How to ensure seed quality
Seed is the foundation of any rice crop. It must be grown, harvested, and processed correctly for best yield and quality results.
Sowing good quality seeds leads to lower seed rate, better emergence (>70%), more uniformity, less replanting, and vigorous early growth which helps to increase resistance to insects and diseases, and decrease weeds. As a result, yield can increase by 5−20%.
While different countries have different standards for seed quality the following factors are used to classify rice seeds:
Good seed is pure, from a single variety.
Varietal purity tests include looking for percentage of (1) germination, (2) other mixed in varieties, (3) weed seeds and other crop seeds, (4) inert material (stones, soil, etc.), (5) red rice seeds, and (6) moisture content.
Seed lot purity
Impurities refer to the degree of contamination caused by (1) weed seeds, (2) seeds of other crops or species, and (3) inert material such as stones, dirt, or twigs. It is expressed as a percentage, by weight.
To measure seed lot purity:
- Randomly select a seed sample. 100 g is a good sample size.
- Weigh the sample. [A]
- Remove all of the weed and other crop seeds.
- Weigh the removed weed and other crop seed matter. [B]
- Compute the weed percentage using the following formula:
- Weigh the inert matter removed from the sample [C], the inert matter percentage is calculated using the formula:
Germination percentage expresses the proportion of the total number of seeds that are alive. Good seeds have more than 80% germination rate.
Many varieties have a dormancy period immediately after harvest. When stored under traditional open systems, the germination rate of most rice seed begins to deteriorate rapidly after 6 months.
To test seed germination:
- Select a number of small random samples from the seed to be planted and select a subset (e.g., 200 seeds) of the combined sample. Soak the seed in water for 24 hours.
- Arrange 100 soaked seeds in a grid pattern on a wet paper towel.
- Place the paper in a closed container or
- Cover the seeds with another moist paper towel and roll together and place the sample in a plastic bag
- Ensure paper remains moist (but not wet to the point of water running off – or seed will rot).
- Count the germinated seeds 3 and 5 days later and record how many have germinated.
- Compute the germination percentage using the following formula:
At least 80 seeds should have germinated to be considered "good seed" (80% germination).
Seed establishment vs germination
It is best to also check seed germination in soil, as emergence can often be 70% or less of germination.
To test, place 2 samples of 100 seeds in a tray filled with soil. Cover lightly with soil (e.g., 5 mm), keep moist and count establishment after 7 days.
Note: Larger seeds tend to establish better then smaller seeds.
Fungi, bacteria, and environmental conditions such as high humidity and temperature (i.e. 27−35°) cause discoloration.
Other rice diseases do not produce consistent discoloration. Any seed having more than 0.5% differently colored or spotted seed surface is considered discolored. Discoloration can occur on fully or partially filled or empty grains.
There are two types of discoloration on rice seeds:
- Spotting, which is caused by certain diseases (brown spot) or insects (rice bug feeding damage)
- Discoloration on a portion or portions of rice seeds can be caused by black kernel fungus, stackburn disease, or sheath rot on certain occasions.
Red Rice kernels are not acceptable in the rice market place. Kernels should not have more than 25% of its surface area colored red or red streaked.
Seed size, plumpness, and/or fullness are generally desirable seed characteristics. These indicate that the seed can potentially produce vigorous seedlings under favorable conditions.
Read: IRRI Rice Quality Assessment Kit
How to obtain good seed
- Buy certified seed that is pure and labeled
- Get farmer-produced good seed, or
- Select your own good seed
High quality seeds are free from weed seeds, seed-borne diseases, insects, pathogens, and other extraneous matter. They should also be free from various types of mechanical injury that reduce germination and seedling vigor.
In case of infected or low quality seeds, treatments can be done.