The Contribution of Breeding to Yield Advances in maize (Zea mays L.)
Introduction
Maize yields began to rise markedly in many countries during the past century, first in the United States in the 1930s and then in other parts of the world in the 1950s and 1960s. For example:
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U.S. yields, level at approximately 1.5 mg ha−1 in the first three decades of the 20th century, started to rise significantly in the 1930s, reaching 8.5 mg ha−1 by the end of the century (USDA-NASS, 2003b). The U.S. yield gains averaged 63 kg ha−1 year−1 from 1930–1960 and 110 kg ha−1 year−1 during the next 40 years (Troyer, 2000).
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Maize yields in Canada tripled during the period 1940–2000, increasing from 2.5 to 7.5 mg ha−1, a linear increase of 80 kg ha−1 year−1 (Bruulsema et al., 2000).
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Maize yields in Germany doubled in the period 1965–2000, going from 4 to 8 mg ha−1 (Frei, 2000).
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Maize yields in France quadrupled in the period 1950–1984, increasing from 1.5 to 6.0 mg ha−1 (Derieux et al., 1987).
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In Argentina, the national mean maize yield increased “at a rate of 2.3% per year from 1970–1992” (Eyhérabide et al., 1994).
Table I summarizes yield gain data for several regions of the world during the period 1961–2002. Globally, maize yields doubled during this time, from 1.9 to 4.3 mg ha−1, a linear increase of 61 kg ha−1 year−1. Different regions varied in the size of annual gain, as well as in average yields at the beginning and the end of the interval, but all showed positive and significant gains with the exception of eastern Europe (highly variable in the past decade) and southern Africa (minimal gain and highly variable during entire period). Yields in south Asia did not start to rise significantly until the 1980s; annual gains since 1985 have averaged 38 kg ha−1 year−1.
These examples and other data show that maize yields have increased significantly in many regions of the world during the latter half of the 20th century, especially in those places where maize is grown as a commercial crop.
Changes in cultural practices have been responsible for a significant portion of maize yield gains. Crop management practices, such as weed and pest control, timeliness of planting, and increased efficiency of harvest equipment, have improved over the years, especially (but not exclusively) in the industrialized countries (e.g., Cardwell 1982, Edmeades 1990).
Perhaps most importantly, the use of synthetic nitrogen fertilizers increased markedly starting in the years after World War II when plentiful and affordable supplies became available, first in the industrialized countries and then in many (but not all) of the developing countries (e.g., Cardwell 1982, Edmeades 1990, Miquel 2001). Total fertilizer applications on all crops worldwide increased fivefold during the period 1961–1992. The linear increase started from an average application of about 20 kg ha−1 in 1962 and reached 105 kg ha−1 in 1992 (USDA-ERS, 2003). However, in some countries, application amounts of synthetic nitrogen fertilizer did not fit this general trend; they began to level off in the 1980s. Application of commercial nitrogen fertilizer to maize plantings in the United States rose from an average of 58 kg ha−1 in 1964 to 157 kg ha−1 in 1985, but since then has stabilized at approximately 145–150 kg ha−1 (Daberkow 2000, USDA-ERS 2003). It would seem, therefore, that yield gains of U.S. maize since the mid-1980s cannot be attributed to application of increasing amounts of nitrogen fertilizer on maize plantings.
Plant density—the number of maize plants per hectare—also increased steadily through the years following World War II in the United States as well as in other countries. The increase was more or less in step with increases in application amounts of fertilizer nitrogen. In the central U.S. corn belt, plant density averaged about 30,000 plants hectare−1 (or less) in the 1930s; it began to increase in the late 1940s and 1950s, reaching about 40,000 plants hectare−1 in the 1960s, 60,000 plants hectare−1 in the 1980s, and is often at 80,000 plants hectare−1 or higher at present (Duvick 1977, Duvick 1984a, Duvick 1992, Duvick 2004b, Paszkiewicz 2001, USDA 1949–1992). During the past 50 years, plant density in the central U.S. corn belt has increased at an average rate of about 1000 plants hectare−1 year−1.
Genetic improvements, as well as cultural improvements, can contribute to an increased yield of maize. Farmer breeders, beginning with the people who first domesticated maize, have selected plants and cultivars to fit their wants and needs and, in so doing, have developed thousands of landraces adapted to a multitude of environments, as well as with a wide range of morphological and quality traits (e.g., Goodman 1988, Grobman 1961, Paterniani 1977). We can assume that a higher yield, or at least an acceptable and dependable level of yield, was always a desired trait for maize cultivars, as well as for those of other staple grain crops.
Although long-term yield trends are not recorded for specific farmer breeding programs, a general observation indicates that when crop varieties are grown in a new environment (e.g., when migrants carry their favorite cultivars to a new land), the cultivars often do not perform as well as intended. Careful selection in the unadapted cultivars, often coupled with hybridization to cultivars from elsewhere, then is used to develop genetically different cultivars that are better adapted to the new environment and therefore yield more (and more dependably) than the first introductions. Examples in U.S. history are the 19th century development of hard red winter wheat (Triticum aestivum L.) cultivars for Kansas (Malin, 1944) and “Corn Belt Dent” maize open pollinated cultivars (OPCs) for U.S. corn belt states such as Illinois and Iowa (Wallace and Brown, 1988).
Farmers developed adapted maize OPCs for the U.S. corn belt states in a relatively short time (Hallauer 1988, Wallace 1988). Within a few decades after settlement of the region in the early years of the 19th century, maize yields and general performance of the new “Corn Belt Dent” cultivars were at acceptable levels in most parts of the region. However, from then on, gains in yield were small or nonexistent. This is evidenced by the lack of gain in U.S. maize yields during the first three decades of the 20th century (Fig. 1).
