Soybeans are primarily self-pollinating crops, which makes them prone to genetic degradation due to factors such as mechanical cross-contamination, unfavorable weather conditions, suboptimal growing environments, and natural hybridization. As a result, seeds produced through self-seeding cannot be used for long-term cultivation. To maintain the quality and purity of soybean varieties, it is essential to replace the seeds every three years. Therefore, consistent efforts in purification and rejuvenation must be made, along with proper breeding and variety management.
The process of purifying and rejuvenating soybean varieties typically involves three key steps: individual plant selection, line identification, and hybrid propagation. These stages help ensure that only the best and most representative plants are retained for future planting.
In the single plant selection nursery, individual plants exhibiting the desired characteristics—such as grain color, shape, and umbilical color—are selected for testing. Those that deviate from the original variety's traits are discarded. The selected plants are then threshed individually to collect their seeds.
Next, in the line identification nursery, each plant is grown separately as a distinct line. This allows for careful evaluation of each strain against the original variety. Any lines that do not meet the standard are eliminated. A control line of the original variety is often included among every 10 to 20 lines to serve as a reference. The best-performing strains are then selected for further breeding the following year.
In the hybrid propagation nursery, the seeds from the previously identified and mixed strains are planted at uniform distances and cultivated under high-density conditions. This helps enhance genetic stability and vigor. The seeds produced from these three nurseries are referred to as "super purified rejuvenation stocks."
Another method, known as mixed selection, involves selecting a number of healthy, vigorous plants during the mature stage that exhibit the typical traits of the variety. These plants are then carefully selected indoors, mixed, and threshed. Only the best seeds are kept for use in the next growing season.
For breeding purposes, one-line selection is also employed. This involves choosing a superior single plant with all the desirable traits of the original cultivar and propagating its offspring. This method expands the availability of single-line seeds and is slightly more time-consuming than simple strain selection.
To scale up production, seed fields are established using the super-purified stock seeds. Since the quantity of these seeds is limited, it's necessary to grow them in dedicated seed farms to produce enough material for commercial use. The size of the seed field depends on the amount of seeds sown and the expected yield per acre. Typically, the seed field should occupy about 4% to 5% of the area of the main production field or the next higher-level seed field.
When selecting plots for the seed field, it is important to choose flat, well-drained land with uniform soil and no heavy clay or waterlogged areas. Organic manure and balanced fertilizers containing nitrogen and phosphorus should be applied according to recommended ratios. Field management should be strict, including timely thinning, weeding, soil cultivation, and maintaining loose, weed-free soil.
During flowering and grain-filling periods, additional top-dressing may be applied once or twice, and irrigation should be provided during dry spells to prevent pests and diseases. Throughout the growing season, strict elimination of off-type plants based on morphological traits—such as seedling development, flowering, and maturity—is crucial.
At harvest time, timely collection and thorough processing of the seeds are essential to avoid mechanical mixing, which can compromise the purity and quality of the final product.
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