Analysis of crop improvement during past decades can elucidate the factors that have contributed to the genetic gain of crop cultivars and the understanding of the factors underlying genetic gain may help to sustain or improve genetic gain in the future. The objectives of this research were to estimate the genetic gain in total and machine‐harvestable grain yield of maize ( Zea mays L.) hybrids representing three decades of commercial maize production in Ontario and to quantify the effect of the plant‐density :grain‐yield interaction on the genetic gain. Nine maize hybrids that had been grown in Ontario during the period 1959 to 1988 were evaluated at two locations during 1987 and 1988 at four plant densities. Total above‐ground dry matter, total grain yield, and machine‐harvestable grain yield were determined at physiological maturity. The genetic gain in total grain yield at the optimum plant density for grain yield was 1.7% yr −1 , whereas the genetic gain in machine‐harvestable grain yield was 2.6% yr −1 . The optimum plant density for machine‐harvestable grain yield increased with year of hybrid introduction. The optimum plant density for total grain yield increased from old to more recent hybrids, but the increase in optimum plant density did not continue for hybrids from the 1970s era to hybrids of the 1980s era. Approximately one‐third of the genetic gain in machine‐harvestable grain yield can be attributed to reduced stem lodging. A comparison of the oldest and most recent hybrids indicates that approximately 15% of the genetic gain in total grain yield can be attributed to increased harvest index. Consequently, increase in total dry matter accumulation attributed 85% to the genetic gain in total grain yield.
M. Tollenaar (Wed,) studied this question.