Crop Science 41:1711-1716 (2001)
© 2001 Crop Science Society of America
CROP BREEDING, GENETICS & CYTOLOGY
Sixty Years of Improvement in Publicly Developed Elite Soybean Lines
James R. Wilcox*
Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907-1150
* Corresponding author (jwilcox{at}purdue.edu)
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ABSTRACT
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Publicly supported soybean [Glycine max (L.) Merr.] breeders evaluate their elite breeding lines in cooperative tests across the northern soybean production area of the USA and Canada. These cooperative tests have been conducted for 60 yr, and virtually all publicly released cultivars have been evaluated in these tests. However, the progress made has not been reported. The objective of this research was to determine progress in elite line improvement in soybean adapted to northern soybean production areas across this 60-yr period. Two-year performance data for the three highest-yielding entries in these tests were regressed on years that entries appeared in maturity group (MG) tests across a 60-yr period. Rates of yield improvement in kg ha-1 yr-1 were 21.6 (MG 00), 25.8 (MG 0), 30.4 (MG I), 29.3 (MG II), 30.6 MG (III), and 29.5 (MG IV). In general, check cultivars were consistent in performance across years in which the checks were included in the tests. Plant height increased slightly for elite lines in MG I, but decreased significantly for elite lines in MGs II to IV. Plant lodging decreased significantly for elite lines in every MG test except MG I. Seed protein concentration decreased significantly for elite lines only in MG I (-0.29 g kg-1 yr-1) and II (-0.27 g kg-1 yr-1). Seed oil increased significantly in MG 00 (0.19 g kg-1 yr-1) and decreased significantly in MG III (-0.11 g kg-1 yr-1). The data demonstrate that soybean breeders have increased seed yield of cultivars by 
1.0% yr-1, while significantly increasing resistance to plant lodging. Rates of yield improvement during the past 20 yr have been equal to or greater than rates of improvement in earlier years.
Abbreviations: MG, maturity group • UTN, Uniform Tests North
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INTRODUCTION
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THE SOYBEAN WAS INTRODUCED into the USA in the late 1700s, and by the late 1800s was widely grown on a trial basis throughout its area of adaptation. In the early years of production, soybean was grown as a forage crop, but production for grain gradually increased, with the acreage harvested for beans exceeding the forage acreage for the first time in 1941. Since then, the crop has been grown almost exclusively as a grain crop (Probst and Judd, 1973).
Soybean cultivars initially grown in the USA were introductions from Asia. In 1936, the U.S. Regional Soybean Industrial Products Laboratory was established at Urbana, IL, in cooperation with agricultural experiment stations of the 12 North Central States (Hartwig, 1973). In 1939, the laboratory initiated two uniform soybean variety and strain tests "to provide a more rapid and accurate method of evaluating new strains developed through the cooperative breeding work." The tests were expanded across the years to include MGs 00 through IV, and have been conducted continuously since 1939. Virtually every publicly developed soybean cultivar released for production in the northern USA has been evaluated in these tests. Data from these tests provide information on the improvement of publicly developed soybean cultivars across a 60-yr period.
Several estimates have been published on the genetic improvement of the yield potential of soybean. Luedders (1977) compared the performance of three groups of cultivars: (i) plant introductions grown in the Midwest during the 1920s and 1930s; (ii) an early group of cultivars developed from hybridization, and grown in the 1940s and 1950s; and (iii) a group of recently developed cultivars from a second cycle of hybridization, grown in the 1960s and early 1970s. He reported a 26% yield increase of 1940 to 1950 cultivars compared with plant introductions, and a 16% yield increase of 1960 to 1970 cultivars compared with those grown in 1940 to 1950. This represented a 1% increase in seed yield per year.
Wilcox et al. (1979) compared yields of recently released MG II and III cultivars with early plant introductions produced in the Midwest. They reported increases in the genetic potential for seed yield of 25%, or 0.5% yr-1. Yield improvements of 1% yr-1 were reported for early maturing (MG 0, 00, 000) cultivars for the period 1934 to 1992 by Voldeng et al. (1997). The authors found evidence that yield increases for these early maturing cultivars were accelerating, from 0.5% yr-1 from 1934 to 1975, to 0.7% yr-1 from 1976 to 1992.
Boerma (1979) reported average yield increases of 0.7% yr-1 for cultivars in MG VI to VIII that were released from 1914 to 1973. Salado-Navarro et al. (1993) evaluated 18 cultivars in MG VII and VIII that were released across a 40-yr period. Cultivars were evaluated in four U.S. and four Argentine environments. Annual rates of genetic gain of 1.59 and 1.87 g m-2 yr-1 were significant and positive in only two U.S. environments.