One could suppose that the lack of yield gain during those decades was because maize-growing areas in the country changed in location and extent over time and therefore were not always equivalent in productivity. However, in the states of Iowa and Illinois, where maize-growing areas and cultural practices were relatively constant during this period, yields were essentially unchanged also. Yields in those states were level at approximately 2.3 mg ha−1 during the years 1900–1930 (USDA-NASS, 2003a). It would seem that farmer breeders in the corn belt, using selection techniques of that time [primarily mass selection based on individual plant performance (Sprague, 1952)], could not raise maize yields further than the levels attained in the initial development of adapted cultivars.
New breeding methods were tried in the late 19th and early 20th centuries. The production of varietal hybrids (first generation crosses of two maize OPCs) was tried and abandoned because of unreliable results (e.g., Crabb 1993, Richey 1922). A few professional breeders in the public sector (USDA) worked on variety improvement in the 1920s using relatively unsophisticated methods of mass selection or ear-to-row breeding (Crow 1998, Russell 1991, Sprague 1946, Sprague 1994). Their efforts did not increase yields either, except when a program provided adaptation to a new environment. These breeders, working in the first decades of the 20th century, lacked access to the present-day knowledge of experimental design, statistical analysis, and quantitative genetics. Lack of these tools must have hindered their progress.
U.S. maize yields started to increase when maize hybrids made from crosses of inbred lines were introduced in the early 1930s. During the next few years the increase in maize yield was correlated with the increase in the proportion of maize area planted to hybrids (USDA 1944–1962, USDA-NASS 2003a). Yields in Iowa increased from 2 mg ha−1 to 3.5 mg ha−1 in the period 1933–1943, as the percentage of maize area planted to hybrids went from 0.7 to 99%. U.S. maize yields rose from 1.5 mg ha−1 in 1933 to 2.4 mg ha−1 in 1950, as the percentage of area planted to hybrids went from 0.1 to 78%. In either case, yield gains took place before a significant increase in use of synthetic nitrogen fertilizers or chemical control of weeds and insects (Cardwell 1982, USDA 1956), so it seems likely that the yield gains primarily were caused by genetic improvements; the new hybrids yielded more than the OPCs that they replaced, and successive hybrids yielded even more.
Maize yields began to rise in conjunction with the introduction of hybrids in other countries as well (Cunha Fernandes 1997, Derieux 1987, Eyhérabide 1994, Frei 2000, Tollenaar 1989), although, as in the United States, improved crop management techniques usually accompanied the introduction of hybrid maize; plant breeding and crop management jointly contributed to the sharp increases in maize yields. The proportion of gain attributed to genetic improvements is treated in more detail in later sections, with emphasis on hybrids and how sequential changes in their breeding and genetics have contributed to increased on-farm yield.
In the United States, the first hybrids were made from inbreds that had been developed by selfing some of the better OPCs of the 1920s. Breeders then worked to develop a second generation of improved hybrids using new inbreds made by selfing the same OPCs. They found that the second round of hybrids yielded little or no more than the first; it seemed that breeders must have selected most of the superior genotypes in the initial round of selfing in the OPCs. Some of the breeders conjectured that it might be possible to make new “synthetic” OPCs, with a potential for production of a superior second generation of inbred lines, by intercrossing some of the best inbreds from the first round of OPC selfing (Baker, 1990).
To this end, the breeders made several “synthetics” by intercrossing the better inbreds of the day. Research in maize quantitative genetics had begun by this time, and some of the populations were subjected to various kinds of selection to make genetic improvements in the populations as such. The selection procedures were based on various assumptions about gene action and genetic variability (Hallauer 1988, Sprague 1946, Sprague 1966). The Iowa State University Stiff Stalk Synthetic (BSSS) (Eberhart 1973, Sprague 1946) is one of the best known of these populations. Sprague (1946) lists the 16 progenitor inbred lines of this synthetic.
Breeders practiced population improvement on other kinds of populations as well, such as locally adapted OPCs, exotic landraces, or composites of exotic landraces and/or inbred lines (e.g., Hallauer 1988, Sriwatanapongse 1985). The name “recurrent selection” was coined (Sprague, 1952) to distinguish these kinds of population improvements from pedigree breeding (i.e., developing improved inbred lines from crosses of proven inbreds). Depending on the prospective end user, breeders intended to develop improved populations that would serve as sources of superior inbred lines or that could be used directly as productive cultivars per se. Results of their work are discussed in a later section.
Section snippets
Previously Reported Genetic Yield Gains
Russell (1991) has summarized 16 independent estimates of genetic yield gains of sequentially released maize hybrids. Most of the estimates are based on comparisons of U.S. hybrids and were reported at intervals during the 20-year period of 1971–1991. Estimates ranged from 25–92 kg ha−1 year−1 with a mean of 57 kg ha−1 year−1. It seems likely that the wide range in values was caused, in part, by differing growing conditions among the several investigations and consequent differential
Comparisons with Genetic Gains in Hybrids
Although the emphasis of this review is on maize hybrids and how successive changes in their breeding and genetics have contributed to increased on-farm yield, recurrent selection to make improved populations has interacted with and sometimes contributed to genetic improvements in hybrids (e.g., via useful inbred lines bred from improved populations). Additionally, for some farmers in some parts of the world, annual purchase of hybrid seed is not an option. For these people, improved
Possible Reasons for Genetic Yield Gains
This review has shown that hybrid maize breeders have consistently increased the yielding ability of hybrids during the past 70 years and that genetic gains in grain yield are still linear. As the yielding ability of the hybrids has increased, other traits have changed as well, in directions that were sometimes intended and sometimes unintended or at least unplanned. Conversely, some traits have not changed (or have changed very little), sometimes at the intention of the breeders and sometimes
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