Changes Of Maize Cultivars And Their Parent Lines Released At Different Years In China

Although six cycles of cultivar replacement have occurred in China since the 1950s, which play an key pole in improvement of maize total yield and yield per unit area. Little information is available on what has happened to improve maize yield and associated characters. Experience from America typifying countries with well-developed maize breeding Retrospective analyses of yield gains may provide an understanding of unexploited genetic potential and reveal the strategies for future yield improvement. The objectives of the study are to reveal(1) Changes of yield and agronomic traits, and the relationship between yield and agronomic traits from the 1950 to the 2000; (2) the genetic yield gain rate and the contribution of breeding to yield gains from 1970 to 2000; (3) Heterosis of yield and other traits from 1970 to 2000, in order to useful experience for breeding in China and elsewhere.The open pollination cultivars (OPVs) in the 1950s, the double-cross hybrids in the 1960s, and the single-cross hybrids, which were used widely in their time, was grown in the study. because experimental material was in the process of collecting and adding, the study could be considered as three stage: (1) The experiment was conducted in Beijing (Shunyi), Xinjiang (Urumiq) and Hainan (Sanya) during 2005-2006. Maize cultivars were grown at three densities of 30 000 plants ha^-1, 45 000 plants ha^-1 and 60 000 plants ha^-1 (2) The experiment was conducted in the same locations with the above-mentioned in 2007. Maize cultivars were grown at three densities of 15 000 plants ha^-1, 45 000 plants ha^-1 and 75 000 plants ha^-1; (3) The experiment was conducted in Beijing, Xinjiang, Hainan, Henan and Liaoning provinces in 2008-2009. Maize cultivars were grown at three densities of 15 000 plants ha^-1, 45 000 plants ha^-1 and 75 000 plants ha^-1. Main results and conclusions were as following: (1)Yield gain increased with increased plant densities, which showed that new hybrids had more resistance to compound stress. Linear regression coefficiences at different environment indexs did not have significant diffeences for 1960s-2000s hybrids, but were reatger than that of the 1950s OPCs. Levels of reponse of all hybrids to higher-yielded environment were similar, which was greater than that of the open pollinated varieties (OPVs). Overall trends in traits of maize cultivars were towards larger plant size and later maturity, especially from the 1980s to the 2000s in China. Newer hybrids showed shorter ASI, relatively stable tassel length, fewer tassel branches, relatively stable leaf number and a more upright plant habit. Tolerance to root and stalk lodging improved greatly.(2) Mean grain yield showed a significant linear increase overseveral decades from the 1970s to the 2000s, an averageof 74 kg ha?1 yr?1. Grain yield gains of newer hybrids were higher than that of older hybrids at different yield levels. The trends were enhanced at stress. This showed that new hybrids Newer hybrids outyielded older onesat all yield levels. The degree of response of each decade's hybrids to increasing site productivity showed no consistent trend. Genetic gain over 40 yr averaged 94.7 kg ha?1 yr?1, and 53% of total yield gain in China could be attributedto breeding. The newer hybrids showed increased tolerance to compound stress (including diseases in particular). We estimated a contribution of increased stress tolerance to genetic yield gain of 46% at 60,000 plants ha?1 and 74% at 75,000 plants ha^-1. Hybrids improved slightly in tolerance to the density of 45,000 plants ha?1 and did not attain greater tolerance to high densities of 60,000 to 75,000 plants ha?1. To continue increasing yield at higher plant densities in China, breeding for improved stress tolerance under high plant populations is required. This can be achieved by breeding inbred lines and their hybrids at high plant populations.(3) Plant height of parent lines was relative stable from the 1970s to the 2000s. Ear height decreased, and the trend was enhanced at higher densities. Days feom seeding to anthesis and from seeding to silking was relative stable, but ASI shorten clearly. Resistance of parent lines to root lodging and stalk lodging increased obviously. Resistance to root lodging improved significantly from 1980-1990, and resistance to stalk lodging improved significantly from 1970-1980. Disease resistance improved significantly from 1980-2000. Tassel length was relative stable, and tassel branch number reduced. Leaf number was relative stable, leaf angle increased, and the greatest leaf area increased. Ear length of parent lines increased, but decreased from 1990-2000. Row number per ear was relative stable. 100 kernel weights increased significantly. Shelling percentage reduced, especially from 1980-2000. Correlation analysis showed that plant height was negative significant correlative with grain yield. Tassel branch number was positive significant correlative with grain yield. Days feom seeding to anthesis and from seeding to silking, ASI and barrent parentage was extremely significantly negative correlative with grain yield. Kernel number per row, row number per ear and shell percentage was extremely significantly positive correlative with grain yield.(4)Absolute heterosis of grain yield increased at a rate of 38 38 kg ha^-1 year^-1 from 1970-2000. The trend enhanced with increased plant densities. Relative heterosis had been relative stable. There were no trends for absolute heterosis of grain yield under different environment. Heterosis varied with different traits and different eras. 100 kernels weight, kernel number per row and ear diameter contributed most to relative heterosis of grain yield. 100 kernels weight and row number per ear contributed most to relative heterosis of grain yield. 100 kernels weight and row number per ear in 1980s. 100 kernels weight and kernel number per row in 1990. 100 kernels weight, ear length and kernel number per row contributed most to relative heterosis of grain yield.