The above studies were based on comparisons among different cultivars when grown in common environments. This should minimize effects of environment, including cultural practices, on estimates of genetic improvement. Recently, Specht et al. (1999) reported U.S. soybean yields increased at a linear rate of 22.6 ± 0.7 kg ha-1 from 1924 through 1997. The increases were attributed to adoption of improved technology, including cultivars and production methods, and increases in atmospheric C02 concentration.
The objective of this study was to determine improvements in publicly developed elite soybean lines adapted to the northern production region during the 60 yr when public soybean breeding programs have been conducted.
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MATERIALS AND METHODS
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Soybean breeders typically entered their elite breeding lines into appropriate MG cooperative regional tests after 2 yr of evaluation within their state. Usually, the lines were evaluated in uniform preliminary tests at 7 to 12 locations, and only the best lines in the preliminary tests were advanced to the Uniform Soybean Tests, hereafter, referred to as Uniform Tests North, or UTN. Test results have been published as: Results of the Cooperative Uniform Soybean Nurseries (1939–1941), Results of the Cooperative Uniform Soybean Tests (1942), Results of the Cooperative Uniform Soybean Tests, Part I, North Central Region (1943–1965), The Uniform Soybean Tests Northern States (1965–1991), and The Uniform Soybean Tests Northern Region (1992–present).
Uniform Tests North were conducted at 7 to 33 locations, depending on MG. Scientists conducting the tests determined the plot size, seeding rate, and number of replications for UTN at the locations they controlled. Plots were typically from 3.0 to 6.0 m in length, and row spacing varied from 0.18 to 1.06 m (Table 1). Prior to 1960, single-row plots were commonly grown at 0.76- to 1.06-m row widths. Later, multiple row plots were used, and row widths were narrowed. Beginning in 1976, only data from multiple-row plots were included in the UTN reports. Four-row plots were commonly grown, with row widths of 0.61 to 1.02 m. Ten-row plots were commonly used for row widths of 0.25 m or less. Plots were seeded at recommended rates for specific row widths. Three or four replications were used for virtually all tests.
Data in the UTN reports have always been in English units. Plant height was measured in inches from the soil surface to the tip of the main stem at maturity. Plant lodging was recorded on a 1 to 5 scale, with 1 indicating almost all plants erect, and 5 indicating almost all plants lodged flat. Seed harvested from each plot was recorded in grams per plot and converted to bushels per acre. Seed protein and oil were determined on a sample of seed, composited across replications for each entry, and reported as a percentage on a moisture-free basis. Prior to 1974, seed oil was determined by butyl extraction or nuclear magnetic resonance, and seed protein by Kjeldahl analyses at the U.S. Regional Soybean Laboratory, Urbana, IL. Subsequent to 1973, seed oil and protein were determined by infrared reflectance, then by infrared transmittance at the current USDA National Center for Agricultural Utilization Research, Peoria, IL.
Data for this report were computed by taking the 2-yr average for the three highest yielding entries, referred to as elite lines, in each MG test. Entries that are included in MG tests for 2 yr are superior in performance, and many of these are eventually released as cultivars. Including data from lower-yielding entries in each test would bias estimates of improvement downward. Data on maturity check entries have been included in each MG test. Maturity checks were often included in the tests for many years, and became progressively lower in yield relative to new, higher-yielding elite lines, so were replaced with higher-yielding checks. The number of days plus or minus the maturity check, that defined a MG, was adjusted when a maturity check was changed. This maintained consistency in MGs across time.
Uniform Tests North data on plant height were converted to m by multiplying by .0254 prior to analyses. Similarly, data in bushels per acre were converted to kg ha-1 by multiplying by 67.25. Protein and oil data were converted from percentages to g kg-1 by multiplying by 10. The number of values included in each mean varied from 42 (3 entries x 7 locations x 2 yr) to 186 [3 entries x (33 locations Year 1 + 29 locations Year 2)], and each value was the mean of three or four replications.
Linear regression analyses were used to determine if traits measured changed across years within MG of the UTN. In addition, similarly calculated mean yields of cultivars used as maturity checks were plotted for years the checks were included in the UTN.
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RESULTS AND DISCUSSION
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Regression analyses demonstrated a linear increase in seed yields of cultivars during the years of evaluation in the UTN (Table 2, Fig. 1)
. Rates of increase were not as great for elite lines in MG 00 and MG 0 as in later MG. Increases in seed yield for elite lines were virtually the same for MG I through MG IV, and averaged 30.0 kg ha-1 yr-1. The estimated increase in seed yield of 30 kg ha-1 yr-1 for MG I through MG IV is similar to the 31.4 kg ha-1 yr-1 estimate obtained by Specht et al. (1999) for mean U.S. yields from 1972 to 1997. This increase is considerably greater than the 22.6 kg ha-1 yr-1 estimate they reported for the years 1924 to 1997.
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Table 2. Regression of agronomic traits for high-yielding soybean entries on years of evaluation in the Uniform Soybean Tests Northern Region.
