Poor plant stands (low populations) are often caused by poor quality seed. The primary causes of reduced seed quality are storage under conditions of high relative humidity and/or high temperature, and damage due to insects or overly rapid drying. Some general rules* about storage conditions for maize:

1. For every 5°C increase in the storage temperature over the range from 0 to 50°C, the life of the seed is reduced by half. 

2. The relative humidity of the storage area should be below 65%. For the temperature range from 6-30ºC, that means that the maize grain should be at a stable (equilibrium) moisture content of 12%. If the relative humidity of the storage area rises to 80%, the grain will absorb water vapor from the air, and the grain moisture will rise to 15%. At that moisture level, fungi can attack the seed and reduce its viability.

3. Storage temperature and relative humidity are related: if the sum of temperature (in °C) plus the relative humidity (in percent) is 80, the seed will begin to deteriorate after 1-5 months. If the sum is 70, you can store the seed safely for 18 months.

4. Seed quality can be reduced if drying is not done carefully, especially if the seed had a high moisture content at harvest. Seed maize should never be dried with air warmer than 45°C. If the seed moisture is above 25%, the seed must be dried slowly (with air temperature no more than 10°C above ambient) to avoid cracking.

Another cause of poor seed quality is diseases which attack the grain either in the field before harvest or while in storage. The main diseases which affect seed viability are ear and kernel rots.

Source: Rules for testing seeds. Association of Official Seed Analysts. Journal of Seed Technology 6:30-35, 1981.

To test seed viability (germination tests)

1. Collect a sample of seed from the farmer near planting time, not just after the harvest. Ask the farmer how long he stores his seed, and examine the storage area. This will help you interpret the results of the germination test. Ask him if he selects only good seeds for planting, or if he sows without removing damaged seeds. This information will allow you to select seed for the germination test which is similar to the seed the farmer will plant.

2. You will need to collect about 500 seeds. If the grain is already shelled, push your hand well into the bag or pile with your fingers straight, and then close your hand to draw out the sample. Collect samples from five different places in the bag or pile (especially from the center). If the maize is still on the cobs, collect at least ten ears from different places in the pile and take the grain from the central part of each ear.

3. Examine the seed for insects, and for holes, cracks, or other damage. If the farmer sows only good seed, you should test only good seed.

4. Count out 400 seeds and divide them into groups of 50. Moisten a paper towel so that it is damp but water does not drip from it when you shake it. Place the seeds on the paper towel in a line along the middle so that they are not touching. Fold the paper over the seeds, and then roll it up loosely. Place the eight samples of 50 seeds in an open plastic bag with the rolls placed vertically in a place where the temperature stays between 20-30°C. Check daily to be sure that the paper towels do not dry out.

   You can also use a dish of wet sand for the test. Plant 400 seeds in groups of 50 about 2 cm deep and be sure that the sand does not dry out.

5. After 4 days, count the number of germinated seeds on each towel or in the dish of sand. You should count only normal seedlings - those which have both roots and shoots. Make a second count on day 6 and your last count on day 7. The percent germination is the total number of seedlings you counted multiplied by 0.25 (because you started with 400 seeds).

6. Remember that the rate of emergence in the field will not be as high as the germination rate, since vigor is also important in allowing the germinating seedling to emerge. Compare the percent germination you observed with Figure 3 to estimate percent emergence in the field, remembering that soil crusting, depth of planting, etc., will also affect the final emergence rate. You can get some idea of field emergence rate by planting seeds in a small box of local soil at the depth the farmers will use.