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From 1950 to 1980, U.S. sorghum [Sorghum bicolor (L.) Moench] grain yield has been increasing at an annual rate of 7% /year. Sorghum yield increased at 11 % from 1950 to 1960, 4% from 1961 to 1970, and 2% from 1971 to 1980. The discovery of cytoplasmic-genetic male sterility, incorporation of new germplasm (Sorghum Conversion Program), selection for disease and insect resistance as well as better production practices have contributed to this gain. Data from State Crop Reporting Services indicated that yields may not have increased as much in more northern as in southern locations.
To obtain information on differences among sorghum hybrids because of long-term improvement of yield, a multi-location performance test and a growth analysis study were conducted in 1981 using sorghum hybrids and their in breds from the 1956–1959 period and those recently available. Hybrids from the earliest period designated as old were AT×378 × RT×7000, AT×399 × RT×7078, and AT×3197 × RT×7078. The new hybrids included AT×623 X RT×430, AT×623 X 77CS256, and AT×2752 × RT×430. A third group comprised of AT×623 X RT×7000 and AT×378 × RT×430 is a combination of an old and a new parent (new × old).
Results showed a 9.0 to 67.7% yield advantage for the new hybrids over the old hybrids with an average of 39 %, which amounts to 1645 kg/ha. The new × old combination hybrids were generally closer in yield to the new hybrids, signifying the positive contribution by the new parent involved in such hybrids. Yield increase of new hybrids reflects a significant increase in kernel number per panicle with little difference in kernel size.
Total plant weight, leaf area, plant height for the new hybrids also increased. No significant differences in either harvest indices or threshing percentages between old and new hybrids were evident.
A comparison of these representative hybrids demonstrated that genetic improvement in sorghum grain yield accounted for 1 to 2% increase/year and has been realized through manipulation of various traits, including increased height, plant weight, leaf area, and kernel number.
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.
Commercial soybean [Glycine max (L.) Merr.] yields in the USA have increased at an annual rate of about 21 kg/ha from 1924 to 1980. This increase in soybean productivity is attributable to improvements in cultivars and agronomic practices. The degree of genetic improvement has been estimated in several previously published reports to be about 10 to 18 kg/ha annually based on yield comparisons of obsolete and modern soybean cultivars. Our 3-year evaluation of 240 cultivars of maturity groups 00 to IV released during the period 1902 to 1977 indicated an average annual rate of genetic improvement of 18.8 kg/ha for the 75 years, ranging from 14 to 29 kg/ha annually within each of the six maturity groups. In comparison, the average annual rate of yield improvement in 19 northern states where one or more of these cultivars are grown was 23.7 kg/ha. A significant portion of the total genetic improvement during the 75-year span occurred as a one-time increase of 15 to 25% in genetic yield potential during the 1940s, when cultivars derived from plant introductions were replaced by cultivars derived from hybridization breeding programs. Consequently, the average annual genetic gain in yield since 1940 has been only 12.5 kg/ha. Soybean breeders have also effected significant increases in 100-seed weight and lodging resistance and decreases in plant height during the 75 years of cultivar improvement.
An analysis of the pedigrees of 136 soybean cultivars that originated from hybridization breeding programs during 1939 to 1981 revealed that only a limited number of ancestral introductions contributed germplasm. Only five introductions were the cytoplasm source for 121 of the 136 cultivars. The ancestry of the nuclear material in these 136 cultivars was also narrow, with 12 introductions contributing about 88% of the germplasm. Traditional breeding procedures that emphasize the mating of elite strains are, in effect, continually recombining the genes contributed by a limited group of ancestral introductions.
Possible methods for increasing the annual rate of soybean genetic improvement include increasing the amount of genetic variability available for selection, increasing the number of lines tested vs. the number selected, decreasing the number of years per cycle of selection, and enhancing heritability by decreasing the phenotypic variance relative to the genetic variance. Increasing genetic variability is an important factor if genetic diversity in soybeans is also to be enhanced. Population improvement methods using recurrent selection have an advantage over traditional hybridization procedures involving two-way crosses in that the former methods more easily facilitate the use of multiparent populations and exotic germplasm to increase genetic variability. Advances in the genetic biotechnology areas of protoplast fusion and recombinent DNA may eventually provide the soybean breeder with the means to exploit genetic variability at the cellular level in addition to the whole plant level.
The purpose of this chapter is to ascertain the contribution of genetics to yield gains of cotton (Gossypium hirsutum L.). The average U.S. lint yield did not increase from 1866 to 1935. Yield increased at a rapid rate of 10.4 kg ha−1 year−1 from 1936 through 1960. From 1961 through 1980, yields declined an average of 0.92 kg ha−1 year−1. Although annual production seems to have plateaued, genetic yield gains were continuous from 1910 through 1980.
Two procedures for estimating genetic contributions to yield were used. First, results of yield tests from 15 regional USDA, state, and private organizations were used. Results were obtained from 63 locations, several per region, and covered periods ranging from 11 to 19 years in the 1960s and 1970s. One or two entries common to all tests within a region were used to provide a covariant to adjust yields of all other entries to a common base. The regression of adjusted yield on years of testing indicated an average genetic improvement of 7 kg ha−1 year−1, or about 0.74 % /year.
The second procedure compared obsolete and modern cultivars in three studies: Stoneville, Mississippi, 1967 to 1968; Stoneville, Mississippi, 1978 to 1979; and Florence, South Carolina, 1979 to 1980. These studies indicated that annual rate of lint increase due to genetic improvement was 10.2, 9.5, and 11.5 kg ha−1year−1, respectively, or an average for all three studies of 10.4 kg ha−1 year−1.
Uniform regional wheat nursery yield data for experimental lines and long-time check cultivars provide a means for determining genetic improvement for yield. Yield data from nine nurseries—six winter wheat and three spring wheat—from 1958 through 1980 were examined. This period contains the largest gains in genetic improvement since the nurseries were established in the early 1930s. Genetic improvement has been the least in those nurseries grown in the harsher climatic regions of the USA and greatest in the more productive wheat regions. When data from all nine nurseries were composited, they showed a consistent yield increase over 1958 to 1980 from a 3-year average of 25% over the long-time check cultivars in 1959 to a 3-year average of 46% in 1979. This represents a total increase of 17%, or an annual rate of gain of about 0.74%, for that period.