DIRECT SEEDING
Direct seeding greatly reduces the cost of establishing a stand of rice. To get maximum yield, the most profitable amount of seeds per unit area must be used for seeding, and the land prepared as required. There are four methods of direct seeding:
- drilling in dry granulated soil
- broadcasting on puddled soil
- broadcasting in water
- drilling on puddled soil
In any of these methods, the optimum seed rate ranges from 80 to 100 kg/ha. Low-tillering varieties usually require a higher seedling rate than high-tillering HYVs. Thus, direct seeding requires more seeds per unit area than does transplanting (IRRI 1968, Mabbayad and Obordo 1970).
Drilling in dry granulated soil
In this method, the soil is plowed and harrowed dry. The soil is pulverized into small clods, of about 0.5-1 inch in diameter. Seed drilling follows, with the assurance of rain and when the soil has enough moisture for good germi-nation. Sow the seeds that are not pregerminated with a grain drill 2-4 cm deep into the soil. Three to four weeks after seeding, flood the field continuously to suppress the weeds and to encourage growth of the rice crop. This procedure requires less water compared with drilling in puddled soil, but seedlings develop more slowly in the earlier growth stage (author’s experience).
Drilling on puddled soil
The same procedure of land preparation is employed as that in the irrigated fields. Plowing, harrowing, leveling, and puddling are all accomplished with irrigation water in the field (IRRI 1980). Drainage canals are provided in case of heavy rains occurring just after seeding.
With a six-row- or eight-row-seeder (IRRI 1976), drill the pregerminated seeds into the puddled soil, which is drained of water up to saturation point. If the soil is well-puddled, the seeds should stick to the bottom of the furrow opened up by the seeder. Two to three days after seeding, let the water gradually into the field to about 1 cm below the tips on majority of the seedlings and increase gradually until a level of 5 cm is reached. With this method, it is possible to operate a rotary weeder between rows.
Broadcast on puddled soil
Land preparation is the same as drilling in puddled soil or the conventional wetland preparation. Pregerminated seeds are broadcast uniformly over the field that has just been saturated with water. Irrigation is done gradually following the growth of the seedling, starting when they are about 2-3 cm tall; the water level is maintained to 5 cm deep during the cropping period. With this method, seedlings develop rapidly but are easily damaged by birds, rodents, and heavy rain.
Broadcast in water
Land preparation is similar to drilling on puddled soil. The field is irrigated up to 3-5 cm depth and the soil particles are allowed to settle before broadcasting the seeds manually or using an aircraft.
Pregerminated seeds are broadcast uniformly over the entire area. The advantage of this method over broadcasting on puddled soil is providing early weed control and protecting the seeds from bird and rat damage (author’s experience).
However, seedling establishment is slow and seedlings would often float when water is too much and the field is muddy. Fields that are not leveled well always result in uneven growth of the crop. Thus, algae grow in abundance and will pose a problem, especially when the field receives basal fertilizers. Water management during the first month of the crop is critical and can affect yield. This is a simple case of poor land preparation, where harrowing or puddling is not done properly and a large amount of organic matter is not thoroughly mixed into the soil.
Advantages and disadvantages of direct seeding
Advantages
The major advantage of direct seeding over transplanting is the elimination of cost related to seedbed nursery preparation, care of seedlings, and pulling and transplanting operations. The savings obtained can reduce cost of production and allow this amount to be used for other inputs. Also, direct-seeded rice matures 7-10 days earlier than transplanted rice and this is important where continued cropping systems are practiced to accommodate right away the next crop. It is possible to seed and harvest five times in 2 years when using the short-maturing 120-day varieties.
Disadvantages
The main drawback of direct seeding is that it exposes the seeds to bird and rat damage before the seedlings get established. Weeds, if not properly managed through proper use of irrigation water, become a major problem as the weeds grow almost simultaneously with the crop, competing for light, water, and plant food. However, straight-row seeding makes it easy for a rotary weeder to destroy the weeds or do a manual operation. Unlike transplanted rice, the direct-seeded crop has a greater tendency to lodge because of poor and shallow root anchorage in the soil (Gorrez 1972).
When there is less than 200 mm of rainfall during the planting month, farmers have no recourse but to engage in dryland preparation and seeding onto dry or moist soil. Wetland preparation is made possible with more than 200 mm of rain for a month. Through years of experience and without other cultural alternatives, the local rainfall pattern becomes the farmers’ gauge. Many Asian farmers in the rainfed areas are engaged in wet-seeded lowland rice culture. Pregerminated seeds are broadcast on puddled fields and later the crop is left at the mercy of the elements. Early-maturing varieties of 100-day duration have proven to be good materials in ensuring even a double cropping in one year (De Datta 1981, Mabbayad and Obordo 1970).
Planting calendar
In tropical Asia, the dry-season crop is exposed to a short daylength at the early growth stages and gradually to a longer daylength at the later stages. Experiments at IRRI during the dry season revealed a strong positive correlation between grain yield and solar radiation during the last 30 days of crop growth. In 12 harvest from six cropping seasons, the highest grain yields were obtained in the month of May (IRRI 1970), when solar radiation was highest during the 45-day period before harvest.
Planting time should be adjusted in such a way that the crop receives maximum solar energy, particularly during the reproductive stage (Moomaw et al 1967). The increase in dry matter following the flowering of rice is primarily in the form of grain. It is dependent also on the amount of photosynthesis, which is primarily associated with the amount of solar energy received by the rice crop during that period (Tanaka 1966).
Another factor is the variability in the amount and distribution of rainfall, thus limiting the grain yield of rainfed rice, which constitutes about 80% of all rice grown in the world. In many rice-growing areas of Southeast Asian countries such as Myanmar, Thailand, Indonesia, Cambodia, South Vietnam, and the Philippines, the calendar year is divided into fairly distinct wet and dry seasons. In most areas in the tropics, the bulk of rice growing occurs in the wet season between April/May and September/October. The amount of rainfall received during the 5-month dry season is usually not sufficient to grow a crop of rice (Moomaw and Vergara 1964).