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.