No. 17/July 18, 1997
Is Corn in Trouble from Drought?
For some fields in Illinois, the answer to this question on July 15, 1997, is "yes"; while in most, it's "maybe, depending on what happens in the next week or two." Without doubt, corn in most fields has been under mild to moderate stress at some time during the past month. There are a number of common themes that are being discussed with increasing anxiety around the state as the rain continues to miss many areas, and to miss parts of the areas that are getting some rain.
How much has the yield potential been reduced already due to dry soils? The answer to this question depends primarily on what stage the crop is in, and on the extent of water deficit the crop has experienced to this point. It is difficult to measure directly the amount of water stress a plant is undergoing, but a number of indicators of the cumulative effects of drought that can be monitored are discussed here.
Water is lost during the day through tiny openings in the leaves called stomata, with more rapid loss when leaf temperatures are higher and when relative humidity is lower. Soils dry out as plant roots extract water from the soil to replenish that lost from leaves. Though the root systems are quite good this year due to early planting and dry surface soils, plant stress is increasing as soil water is extracted by the roots. It is not easy to measure soil drying directly, but one of the earliest signs that a corn crop is experiencing stress is a silvery gray appearance of the upper leaves, especially on their lower surface.
Leaf rolling for a few hours during late afternoon is an indicator of mild stress. Leaves roll earlier in the day as stress increases, to the point under extreme stress at which leaves may not unroll completely at night. Thus, the time of day when leaf rolling starts can give some indication of the degree of stress. There is, however, a fairly strong genetic influence on leaf rolling, and so it may not be a good indicator for all hybrids. Although the tendency to roll early probably does prevent water loss as the leaf area exposed to the sun is decreased and the humidity inside the rolled leaf rises, leaf rolling also means the leaf has stopped photosynthesizing. It is not clear, therefore, if rapid leaf rolling helps a plant survive drought.
One of the best indicators of the cumulative effect of dry soil is plant height and, in more extreme cases, leaf size. The longer the plants have been under stress, and the more severe the stress has been, the shorter the plants are. Plant height, and to some extent leaf area, increases almost entirely at night. High evaporative demand during the day means that young, expanding cells cannot compete for water until water loss through the stomata stops at night. If soils are dry and plants need to rehydrate at night before cell expansion begins, there is less time for cell expansion, and thus less growth.
Leaf firing--loss of chlorophyll and proteins, with yellowing and eventual death of leaf tissue--is an indicator of fairly serious drought. Firing starts in the leaves at the bottom of the plant and moves up with increasing dryness. Although this may allow the plant to better provide nutrients to the upper leaves, firing is irreversible, and it reduces yield in direct proportion to the extent to which it decreases the ability of the plant to maintain light interception and photosynthesis during grainfill.
Slow emergence of silks, or failure of the silks to emerge at all during the pollen-shedding period, can be a most devastating consequence of very dry soils. Like the rest of the plant, silks grow primarily at night. But they are more sensitive to dry soils (that is, less able to compete with the soil and air for water) than are the stem and other plant tissues. If silks fail to emerge by 2 or 3 days after the first pollen is shed, then they may be growing more slowly than normal. One way to monitor this directly is to cut silks of a few plants off at the tip of the ear in late morning, then to see how much new growth appears in 24 hours. Silks usually grow 2 inches or so per night, so less than an inch of growth means that silks are probably being delayed. Look also for the number of silks that merge each day: If there are only a few dozen, then problems with final kernel number are likely.
For high yield potential, a normal ear needs to have 600 to 800 silks emerge to receive pollen before pollen-shedding ends. If pollen-shedding ends before silking is complete, silks may continue to emerge and elongate, with very slow browning. One can also check the success of the fertilization process by removing the husks and pulling gently on the silks; those from kernels that have been fertilized detach easily from the kernels.
Continued dry weather after pollination often results in kernel abortion, which is death of kernels that have been fertilized. Such abortion affects kernels near the tip of the ear first and can reduce kernel number substantially. The effect of such kernel loss on yield is usually proportionately less than the percentage loss of kernel number--because remaining kernels usually fill to a larger size.
Pollination is a critical time in the life of the corn plant; the success or failure of pollination and kernel establishment directly affects kernel number and yield potential. Unfortunately, because the pollination period is the time when vegetative growth is concluding while rather involved reproductive growth processes are taking place, shortage of photosynthetic products (sugars) in the plant can be devastating. Studies have shown this to be a period of low sugar content even under good conditions, and so lowered levels of photosynthesis for any reason (drought, hail, disease, or insect damage) can seriously reduce yield potential.
With the generally good color of the corn canopy and the reasonable amount of growth so far in most fields signaling the efficient extraction of soil water by the plants, there is reason for cautious optimism that pollination and fertilization will proceed reasonably well in most fields. If dry weather continues, along with high temperatures, however, the crop will deteriorate rapidly, especially in soils with lower water-holding capacity.
Emerson Nafziger, Department of Crop Sciences, (217)333-4424