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Soybeans are self-pollinating crops, which can lead to a decline in variety quality due to factors such as mechanical cross-contamination, unfavorable weather conditions, and suboptimal cultivation practices, along with natural hybridization. As a result, seeds that are self-seeded cannot be used for long periods. To maintain the integrity of high-quality soybean varieties, it is essential to regularly renew the seeds, typically every three years. This requires consistent efforts in purification and rejuvenation, as well as effective breeding and selection strategies.
One common method for purifying and rejuvenating soybean varieties involves three key steps: single-plant selection, strain identification, and mixed propagation. In the single-plant selection nursery, plants that exhibit desirable traits—such as grain color, shape, and umbilical color—are selected for further testing. Each selected plant is then threshed individually, and those that do not match the original variety’s characteristics are discarded.
In the strain identification nursery, each single plant is grown as a separate line. These lines are then evaluated against the original variety, and any that deviate are eliminated. The planting design usually includes 10 to 20 lines per control line (the original variety). Only the most promising strains are chosen for further breeding.
During the mixed propagation phase, the identified strains are planted at equal intervals and harvested together. The resulting seeds are then sown in large quantities to produce high-quality breeding stock.
Another approach involves selecting mature, vigorous plants that display the typical traits of the variety. These are carefully inspected indoors, threshed together, and stored separately for use as breeding seeds the following year. The breeding field employs advanced agricultural techniques and strict weed and impurity removal to ensure seed purity.
A single plant with the desired traits is selected, and its offspring are carefully bred and propagated. This method expands the number of single-line seeds produced, though it takes slightly longer than traditional strain selection.
To support large-scale production, a dedicated seed field must be established after purification and rejuvenation. Since the initial quantity of purified seeds is limited, a sufficient amount of breeding seed must be produced. The size of the seed field is generally around 4% to 5% of the area of the main production field or the next higher-level seed field.
For the seed field, it is important to choose plots with flat terrain, good drainage, and uniform soil. Avoiding heavy or poorly drained land is crucial. Organic manure and balanced fertilizers, particularly nitrogen and phosphorus, should be applied. Field management should include timely thinning, weeding, and soil cultivation to keep the soil loose and free of weeds. During flowering and grain filling, additional top-dressing may be applied, and irrigation should be provided during dry periods to prevent pests and diseases.
Throughout the growing season, plants are closely monitored, and any that show undesirable traits during the seedling, flowering, or maturity stages are removed. After harvest, careful handling during cutting, threshing, and storage is essential to avoid mechanical contamination.