a Director, Asia and Pacific Seed Association,
b General Manager, Hybrid Rice International, Hyderabad, India
Rice is the staple food of more than 60 percent of the world population. About 90 percent of all the rice grain in the world is produced and consumed in Asia. In India alone, rice is cultivated on more than 44.6 million ha (Mha) with an average productivity of 3.0 t/ha. In countries such as China, where rice is cultivated on nearly 30.5 Mha, average productivity is nearly 6.2 t/ha. Such differences in productivity depend mainly on the genotypes used, the quality of the seeds, the management practices adopted and the climatic conditions during the crop growth period. Since high quality seed is free from various diseases and has better seed health, it tends to produce healthy seedlings with no initial disease inoculums. Asuperior quality seed not only increases productivity per unit area, but it also helps produce uniform crops without any admixtures - important for obtaining high prices on the market. High-quality produce results in high milling recovery and better quality rice, which translates into increased profits.
In order to increase crop productivity, significant efforts in breeding new conventional varieties and hybrids have been made. Given the very large area cultivated under conventional varieties and the vast potential of hybrid rice in Asia, it is increasingly important to have genetically pure and good quality seed. Quality seed production procedures for conventional varieties are well known for different classes of seed. Since rice is a self-pollinated crop, maintenance of seed purity is much easier with varieties than with rice hybrids. This paper deals primarily with hybrid rice seed production procedures, which are different from the procedures adopted for varieties. It should be noted, however, that for parental line seed production the procedures are similar to those for varieties.
SEED PRODUCTION OF HYBRIDS THROUGH THE THREE-LINE SYSTEM
The success of hybrid rice cultivation depends on the success of the hybrid rice seed production programme which enables seed producers to produce high quality seed at an economical price. Hybrid rice seed production requires specialized techniques which must be fully understood by the production staff.
Hybrid rice seed production using the CMS system, i.e. the three-line system: A line (female), B line (maintainer) and R line (restorer) involves three steps:
1. multiplication of A line
2. multiplication of B and R lines
3. production of hybrid seed (A x R)
In order to obtain the best quality F1 seed in the hybrid seed production programme, high genetic and physical purity of the parental lines is a prerequisite. Even a very low percentage of impurity or contamination in a parental line can cause the production plots to reject the seed. A slight impurity in parental lines can also lead to high costs because of the enormous efforts in rouging which can be very costly for the hybrid seed producer. Impure parental lines lead to variation in plant type, duration, plant height and grain size, and ultimately the quality of the F1 hybrid is affected. It is therefore essential to adopt methods to ensure quality seed production. Quality seed should have the following characteristics:
true-to-type genetic purity
no contamination or admixture in the seed
high germination capacity
free from disease
free from weeds, soil particles, sand and stones
no broken seeds
In order to achieve higher seed yields, rice seed production should be undertaken in the most favourable areas where irrigation is guaranteed, and with adequate and balanced use of fertilizers together with integrated nutrient and pest management. Some standardized seed production practices are described below:
Selection of land
The land selected for seed production should be fertile, preferably light-textured, with adequate irrigation and a proper drainage system. The field should be free from weeds and volunteer plants from the previous paddy crop. In order to achieve synchronous flowering, a homogenous plot with an even topography is required. The field should not be infested with serious pests and diseases. Hybrid rice seed production fields should be isolated as rice pollen can travel longer distances with the wind; negligence leads to impurity of F1 seed. When selecting for isolation of land, the following points must be considered:
Space isolation: Space isolation of at least 100 m from seed production plots to other rice varieties is normally satisfactory for quality hybrid seed production. It is safer to have an isolation distance of up to 200 m for male sterile (A line) multiplication, while for B and R line multiplications in varieties, an isolation distance of 3 to 5 m is sufficient.
Time isolation: When space isolation is not possible, time isolation of about 30 days is satisfactory. This means that the flowering stage of the parental lines in the seed production field should be at least 30 days earlier or later than that of other varieties grown within the area to avoid contamination by pollen.
