Post–Green Revolution Trends in Yield Potential of Temperate Maize in the North-Central United States

farmers’ fields, it is not clear that breeders have been successful in achieving greater yield potential as defined This paper addresses the question of whether there has been an by Evans (1993), which is the yield of a cultivar when increase in yield potential of maize (Zea mays L.) hybrids released in the north-central United States since the advent of the “Green grown in environments to which it is adapted, with nonRevolution” that began in the late 1960s. Because there are few limiting nutrients and water, and effective control of published data about hybrid growth rates and yield-determining plant pests, diseases, weeds, lodging, and other stresses. traits when grown at yield potential levels, we attempt to address this Unfortunately, there are few published studies in issue indirectly by evaluation of maize breeding efforts, changes in which plant traits and physiological processes that govplant traits of commercial hybrids, and by comparison of statewide ern maize growth and development were measured in average yield trends and yield trends in sanctioned yield contests. On fields that attain yield potential levels of modern hythe basis of these sources of information and a definition of yield brids. Although on-farm yields of 21 000 to 22 000 kg potential as the yield that can be achieved with an adapted hybrid ha21 have been regularly reported in the north-central when grown without obvious stress of any kind, we found that there United States since the mid 1970s (Robertson et al., is conflicting evidence to support the hypothesis that maize yield potential has increased. We recommend experimental approaches to 1978; Nelson and Reetz, 1986), there are few data on quantify and investigate the determinants of maize yield potential in plant performance at these yield levels. Because direct the north-central United States and for use in breeding hybrids with measurements of maize growth and development at greater yield potential. yield potential levels under field conditions are lacking, we attempt to identify the factors that have contributed to the maize yield increases in the post–Green RevoluA there is considerable uncertainty in tion era by evaluating maize breeding efforts, changes predictions of global requirements for food and in plant traits in a historical series of commercial maize feed grains during the next 30 yr, there is no doubt that hybrids widely used in the north-central United States, total requirements will increase substantially. Expanand trends in the highest yields obtained by farmers in sion of cereal production on land not presently under rainfed and irrigated yield contests. cultivation is limited by the need to preserve remnant natural ecosystems and by losses of arable land to urban, Breeding Methods and Investment industrial, and recreational development—trends that in Maize Improvement are expected to continue as population increases. Given these constraints on the availability of arable land, crop About 95% of total expenditures for maize improvement in the United States are made by private-sector yield potential will be a primary factor governing the nature of agricultural systems in the next century. At seed companies (Frey, 1996). By far the most important selection criteria used by commercial maize breeders issue is the degree of intensification in crop production systems that will be possible, which in turn, will deterare yield and yield stability. While other agronomic traits such as pest resistance, plant height, and lodging mine the amount of land and natural resources that can be spared for other uses (Waggoner, 1994). are also taken into account, primary selection emphasis is given to direct measurement of yield from individual One global food supply–demand model predicts that global demand for maize will increase from 526 000 000 performance trials, with increasing numbers of sites and years in the testing process as new hybrids move closer to 784 000 000 t from 1993 to 2020, with most of the increased demand coming from developing countries to potential commercialization. A key point here is that there are no proxies for direct yield measurements at (Rosegrant et al., 1999). Assuming no increase in maize production area, an annual growth rate in maize yield all stages of hybrid development. Hybrid development and commercial release is a seof ≈1.5% will be needed to meet this demand; however, from 1982 to 1994, the yield growth rate for maize was quential selection process that depends on numbers and scale. Inbred lines are produced with rigorous selection 1.2% worldwide, but only 1.0% in developed countries as a group, which account for the majority of total maize for multiple traits with additive inheritance. New inbred lines are continually being developed from a large germproduction. Understanding the factors contributing to these yield trends is therefore fundamental to efficient plasm pool with sufficient diversity for development of improved hybrids (Duvick, 1981). The inbred lines are allocation of research investments to sustain the needed increase. Although it is clear that genetic improvement used to make thousands of experimental hybrids which are grown in small-plot yield trials for a period of 3 to has contributed significantly to maize yield advances in 5 yr at 25 or more on-farm locations per hybrid per D.N. Duvick, Dep. of Agronomy, Iowa State University, P.O. Box year. The survivors (not more than one per several thou446, Johnston, IA 50131; K.G. Cassman, Dep. of Agronomy, Univ. sand) are conditionally released on a small scale for of Nebraska, Lincoln, NE 68583-0915. Received 28 Dec. 1998. *Corresponding author ([email protected]). Abbreviations: GDD, growing degree days; OPV, open-pollinated variety. Published in Crop Sci. 39:1622–1630 (1999).