THE methods of stand establishment can be broadly divided into direct sowing of pre- or un-germinated seeds and transplanting of seedlings. Traditionally, nursery seedlings are raised which are then transplanted in standing water. This traditional transplanting system not only helps in controlling the weeds but also ensures a premium quality paddy.
But the looming water crisis and increasing labour cost are inducing farm experts to find out alternate ways of seedling establishment. Direct sowing of germinated or un-germinated seeds is one of the alternatives.
The most suitable planting technique depends on locality, soil type, and crop ecosystem. Crops can be direct seeded or transplanted. Similarly, transplanted crops can be established manually or by machine. Direct seeded crops tend to mature faster than the transplanted crops but have more competition from weeds.
Direct seeding method is becoming more popular among rice farmer as it is economical than transplanting. The yields are also comparable with transplanted rice if crop is properly managed. Direct seeding methods could be divided into wet seedling and dry seeding.
Wet seeding: In wet seeding pre-germinated seeds are broadcasted into puddled and levelled field which are free from standing water. At the time of puddling basal fertilizer mixture should be added.
After germination of seed, seedling desiccation due to water stress should be avoided by intermittent wetting of the field. When seedlings are of about 5cm tall (about a week after sowing) water is impounded to prevent germination of weeds and desiccation of seedlings. The stand establishment by this method varies with the quality of land preparation, weed competition, water management and rainfall during the initial period after sowing.
Row seeding of germinated seeds could also be done but it is practiced on a limited scale because of the cost and the difficulty in obtaining implements. This method of sowing will help in controlling weeds, especially mechanical control and management of the crop. This system also helps in maintaining optimum density of seedlings, whereas random broadcasting often leads to low or high seedling density. Selection of a suitable variety for direct seeding is important as there is a genotypic variability in germination under submerged conditions.
However, if field can be maintained at or below field capacity for about five days, focus should be on varieties which process good initial seedling vigour. Seedling vigour is mainly determined by the seed quality and other cultural practices. Stand establishment is often poor with direct seeding because of poor quality seed paddy, poor land preparation, weed competition, poor water management, unfavourable environmental conditions and physical damages.
Seed priming techniques have the potential to enhance the uniform stand establishment. Seed rates should be adjusted accordingly to have the desired panicle number. Components of yield could be divided into panicle number, seeds per panicle and seed weight. Panicle number is mostly determined by the tillering ability of a variety which is a function of the number of seedlings per unit area. Thus seed rate should be adjusted accordingly to meet this requirement.
Decreasing seed rate would increase unproductive tillering. Increasing seed rate would also increase density, which increases unhealthy seedlings with small panicles due to competition for resources, and increase susceptibility to pest and diseases.
Dry seedling: In this method dry seeds are sown to moist soil either in rows or in random (just like wheat). Seed rate generally vary with the severity of the environment and the type of physical damages to the seeds.
Depending on the level of weed infestation in dry seeded rice the seed rate should also be increased. However, if conditions for rice seed germination and subsequent operations are favourable, the seed rate for dry seeding could be reduced. Direct seeded crops can be established using dry, pre-germinated or primed seed. They are broadcast by hand or planted by machine.
Transplanting: The extent of transplanted rice is decreasing in most of the rice growing countries due to the scarcity of labour and other resources. Transplanting will also decrease rice plants ability to withstand moisture stress. It has been reported that transplanting increase the yield of long age varieties when compared with broadcasting because transplanting reduces the excessive build up of vegetative biomass due to transplanting shock.
In transplanted rice, spacing between hills varies with the variety and seedling age. A spacing of 20 x 20cm is recommended. A hill should be planted with two healthy seedlings. For transplanted rice seedling, age is a major factor in determining yield.
Transplanting shock, which is a setback to growth due to uprooting and replanting of seedling, increases with the increased age of seedling. In general, the effect of transplanting on yield increases with the decreasing age.
Seedling age (in calendar days) also vary with the environmental condition and the type of nursery. The physical and bio-chemical factors would set a minimum and maximum age for a particular nursery. Minimum age of a seedling for transplanting would be about 15-20 days. However, ideal seedling age is about 30 days; tillering capacity is reduced if older seedlings are transplanted.
Methods of raising nursery seedlings: Rice, which is to be transplanted into puddled soil, must first be nursed on seedbeds. The main reason for raising nursery is to provide the seedlings a substantial head-start on weeds. Four types of nurseries are used in the world - the wet bed nursery, the dry bed nursery, the dapog and mat type in trays.
Wet bed and the dry bed methods are practiced in Pakistan; dapog is restricted to South East Asia, while mat type is being introduced in the country. Each type has some advantages and disadvantages. It should always be kept in mind that it is really very easy to raise healthy seedlings if one is prepared to take enough time to do the job properly.