Barrier isolation: Tall and compact trees or bushes or some tall crops (e.g. sorghum, pigeon pea and sugar cane) with 30-40 m distance can serve as barrier isolation.
Space isolation is the most important factor to be considered for the production of quality seed.
Seeding of the parental lines should be planned in such a way that flowering coincides with the most favourable climatic conditions listed below:
daily mean temperature of 24°-30°C
relative humidity of 70-80%
differences between day and night temperature of 8°-10°C
sufficient sunshine with moderate wind velocity
no 3 days continuous rain during flowering period
These conditions are well met in the dry season.
Nursery bed preparation and sowing
Given the high cost of seed, it is essential to raise the nursery in a well-managed field if healthy and robust seedlings are to be obtained. Optimum seed rate should be applied and every seed must be utilized by adopting good nursery management practices. A sparse well-managed nursery gives healthy seedlings for the main field. The normal recommendation is 1 kg of parental line seed in an area of 40 m2. For 1 ha of main field, 12.5 kg of A line seed and 5 kg of R line seed are required.
Staggered sowing of parental lines for flowering synchrony
Hybrid seed set on the female line depends primarily on its flowering synchronization with the R line; the sowing of male and female lines must therefore be planned to achieve this. For example, if the duration of the male line is 10days more than that of the female line, the male line is sown in 2-3 staggered sowings so as to ensure a continuous pollen supply. In such cases, 3 sowings of the R line (i.e. 13, 9 and 5 days ahead of the female line) are carried out. However, in countries such as China where the technology has been perfected, only 1 or 2 sowings of the male line are necessary.
Conventional high-yielding varieties may be transplanted once the nursery crop is 25-30 days old; but in hybrid seed production plots transplanting may commence (depending on the difference in duration of the A and R lines) when 21-35 days old. Timely transplanting ensures good picking of parental lines. Transplanting of too young or very old seedlings may either delay or accelerate flowering and affect tiller number.
While pulling out the nursery and during transplanting, special care should be taken to avoid mixing seedlings of male and female parents. It is also important to avoid mixing seedlings of different ages of the male parent, which could affect the uniform distribution and availability of pollen. The long- duration parental line must be transplanted first in order to obtain good synchronization at flowering.
Transplanting R lines
Paired rows with 15 cm spacing between plants.
Seedlings of different ages transplanted in a sequential order (e.g. I, II, III, then again I, II, III).
Single seedlings per hill with row-to-row spacing of 15 or 30 cm (as per recommendation) in the main field.
Transplanting A lines
Six rows with 15 cm spacing between the paired rows of R line seedlings is the normal recommendation in many Asian countries.
One seedling per hill with a spacing of 15 x 15 cm.
In India, spacing of 30 cm between A line and R line rows to facilitate bumper male growth and supplementary pollination. In China, A: R row ratio varies from 2: 8 to 2: 14.
Row ratio, spacing and direction
Years experience in hybrid rice seed production indicates that row ratio and spacing play a major role. Seed parents and pollen parents planted in a specific row ratio and with specific spacing have a marked effect on seed yields. A row ratio of 6: 2 seed parent to pollen parent has proven very effective. Row direction perpendicular to the prevailing wind direction at flowering stage allows easy pollen dispersal on the seed parent.
Use of chemical herbicides within 3 to 4 days after transplanting the male lines is recommended.
Optimum fertilizer management
Application of farmyard manure (FYM) at a rate of least 10 t/ha and a fertilizer dose of 120: 60: 40 kg/ha NPK is recommended in the main field. In order to achieve optimum fertilizer-use efficiency, it is recommended that the fertilizer be placed in the root zone with split application: one basal and two top dressing, at tillering and at panicle initiation. In general, split application of N will prolong the pollen supply of the male line and increase tillering capacity. One heavy N application results in a leafy crop, whereas controlled N levels during middle and late crop growth stages prevent excessive growth of flag leaf and provide good aeration and sunshine, which is good for pollen spread.
