Key Cultivation and Management Techniques for Spring-Sown Rapeseed
Rapeseed, also called oil bok choy or bitter greens, is a plant of the Brassicaceae family and Brassica genus, native to our country. Its stems are dark green, and its leaf stalks resemble those of bok choy. It is a variety of Brassicaceae bok choy, and its flowers are yellow. In agronomy, the various plant species whose seeds contain oil are collectively called rapeseed. At present, the main cultivated (variety) types of rapeseed are: Brassica rapa (bok choy type), Brassica juncea (mustard type), and Brassica napus (cabbage type). They are mainly distributed in Anhui, Henan, Sichuan and other places. Rapeseed is rich in nutrients, among which the vitamin C content is very high. Rapeseed generally grows in relatively humid climates, such as in southern China. Rapeseed also has many uses; for example, rapeseed flowers can be eaten when the buds are still unopened; when rapeseed flowers are in full bloom, they also form a beautiful scenic line (Hanzhong City in Shaanxi, China, has a rapeseed flower sightseeing tourist area); after the flowers fade, rapeseed seeds can be pressed for oil.
Growing Conditions for Rapeseed
Temperature: Rapeseed is a crop that prefers cool conditions and has relatively strong cold resistance. According to experiments, the minimum temperature for seed germination is 4-6°C; at 20-25°C, seedlings can emerge in 4 days; the flowering stage is 15-19°C; the pod development stage is 12-15°C, and a large day-night temperature difference favors flowering and pod development and increases the accumulation of dry matter and oil.
Water: Rapeseed has a long growth period, a large vegetative body, and many seed-bearing organs, so it requires more water. The water requirements at each growth stage are: during the germination and emergence stage, soil moisture should generally be maintained at about 65% of field capacity; during the bolting stage and flowering stage, it should be 76-85% of field capacity; during the pod development stage, it should be 60-80% of field capacity.
Fertilizer: According to measurements, for every 100 kg of rapeseed produced, the ratio of nitrogen, phosphorus, and potassium is 1:0.35:0.95, and the demand for these three elements is more than 3 times that of cereal crops. In addition, rapeseed is relatively sensitive to the trace element boron, and deficiency easily causes flowers but no seeds.
Soil: Rapeseed is a taproot crop with a relatively developed root system, a main root that penetrates deeply into the soil, and many lateral and fine roots. It requires deep soil with good structure and rich organic matter, a loamy soil that both retains fertilizer and water and is loose and well aerated. In weakly acidic or neutral soil, it is even more conducive to increasing yield and raising the oil content of the seeds.
Cultivation and Management of Spring-Sown Rapeseed
After the beginning of spring, rapeseed enters the stages of vegetative growth and reproductive growth. At this time, doing a good job of field management is very important. In the management of rapeseed after the beginning of spring, the first task is to apply bolting fertilizer heavily.
After rapeseed buds appear, it begins to change from the vegetative growth stage to the reproductive growth stage. At this time, rapeseed has a developed root system and absorbs a large amount of nutrients. If fertilizer is insufficient, it will affect stem and leaf growth and flowering and seed setting, so more fertilizer should be applied. This is called bolting fertilizer. Especially for bok choy-type rapeseed, because its flowering and seed-setting period is shorter and its fertilizer tolerance is poorer, bolting fertilizer should be applied even earlier, generally applying the fertilizer to the field before bolting. After flowering, fertilizer should not be applied again, to prevent delayed senescence and disease.
During the flowering period of rapeseed, waterlogging prevention must be done well. The bolting period of rapeseed requires a large amount of water. As the farming proverb says, “If you want oil, let the ditch water flow in February.” If wilting of the lower rapeseed leaves is found at noon on a sunny day, irrigation should be carried out in combination with fertilization. When irrigating, note that it is enough for the soil to become moist. It is not suitable to irrigate too much.
Field Management
1) Do a good job of clearing ditches and draining water to reduce field humidity.
2) Remove weeds in the field, promptly remove diseased, old, and yellow leaves from the lower part of the plants, and carry them out of the field for centralized treatment, so as to reduce the base number of disease and pest sources.
Spring is also the main season for the occurrence of diseases and insect pests in rapeseed. A common disease is white rust. White rust is most favored by a temperature of 10°C. It is most likely to occur during continuous spring rain and when field drainage is poor, and rapeseed is most susceptible during the bolting and flowering stages. Rapeseed that has suffered freeze damage and has weakened disease resistance is also prone to the disease; early-maturing varieties suffer more severely; in continuous cropping rapeseed fields, the increase in inoculum also makes it more severe. In addition to selecting cabbage-type rapeseed with relatively strong disease tolerance, strengthening cultivation management, paying attention to clearing ditches and draining water, and reducing field humidity, do a good job of cleaning the garden and thoroughly remove diseased residues, and avoid continuous cropping with Brassicaceae crops. In the early stage of the disease, use ① 75% chlorothalonil wettable powder at 100 g per mu, diluted with water to 60 kg for spraying; ② 80% mancozeb wettable powder at 100 g per mu, diluted with water to 60 kg for spraying; ③ 70% thiophanate-methyl wettable powder at 60 g per mu, diluted with water to 60 kg for spraying. In dry seasons, aphids often occur in large numbers. After they are found, use 40% dimethoate emulsifiable concentrate at a concentration of 1:500 and spray 200 jin per mu.
The following key points should be noted when spring sowing rapeseed:
1. Sow at the appropriate time. In March, according to soil moisture and ground temperature, pay attention to grasping the sowing time well.
2. Sowing: When the daily average temperature is stable at 2-3°C and the soil has thawed 5-6 cm, sowing can begin. The sowing rate is 500-800 g/667 sq m. Apply 4-5 kg/667 sq m of diammonium phosphate and 1-2 kg/667 sq m of urea. Achieve accurate sowing rate, uniform seeding, straight sowing rows, and firm soil covering, with no repeated sowing or missed sowing.
3. The sowing depth should be controlled at 3 cm to 5 cm. The sowing quality should achieve three uniformities (row spacing, plant spacing, and sowing depth).
4. Irrigation: The bolting stage is the critical water period for spring rapeseed, and especially the flowering stage has an urgent demand for water. Depending on the growth of spring rapeseed and the field water-holding capacity, generally irrigate 2 times. Irrigate once after rapeseed bolts and before flowering, and irrigate once with light flowing water in the late flowering stage.