than for their single cross hybrids. Midparent heterosis for grain yield increased at a linear rate of about 40 kg ha−1 year−1.