Primary symptoms
Optimum density varies with variety and growing conditions.
The sunlight that reaches the ground through the crop canopy is often a good indicator of whether density is adequate. By flowering, the crop should be intercepting at least 80% of the sunlight in most environments where maize is not intercropped and drought is not frequent.
Confirm the problem by checking the tables below.
Summary
| Causes of low plant density | Additional evidence required |
|---|---|
| Farmer plants too few seeds. | Ask the farmer how much seed was planted and why. Look at rainfall data to see if drought is common. The farmer may lower density to adjust for drought. |
| Seed planted not viable. | Test seed viability. Ask how the seed was stored, and compare with "General rules for seed storage." |
| Poor seedbed preparation. | Ask farmer about land preparation. |
| Plants lost after planting (during germination and afterwards). | Look for evidence of disease, pests, waterlogging, drought stress, birds or rodents. Ask the farmer about these problems. If the farmer planted dry, ask about how many days after planting the rain came. Examine temperature records for that period. If soil temperatures were above 40°C for more than 2 days, the seed may have been damaged. (Recall that the maximum temperature of bare, dry soil can be several degrees higher than the maximum air temperature.) |
| Farmer destroyed some plants while cultivating. | Ask farmer about cultivation practices. Look for pieces of broken or uprooted plants. |
The optimum density under non-limiting conditions will be different for different varieties, and it should be determined for the important varieties in your region. The optimum density could be related roughly to plant height and maturity for CIMMYT lowland tropical germplasm growing in one environment (Table 5 below).
The optimum density at harvest for a variety is that which yields the most grain when the crop is grown under non-limiting conditions, which are almost never found in farmers' fields. Therefore, the recommended density at harvest (that which will give the best grain yields in farmers' fields) will be different from the optimum density. In addition, the best density for grain yield in a farmers' field will vary from year to year, depending on the climate and crop management. An agronomist working in an area needs to find the population density which will give the farmer the best return in bad years as well as in good years. It has been shown that a reduction in population density of 30% below the optimum will only reduce yields by about 5% in good years, and the lower density should increase yields when stress occurs. Therefore recommended densities are usually about 20-30% below the optimum density.
If drought stress is very common in your region, you may need to adjust your recommended densities downward even more than 30% from the optimum in order to increase the water available to each plant. In semi-arid regions, the risk of crop failure increases sharply as density increases. If the density is reduced to adjust for drought, remember that weed control might also require more attention, since the low density maize will provide less shade.
Once you have decided on a recommended density, you will need to calculate the amount of loss you expect from planting to harvest in order to get a recommended seeding rate. Compare the harvest density in the area with the number of seeds the farmer plants. In many environments, the loss of plants from planting to harvest is around 20%. Divide your recommended density by one minus the % loss (1 % loss) to get the recommended seeding rate. For example, if you were growing a material with an optimum density of 85,000 plants/ ha, the recommended density for farmers' fields might be:
85,000 - (85,000 x 0.30) = (approximately) 60,000 plants/ha
(Note: the 0.3 is to adjust planting density down by 30% to allow for drought)
However, you would expect about 20% of the plants to be lost between planting and harvest due to insect attack and disease. The recommended seeding rate is thus
60,000/(1 - 0.20) = 75,000 seeds/ha
If there are 3,500 seeds in a kilo, that rate equals 21.4 kg of seed per hectare.
Evidence: measurements.
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If the crop is planted in a row pattern with plants in hills (individual planting stations), count the number of hills in 5-m lengths of row at ten randomly chosen locations in the field. Count the number of plants as well.* At each location, measure the distance between the rows. Now calculate the number of plants per hill (plant distribution) and the number of plants per hectare (population density).
Plants/hill = number of plants in 5 m / number of hills in 5 m
Plants/ha = (number of plants in 5 m / 5 m x distance between rows in m) x 10,000
* This procedure should not require more than 30 seconds at each sampling point. Hook a 5-m piece of string to a plant and walk down the row, counting the number of hills you pass until you reach the end of the string. Now walk back to the starting point while counting the number of plants. In this way these important data can be collected quickly and easily.
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If the crop is planted in rows but not in hills (for example, when a mechanical planter is used), count the number of plants in 5-m lengths at ten randomly chosen locations in the field. At each location, measure the distance between the rows. Now calculate the number of plants per hectare (population density).
Plants/ha = (number of plants in 5 m / 5 m x distance between rows in m) x 10,000
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If the crop is planted with no clear pattern (broadcast), count the number of hills and the number of plants in ten randomly chosen 20 m2 areas.
Plants/hill = number of plants in 20 m2 / number of hills in 20 m2
Plants/ha = number of plants in 20 m2 x 500
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Compare the population density with the optimum for the variety. If you do not know the optimum density for your variety, use the estimates in Table 5 below.
Table 5. Optimum densities and recommended densities (optimum - 30%) for CIMMYT lowland tropical maize.
| Plant height (m) | Days to 50% male flowering | Optimum density (plants/ha) | Recommended density (plants/ha) |
|---|---|---|---|
| 1.6-1.8 | < 50 | 85,000 | 60,000 |
| 1.8-2.0 | 50-55 | 78,000 | 55,000 |
| 2.0-2.2 | 56-60 | 70,000 | 50,000 |
| 2.2-2.4 | > 60 | 65,000 | 45,000 |
Evidence: observations.
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Look over the field. Are the plants uniformly distributed, or are there patches of missing plants? Problems such as soil insects, rodents, and waterlogging can reduce population density in spots that you may overlook in your sampling but which are important for the farmer.
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Examine the plants carefully at each location where you take your measurements. Are the stems thin and weak? That may indicate a density that is too high (or too many plants per hill).
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Look at the amount of sunlight which reaches the ground between 11 a.m. and 1 p.m. By flowering, no more than about 20% of the sunlight should reach the ground, (the maize should intercept at least 80%) unless the maize is grown in an environment where drought stress is common or with an intercrop. If the maize is intercropped, most of the sunlight which the maize lets pass should strike the leaves of the intercrop.
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If the crop is past flowering, look at the number of ears per plant. (Count the number of ears in the sample, and divide by the number of plants.) Is the number of ears per plant below 0.9? The density (or number of plants per hill) may be too high
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At harvest: if the average weight of individual dry ears is above 180 grams, the density may have been too low.
Possible solutions
- Increase seeding rate.
- Improve seed quality or do a germination test before planting and adjust the seeding rate accordingly.
- Improve land preparation or cultivation practices.
- Fungicide treatment of seed or crop rotation to reduce disease losses.
- Insecticide treatment of seed to reduce insect losses.
- Increase planting depth or apply mulch to avoid early drought or high soil temperatures.
- Improve covering method and/or firm the soil over the seed to improve seed-soil contact in areas with marginal soil moisture. These measures may also reduce losses to birds or rodents.
- Level field to avoid waterlogging.
For the following figures, the sunlight that reaches the ground through the crop canopy is often a good indicator of whether density is adequate. By flowering, the crop should be intercepting at least 80% of the sunlight in most environments where maize is not intercropped and drought is not frequent. The photos show 40, 70, and 90% light interception.