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Fig. 1. Regression of yields of elite soybean lines in Maturity Group (MG) 00 through MG IV on years of inclusion in the Uniform Tests North.
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The percentage yield increase within MG was determined by dividing the rate of yield increase by the mean yield of all elite entries for the duration of each test. Increases in seed yield per year for elite lines were 0.95 (MG 00), 1.05 (MG 0), 1.14 (MG I), 1.02 (MG II), 1.09 (MG III), and 1.08% (MG IV). Percentage increases are similar to values reported by Luedders (1977) for the years 1920 to 1970, and about twice as great as the 0.5% reported by Voldeng (1997) et al. across a 58-yr period, and by Wilcox et al. (1979) across a 40-yr period.
Seed yields of elite lines were compared with yields of the check cultivars for MG 00 from 1959, when 2-yr data were first available, to 1999 (Fig. 1). Yields of the check cultivar Flambeau were similar to yields of elite lines during the years Flambeau was included as a check. McCall was included as a check cultivar from 1973 through 1999. Initially, yields of McCall were equal to or greater than yields of elite lines in this test. Beginning in 1988, yields of elite lines exceeded those of McCall, and the difference became progressively greater as new elite lines were evaluated.
Grant, Evans, and Lambert were used as check cultivars in MG 0 (Fig. 1). Yields of Grant and Evans did not consistently increase across the years as did the yields of elite lines. During the years Lambert was included as a check, yields of this cultivar increased across the years more rapidly than did yields of elite lines. This suggests that during this period, environment affected yield increases of Lambert more than genetic improvement affected yields of the elite lines.
The check cultivars Mandarin, Chippewa (including Chippewa 64), Hodgson, and Parker were each fairly consistent in yield during the time they were included in the MG I tests (Fig. 1). Elite lines were similar in yield to these checks when the checks were initially used in the tests. As higher-yielding elite lines were developed, yields of these improved lines were progressively greater than yields of the checks. This suggests that in MG I, most of the yield increases can be attributed to genetic improvements.
Similar patterns of improvement of elite lines compared with check cultivars are evident in the MG II and MG III tests (Fig. 1). Dunfield, the first check cultivar used in the MG III tests, was also included in these tests in 1987 and 1988. The 2078 kg ha-1 yield of Dunfield in 1987 to 1988 is essentially the same as the 2058 kg ha-1–yield of Dunfield in the 1955 to 1956 MG III tests. This indicates that most of the improvement in seed yields of elite lines can be attributed to genetic improvement for yield potential during this period.
Data for the MG IV tests include the check cultivars Chief, Kent, Spencer, and KS4694 (Fig. 1). Yields of Chief and Spencer increased somewhat during the years of inclusion in MG IV, however, the increases were not as great as those for the elite lines. The performance of Kent was fairly consistent across years. Kent was higher-yielding than the elite lines when first included in MG IV, but by the time it was dropped as a check, yields of elite lines were consistently greater than those of Kent. Yields of Spencer increased with successive years in MG IV, but yields of elite lines increased more rapidly. KS4694 was the highest-yielding entry in MG IV when first evaluated in this test, but yields of elite lines gradually exceeded those of this check.
Two studies have reported that yield increases in soybean have not been consistent with time, and that in recent years, rates of yield improvement have increased (Specht et al., 1999; Voldeng et al., 1997). In this study, seed yield of elite lines in MG 0 through MG IV were regressed on years for three periods of 20 yr each (Table 3). Data for MG 00 were not included because of the very erratic performance of lines during recent years. These analyses showed consistent yield increases for the three periods in MG 0. In MG I, yield increases were greater and more consistent during the past 40 yr than the previous 18 yr. MG II yield increases of elite lines averaged 40.1 kg ha-1 during the past 20 yr, considerably greater than the average of 27.4 kg ha-1 for the previous 40 yr. In MG III, yields of elite lines increased more rapidly from 1941 to 1960 and from 1981 to 1999 than in the intervening years.
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Table 3. Regression of seed yield on years for 20-yr periods for elite soybean lines grown in the Uniform Soybean Tests Northern Region.
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Data for MG IV show virtually no increase in yields of elite cultivars entered in this test during the period of 1961 to 1980. This is also apparent in the regression for MG IV (Fig. 1). However, during the first 20 yr and the most recent 20 yr, seed yields of elite lines increased by 49.8 and 45.7 kg ha-1, respectively. Had this study been terminated in 1980, there would have been a strong indication of a yield plateau for MG IV breeding lines and cultivars.
Data on plant height demonstrated small but significant increases in plant height of elite lines in MG I (Table 2). Plant height of elite lines decreased across years in MG II, and at a greater rate in MG III and MG IV. In MG III, two short, determinate lines were among the three elite lines for seed yield in 1990, and this is reflected in the very short plant height of elite lines in that year (Fig. 2)
. A short, determinate selection was among the elite lines in 1995 and 1996, and this affected mean plant height during these years.