Rice crop normally suffers from zinc deficiency; application of zinc sulphate at a rate of 50 kg/ha provides the necessary supplement.
Following transplanting, the main field should be irrigated or drained based on the growth stage of the crop:
Up to the third stage of panicle development: shallow (2-3 cm).
From the third stage of panicle development to heading: about 5 cm.
From heading to grain filling: no shortage of water.
One week before harvesting: water drained out.
For optimum synchronization of flowering, the female parent should flower 2-3 days earlier than the male parent.
If A and R lines have the same growth duration, the A line should flower 1-2 days earlier than the R line in all panicle developmental stages.
If the A line has shorter duration than the R line, the R line should be one stage earlier than the A line during the first three panicle development stages.
If the A line is longer than the R line in growth duration, the A line should be 2-3 days earlier than the R line during the first three panicle development stages.
Adjustment of flowering
If the difference in predicted flowering is more than 3 days between the parental lines, measures should be taken to synchronize flowering. The application of quick-releasing N fertilizers on an early-developing parent in the early panicle development stages tends to delay flowering. Similarly, spraying phosphatic solution (1%) on the later-flowering parent tends to enhance flowering by 2-3 days. If the pollen parent (R line) reveals a tendency towards heading earlier than the seed parent (A line) after the third stage of panicle initiation, root zone placement of N fertilizer is helpful in delaying panicle development.
Leaf clipping of A and R lines is helpful for better out-pollination and seed set. Long and erect flag leaf may obstruct pollen dispersal from the R to the A line and affects the outcrossing rate. Flag leaves should be clipped off in such cases, when the main culms are still in the boot leaf stage. Flag leaf clipping gives uniform distribution of the pollen over A line plants. However, it is not advisable to perform leaf clipping in areas where diseases such as bacterial leaf blight, sheath blight and bacterial leaf streak prevail, as they may spread further and reduce seed yield.
Use of GA3
GA3 is used to enhance panicle exertion. Female lines with WA cytoplasm have poor or incomplete panicle exertion. Spraying GA3 not only helps exert the panicle but also increases the duration of floret opening, improves stigma exertion and stigma receptivity, and widens the flag leaf angle. Spraying GA3 increases plant height by 10-15 cm and it can also be used to adjust the plant height, in particular of R line in relation to A line. In India, a dose of 50 g GA3/ha has been found to be optimum (30 g GA3 sprayed at 5-10% heading and another 20 g GA3 1 day later, i.e. a 1-day gap between the two sprayings). If the male line is no higher than the female line, it is advisable to give one extra dose of GA3 to the R line to increase its plant height. GA3 should preferably be sprayed in the evening (15.00-18.00 hours) and on sunny days.
Rice is basically a self-pollinated crop. Supplementary pollination serves to enhance the outcrossing rate in order to increase seed set. Supplementary pollination should be done by shaking the pollen parent with the help of ropes or sticks so that the pollen is shed effectively on A line plants. Supplementary pollination needs to be done 3-4 times at 20- to 30-minute intervals and should be continued for 10-12 days during flowering. With improved management of parents and effective supplementary pollination, hybrid seed yield can be increased significantly.
Purity of the hybrid seed is top priority for the production of quality seed. Roguing of off-types and voluntary plants at several stages is essential for obtaining physical and genetic purity. Roguing is the removal of undesirable rice plants from both parents (male and female). Undesirable plants include off-types (eg. maintainer or B-type plants in A line). Off-type plants can be identified by their morphological characters (eg. height, leaf size, leaf shape and colour, panicle shape, panicle size and pigmentation) in the late vegetative/early flowering period. B line plants with similar morphological features to A line plants can be identified by their plumpy anthers, completely exerted panicle and 3-4 days earlier flowering compared to the A line. These plants in the A line row must be uprooted as soon as they are identified. Roguing at an appropriate time (flowering initiation) ensures good seed quality. Roguing is normally done from the vegetative to the flowering stage.
