Hybridization is performed between superior parents selected on the basis of breeding objectives. These important steps (the selection of parents and hybridization) in breeding programmes have already been described above. In a successful cross, a small pod can be seen after a week. The success rate of hybridization varies from 10% to 75% in soybean, depending upon the experience of the breeder (Fehr, 1987). Generally, hybridization is performed during the normal soybean growing season in the field, although some private companies hybridize in greenhouses throughout the year. After hybridization, the next step is generation advancement to produce the inbred lines. Soybean is a self-pollinated plant. Simply growing different generations will result in selfing to advance the generation to produce inbred lines. Recombinant inbred lines are grown at shuttle breeding stations to minimize the time required to achieve homozygosity in the population. In a large country such as the USA, shuttle breeding can be performed by growing soybean in the south of the country (e.g. Florida) during the winter and in the north during the summer. In smaller countries, the same effect can be achieved by collaboration between countries with appropriate environments. Furthermore, three generations can be grown near the equator, where the photoperiod remains constant throughout the year, or where about 12-14 h of light are available. After growing for five generations a soybean line will be at an almost homozygous state, after which selection can be made in homozygous lines.
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