Tall seedlings are one of the most common and easily formed abnormalities in rapeseed fields. They typically occur when there is an excessive seeding rate, too much fertilizer and water, or when seedlings are left in the seedbed for too long. This causes the main stem to elongate prematurely, especially in early and mid-season varieties. These tall seedlings are prone to falling over after transplanting, losing their lower leaves, and exposing the stem base. In the event of a cold snap, the stems can be damaged or even frozen and cracked, leading to weak growth and poor yield. To avoid this, it's important to manage the timing of planting and apply growth regulators. At the 3-leaf stage, spraying with paclobutrazol (15% concentration) at a rate of 50 grams per acre mixed with 40–50 kg of water can effectively prevent the formation of tall seedlings.
Planting too early or too late can also lead to issues. If sown too early, the temperature may be higher, causing rapid leaf growth and longer seedlings, which makes them more vulnerable to frost damage during winter. On the other hand, planting too late results in slower growth due to lower temperatures, which can hinder development during the autumn and winter seasons.
Leggy seedlings often appear when there is an overuse of nitrogen fertilizer combined with high temperature and humidity conditions. This leads to excessive vegetative growth, making the plants weak and tender. After transplanting, these seedlings tend to wilt, take longer to green up, and are more susceptible to cold stress, which negatively affects their ability to survive the winter and thrive in spring. To prevent this, proper sowing time and controlled fertilization are essential. For seedlings showing signs of excessive growth, spraying with chlorothalonil at a concentration of 50–100 ppm when they have four leaves and one heart can help control the problem.
Thin and weak seedlings, also known as "shady foot seedlings," usually result from poor seedbed conditions such as overcrowding, lack of light, and insufficient nutrients. These factors cause the plants to grow thin and weak, with reduced chlorophyll content. To prevent this, it's important to prepare sufficient seedbeds—generally around 20% of the total planting area—with well-drained, loose soil and good fertility. After emergence, timely thinning should be done, and any weak seedlings should be removed. At the three-leaf stage, applying a small amount of fertilizer and water helps promote strong growth and healthy development.
Dwarf seedlings, also called stiff seedlings, occur when the seedbed soil is infertile, or when there is inadequate water and fertilizer. Prolonged waterlogging, delayed sowing, or exposure to cold waves can also contribute to this issue. These seedlings have short stems, thin roots, and small, pale leaves, resulting in weak photosynthesis and stunted growth. After transplanting, they struggle to recover and often fail to develop properly during the winter and spring seasons, leading to low yields. To prevent this, it's crucial to maintain appropriate seedling age—around 35–40 days for early and mid-season varieties—and ensure proper watering and drainage. If the seedbed lacks nutrients, foliar sprays or top-dressing with fertilizer can be applied to support healthy growth and avoid the formation of weak, underdeveloped seedlings.
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