Primary symptoms
Poor crop stand. Rolled wilted leaves, early senescence, dull grey-green color.
Confirm moisture stress by checking the additional evidence required (see below).
Summary
Water is the most common limitation to maize production in the tropics. Drought during the crop establishment stage can kill young plants, reducing the plant density. The main effect of drought in the vegetative period is to reduce leaf growth, so the crop intercepts less sunlight. Around flowering (from about two weeks before silking to two weeks after silking), maize is very sensitive to moisture stress. Grain yield can be seriously affected if drought occurs during this period. During the grain-filling period, the main effect of drought is to reduce kernel size.
In general, maize needs at least 500-700 mm of well- distributed rainfall during the growing season. Even that amount of rain may not be enough, however, if the moisture cannot be stored in the soil because of runoff or shallow soil depth, or if the evaporative demand is very large due to high temperatures and low relative humidity.
The occurrence of moisture stress usually varies greatly from year to year. If you observe symptoms of stress one year, you still need to examine weather records and talk to farmers to find out if it is a common problem. If drought is a common problem, it will reduce yield by more than 20% in one year out of four.
| Causes of moisture stress | Additional evidence required |
|---|---|
| Insufficient rainfall, or very large environmental demand due to high temperatures and low relaltive humidity. | Check weather records for rainfall and evaporative demand. |
| Shallow or compacted soils. | Measure soil depth. |
| Soils have low water holding capacity. | Examine soil texture and predicted water holding capacity. |
| Restricted root growth due to soil chemical properties. | Examine roots. Check soil pH. |
| Rain is lost to runoff. | Estimate soil slope. Look for signs of erosion or crusting. |
| Weeds or an intercrop are using the water. | Estimate the percentage of sunlight that is falling on weeds or the intercrop rather than on the maize or soil. At least this proportion of the available water is being used by weeds or the intercrop. |
Is moisture stress a problem?
Evidence: measurements.
Is the plant density low? Inadequate moisture shortly after planting could be the reason. How many ears per plant do you count? Drought near flowering can cause barren plants (ears per plant < 0.9). If you visit the field at the R1 stage, is 50% silking delayed more than 4 days after 50% pollen shed? An increase in the anthesis-to-silking interval is very common when drought occurs near flowering, and yield tends to decline by about 9% relative to unstressed plots for each day that silking is delayed.
Evidence: calculations.
You can use soil moisture content and weather data to estimate the probability of moisture stress at the time you are making your observations. Estimate available soil moisture. If the available moisture is less than 25% in humid areas or less than 50% in dry areas, the crop is probably already suffering from moisture stress. You can also do the following calculation to see approximately how many days the crop can go without stress if no significant rain falls.
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How much soil water is present to be used before stress begins? This equals (mm current soil moisture) - (mm soil moisture present when the soil reaches 25% available moisture in humid areas, or 50% available moisture in dry areas). This is calculated for the current rooting depth of the crop.
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How much water is used by the crop each day? This equals (evaporative demand in mm/day from weather data or Table 2) x (the crop coefficient for the growth stage from Table 3).
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The number of days the crop can go without significant rainfall before stress begins is:
mm water in soil to be used (from step one) / mm water plant uses per day (from step two)
Table 2. Approximate evapotranspiration (ET) values for different environments. Values are in mm/day.
| Average daily temperature (degree Celsius) | |||
|---|---|---|---|
| 10-16 | 17-23 | 24-30 | |
| Humid tropics | 3-4 | 4-5 | 5-6 |
| Subhumid tropics | 3-5 | 5-6 | 7-8 |
| Semi-arid tropics | 4-5 | 6-7 | 8-9 |
| Arid tropics | 4-5 | 7-8 | 9-10 |
Source: FAO Irrigation and Drainage Paper 33.
Table 3. Values of the crop coefficient for maize grown at normal densities.
| Growth stage | Crop coefficient (kc) |
|---|---|
| Crop establishment (VE-V5) | 0.4 |
| Early vegetative (V6-V10) | 0.8 |
| Late vegetative to flowering (V11-R2) | 1.1 |
| Early grain filling (R3-R4) | 0.9 |
| Late grain filling (R5-R6) | 0.6 |
Source: FAO Irrigation and Drainage Paper 24.
Evidence: observations.
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Are the leaves of the plants rolled (before flowering) or wilted (after flowering)? Do the plants have a dull, grayish color rather than a bright green color? Are there symptoms of photo-oxidation (sections of leaf appear bleached and yellowish) or tassel blasting (See sun and temperature effects)? Are they warm to the touch when in full sun? These symptoms all indicate current moisture stress.
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Examine the internodes above the ear. Are they much shorter than the internodes just below the ear? This can indicate drought stress in the later vegetative stages, but can also be a symptom of stalk borer damage. Cutting open the stalk will allow you to see if the cause is borer damage.
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Compare the heights of plants on the edge of the field with those having full competition. The plants with less competition will tend to be taller than the plants, which are completely surrounded if drought or nutrient stress occurred.
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Examine the amount of sunlight that reaches the ground around noon (See plant density). If the density is appropriate for the variety but more than about 20% of the light reaches the soil surface, the leaf area of the crop might have been reduced by drought stress prior to flowering. During the grain-filling period: look at the leaves below the ear. Are they dying more rapidly than you would expect? Senescence due to drought looks somewhat different than normal senescence; in normal senescence, the yellow color tends to follow a "v" shaped pattern beginning from the leaf tip. When leaves die in response to drought, the yellow color moves along the leaf margins or in a fairly straight line up the leaf, and the leaves rapidly become brown and dry.
Possible solutions
- Improve soil water storage by mulching, contouring, or tied ridges to improve infiltration, or by subsoiling. Use conservation tilllage to reduce soil water loss.
- Reduce water demand by the crop by planting at reduced density. (See plant density)
- Plant at a different time of the year to reduce the risk of drought, or use early varieties to avoid drought.
- Lime to make pH more favorable for root growth.
- Improve weed control.
- Reduce the density of the intercrop, or plant the intercrop later in the growing cycle.