In order to maintain high purity, extreme care should be taken at harvesting and threshing. Just before harvesting, check female rows for left-over pollen shedders (i.e. maintainer plants), off-types and male parent plants (restorers). After confirmation that the field does not contain unwanted plants, the male rows should be harvested first and all panicles carefully removed. Move the harvested male plants to a separate threshing floor where only male parents should be threshed. The female rows which are ready for harvest should be carefully rechecked for left-over plants or panicles of male (restorer) parents. Female plants should then be harvested and threshed on a separate threshing floor. The threshing floor should be free from seed of the previous crop and must be very clean. Threshing can be done by a harvest combiner, thresher, tractor or bullocks. With the harvest combiner or thresher, extreme care must be taken to clean the machine thoroughly before use, so as to avoid admixture with other varieties of paddy.
After threshing, the seed should be properly dried and bagged. The bags need to have labels inside and outside, listing all necessary details, such as lot identification or lot number, date of harvest and quantity of lot.
Seed is an important and basic input for achieving higher crop yield and increasing a countrys agricultural economy. Thus it is very important to maintain seed quality by understanding the right mechanism. Seed quality means preserving and maintaining physical and genetic purity. Normal standards for genetic purity are given in Table 1.
Standards for genetic purity
Class of seed
Minimum genetic purity in percentage
Commercial hybrid seed - physical purity
Seed arrivals and processing
As soon as seed arrives at the seed processing plant from seed production areas, lot-wise seed weighing of every bag should be recorded and lot-wise representative samples drawn. These samples should be used for laboratory testing of germination percentage, moisture percentage, physical purity and seed viability. Based on the test results, the seed from good lots should be processed in the seed conditioning plant.
Seed conditioning or processing should be done one hybrid at a time. Extreme care should be taken before beginning work in the seed processing plant. The processing plant must be cleaned thoroughly for any left-over seed from an earlier processed variety or hybrid, so as to avoid contamination. In order to maintain good quality standards, the seed is passed through several machines:
Pre-cleaner: eliminates inert matter (chaffy seed, big stones etc.) and reduces the workload of the cleaner.
Cleaner: eliminates remaining inert matter in the seed (sand, fine dust, chaffy seed and undersized or small seed).
De-stoner: separates mud balls, small stones etc., which are heavier than the seed, using higher specific gravitational force.
Gravity separator: separates light from heavy seed; seed is then of uniform size and clean.
Treator: seed is treated with the right proportions of chemicals (Thiram, Chlorophyriphos, Sevin, K-Obiol etc.).
The seed is then packed as truthfully labelled. The label contains information such as: physical purity percentage (min.); genetic purity percentage (min.); inert matter percentage (max.); other crop seed percentage (max.); weed seed number/kg (max.); germination percentage (min.); and moisture percentage (max.).
SEED PRODUCTION OF A LINE
For nucleus seed production of A line, the originating breeder hand-crosses true-to-type A/B plants.
For breeder seed production of A line, the breeder plants A/B rows in a ratio of 2 female: 2 male rows with a minimum isolation distance of 200 m. Timely roguing of the plot is carried out at various crop growth stages from vegetative to harvesting.
For foundation seed production of A line, the process is supervised by a foundation seed expert. The seedlings are planted in a ratio of 4 female: 2 male rows with a minimum isolation distance of 150 m. There are normally two sowings of the male line, sowing the B line 3 and 6 days after the female line.
SEED PRODUCTION OF B AND R LINES
Nucleus, breeder and foundation seed of B and R lines are multiplied adopting practices similar to those used for conventional varieties. However, care must be taken not to harvest B and R line seed from A/B or A/R seed production plots. Separate multiplication of B and R line plots helps maintain purity and produce quality seed.