Success in raising healthy rice seedlings depends mainly on constant supervision of seedbeds and proper management. Different types of nursery raising techniques are given in the following lines.
The wet bed nursery: The wet bed method of raising rice seedlings is the most popular worldwide. It is mainly used in areas where water is adequate for nursery establishment. Before sowing of germinated seeds, soil is thoroughly puddled and levelled. Addition of organic manure (decomposed) and small amount of inorganic fertilizer as basal dressing increase easiness of uprooting of seedlings and seedling vigour.
Total seed bed area is about 1/20 of the area to be transplanted. Nursery site should be with adequate irrigation and drainage facilities. Quality rice seeds should be soaked in clean water for a minimum period of 24 hours and incubated about 24 hours.
Sprouted seed should then be broadcasted uniformly on the nursery bed. Before seeding the nursery should be drained completely. There after nursery should be maintained in moist condition for about five days. Once the seedlings are established, the nursery is impounded with water. Water level is then raised gradually.
The best stage of transplanting seedling is about 20-25 days. Younger seedlings are always preferable, as they establish themselves more quickly. The “fourth leaf” stage is generally regarded as optimal. Remember to flood the beds completely to minimize damage to the seedlings while uprooting. Uproot seedlings by holding a few at a time between thumb and forefinger at the base of the culm and pulling sideways.
Always handle seedlings with extreme care. Seedlings which are handled gently during uprooting and transporting recover much more quickly when transplanted than those which are crushed, bruised, or allowed to dry out.
Seedlings in the wet bed nursery can be attacked by insects, although pest infestation is generally not a big problem. If it becomes necessary to use a chemical pesticide, do not spray until the seedlings are at least five days old. Nursery should be free from weeds, disease incidence and nutrient deficiencies. If such conditions occur it must be treated at the nursery level.
The dry bed nursery: This system of nurseries is practiced in dry soil conditions. Seed beds of convenient dimensions are prepared by raising the soil to a height of about 5-10 cm. A thin layer of half burnt paddy husk could be distributed on the nursery bed mainly to facilitate uprooting. In this method dry or in just sprouted seeds are sown. The site should be free of shaded and with adequate irrigation facilities.
Nursery area should be about 1/20 of area to be transplanted. Seed rate should be higher than for wet-bed method. Dry bed seedlings will not grow as fast as wet bed seedlings. Seedlings on the dry bed should be ready for transplanting 30-35 days after sowing. Nursery should be maintained without any moisture stress.
A basal fertilizer mixture could be applied and incorporated between rows if the soil nutrient supply is low. The advantage of this method is that seedlings are short and strong, has longer root system than wet bed and can be raised even during heavy rains which are not possible with wet bed.
However roots may get damaged during pulling. Seedlings of upland nurseries may also get infected with blast and are more prone to pests such as rodents etc. Water the bed to saturation before uprooting, to make sure the soil is moist and loose.
The dapog nursery: The dapog method of raising seedling originated in the Philippines and is now fairly common in South and Southeast Asia but still not practised in Pakistan.
The dapog nursery is constructed for the raising of seedlings without any soil. Rice seeds contain sufficient food in the endosperm to permit the young seedlings to grow for up to 14 days without receiving any outside nutrients except air, water, and sunlight.
Consequently, it is possible to nurse seedlings without actually sowing them in soil. The dapog nursery can be located anywhere convenient, as long as it is near a reliable water supply. It is ideal to raise dapog nurseries where they can be watched at all times, since they require constant watering and are very susceptible to birds (including chicken) attack.
Construct a raised earthen bed roughly one meter square - the exact shape is not too important and covers it with green banana leaves, or better with plastic sheet. Keep the surface of the bed as level as possible, but construct a low raised border to hold the seeds in place after they have been sown and to prevent water from escaping.
Soak and pre-germinate the seed as with the other types of nursery. Pre-germinate a little more seed than usual, because dapog seedlings are quite small when transplanted and difficult to separate; almost inevitably, the farmer will need more of them.
Spread the seed in a solid, even layer on the dapog to a depth of approximately 2cm (5-6 seeds thick). Gently sprinkle water over the seeds, taking care not to dislodge them. Keep the seeds moist all the time, without insulation of a layer of soil. The seeds will dry out very quickly if they are not watered constantly. From about the third day, keep the bed continuously flooded with a thin layer of water.
Regardless of the duration of the variety, dapog-raised seedlings are usually ready for transplanting after 15-20 days. By then, the food material in the endosperm exhausts, and the seedlings will quickly begin to die off. On the day of transplanting, simply roll up the entire seedling mass (the roots will have matted together to form a soft of rug), throw it over the shoulder and head for the field.