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Fig. 2. Regression of plant heights of elite soybean lines in Maturity Group (MG) III on years of inclusion in the Uniform Tests North.
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Lodging of elite lines decreased in successive years in each MG test except MG I (Table 2). This can be attributed to rigorous selection for lodging resistance by soybean breeders in the North Central states. Lodging scores of elite lines consistently decreased across years of inclusion in MG III of the UTN (Fig. 3) . Lodging scores for Dunfield were 2.5, averaged across the years 1940 and 1941, and 2.7, averaged across 1987 and 1988.
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Fig. 3. Regression of lodging scores of elite soybean lines in Maturity Group (MG) III on years of inclusion in the Uniform Tests North.
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Changes in seed oil across time were minimal, and decreased significantly only in MG 00 and MG III. Seed protein concentration among elite lines decreased significantly only in MG I and MG II. In spite of the typical negative relationship between seed protein and seed oil in soybean (Burton, 1984), no significant decreases occurred in either protein or oil associated with increases in the other seed component.
These data demonstrate that publicly-funded soybean breeding programs have been very successful in developing cultivars with improved yield potential for production in the major soybean growing areas of the northern USA and southern Canada. Yield improvements have been accompanied by improved lodging resistance. During this time, adequate seed protein and oil concentrations of elite lines have been maintained. Additionally, many improved cultivars have genetic resistance to major pathogens affecting soybean production. There is good evidence that there is still adequate genetic variability in soybean breeding populations used by public breeders to continue progress in yield improvement.
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ACKNOWLEDGMENTS
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The Cooperative Uniform Soybean Nurseries reports were compiled by various individuals, including D.I. Allen, A. Arneson, J.L. Cartter, F.I. Collins, C.V. Feaster, G.E. Geeseman, D. Heusinkveld, R.R. Kalton, O.A. Krober, R.W. Lawrence, W.J. Morse, A.H. Probst, L.C. Saboe, C. R. Weber, and L.F. Williams, from 1939 through 1944. Compilation was by the staff of the U.S. Regional Soybean Laboratory under the direction of J.L. Cartter from 1945 through 1964, and under the direction of R.L. Bernard from 1965 through 1973. Data compilation and test coordination were under the direction of J.R. Wilcox from 1974 to the present. The efforts of UTN participants in both the USA and Canada are gratefully acknowledged.
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NOTES
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Journal Paper no. 16414 of the Purdue Univ. Agric. Res. Programs.
Received for publication December 11, 2000.
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REFERENCES
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- Boerma, H.R. 1979. Comparison of past and recently developed soybean cultivars in maturity groups VI, VII, and VIII. Crop Sci. 19: 611–613.[Abstract/Free Full Text]
- Burton, J.W. 1984. Breeding soybeans for improved protein quantity and quality. p. 361–367. In R. Shibles (ed.) Proc. of the World Soybean Res. Conf., 3rd, Ames, IA. 12–17 Aug. 1984. Westview Press, Boulder, CO.
- Hartwig, E.E. 1973.Varietal Development. p. 187–210. In B.E. Caldwell (ed.) Soybeans: improvement, production, and uses. Agron. Monogr. 16. ASA, CSSA, and SSSA, Madison, WI.
- Luedders, V.D. 1977. Genetic improvement of yield in soybeans. Crop Sci. 17:971–972.[Abstract/Free Full Text]
- Probst, A.H., and R.W. Judd. 1973. Origin, U.S. history and development, and world distribution. p. 1–15. In B.E. Caldwell (ed.) Soybeans: improvement, production, and uses. ASA, CSSA, and SSSA, Madison, WI.
- Salado-Navarro, L.R., T.R. Sinclair, and K. Hinson. 1993. Changes in yield and seed growth traits in soybean cultivars released in the southern USA from 1945 to 1983. Crop Sci. 33:1204–1209.[Abstract/Free Full Text]
- Specht, J.E., D.J. Hume, and S.V. Kumudini. 1999. Soybean yield potential—a genetic and physiological perspective. Crop Sci. 39: 1560–1570.[Abstract/Free Full Text]
- Voldeng, H.D., E.R. Cober, D.J. Hume, C. Gillard, and M.J. Morrison. 1997. Fifty-eight years of genetic improvement of short-season soybean cultivars. Crop Sci. 37:428–431.[Abstract/Free Full Text]
- Wilcox, J.R., W.T. Schapaugh, Jr., R.L. Bernard, R.L. Cooper, W.R. Fehr, and M.H. Niehaus. 1979. Genetic improvement of soybeans in the Midwest. Crop Sci. 19:803–805.[Abstract/Free Full Text]
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ABSTRACT
INTRODUCTION
