What is a good seed?
The rice grain or seed has a hard protective covering commonly called the rice husk; the endos-perm is made up mostly of fat, starch, and sugar that serves as a storage facility; and the embryo, at germination stage, develops into shoots and roots. The rice grain is considered good when it is big and full. This indicates that large amounts of food are avai-lable for healthy growth of the seedlings. Small and lightly shriveled seeds are to be discarded and should not be used for planting because the resulting plant will always be weak and unhealthy.
Rice is a self-pollinated plant. Carelessness in manual handling of seed can result in mecha-nical mixture of different varieties during post-harvest operations, such as drying, threshing, bagging, and storing, especially when using the same facilities. Theoretically, one seed alone can produce as much as 2,000 – 3,000 new seeds in one cropping season. Mixtures of seeds lead to deterioration of planting materials and to poor-quality seeds not capable of producing maximum yield.
Salt water treatment to select good seeds
Heavier seeds can be separated from lighter seeds before sowing by using air for winnowing, a method by which lighter seeds are blown away and separated, or using a water salt solution having a specific gravity of 1 in order to float lighter seeds (Xuan and Ross 1972).
The procedure is as follows. Weigh exactly 1 kg of rice seed. Mix 1.65 kg of common salt in 10 liters of water or use 2.20 kg of ammonium sulfate (21% N) instead of the salt. Both solutions will give a specific gravity of 1.08 and this can be checked by floating a fresh egg. (The specific gravity is about 1 when one end of the egg barely touches or breaks the surface of the solution.)
Satisfactory results are often obtained with clean water alone instead of the salt solution. Merely stir the seeds thoroughly in the clean water and allow them to settle. Remove the seeds that floated and immediately dry the heavier seeds that are left at the bottom. The same procedures in separating the light seeds from heavier ones are followed for the salt or ammonium sulfate solution. To calculate the percentage of unfilled seeds (dry), divide the weight of unfilled seeds by the initial weight of the whole sample and multiply the quotient by 100.
Germination tests
Conduct germination tests 1-2 weeks before sowing to allow ample time to make adjustments in case germination is found to be poor (i.e., less than 80%). Without knowing the percentage germination, it is hard to guess the amount of seeds required to plant a field at the recommended seeding rate.
Two common methods are used to test the viability or germination percentage of seeds. These are the rag doll and seed box methods.
Rag doll method: Take a given number of seeds at random, usually 100 from a bulk supply. Distribute the seeds evenly in rows on a piece of cloth. Roll the cloth with the seeds around a stick for support and tie at both ends. Keep it moistened and check germination in 5 days.
Seed box method: Take a given number of seeds at random, usually 100 from a bulk supply. Prepare a small box with sandy soil. Sow the seeds in straight rows and cover thinly with soil until no seeds are visible. Water regularly and count for germination percentage after 7 days. If germination is lower than 60%, do not use the seeds. If it is above 60%, make calculations to increase sowing rate and adjust accordingly to make up for poor germination. The same recommendation applies to the rag doll method.
Computation of seed rate
The recommendation on seed rate has not been followed properly if percentage of unfilled grains and percentage germination are not determined. As a whole, farmers always end up with poor yield due to the low rate of seeding and to poor germination. Left to his own devices, the farmer will just increase seeding rate on a “hit and miss” basis; consequently, too much seeds are wasted or there may not be enough to meet the planting requirement.
The seed requirement (kg) can be calculated for a given area if seeding rate (kg/ha), area to be planted, percentage germination, and percentage of unfilled grains are known.
For example, if a given variety has 90% germination and 3% unfilled grains, what is the required weight of seeds to plant a 500-m2 area at a seeding rate of 50 kg/ha?
The formula is
Seed weight required (kg) = [seeding rate (kg/ha) x area to be planted (sq.m2)] divided by [% germination x % filled grains].
Solution: 50 (kg/ha) x 500 (sq.m2) ÷ 98% x 97% = 2.83 kg