Although we've spent most of the first half of the growing season praising the condition of the corn crop, we're now two-thirds of the way through July, and there has been little if any rain this month in some parts of Illinois. The conventional wisdom is that corn yields are driven mostly by water supply, and that the most important time for water supply to be adequate is during the month of July. Concern levels are rising as warm, dry weather continues in many areas this week. How much worry is warranted? |
As water stress develops during the month of July, small differences in the developmental stage and the condition of the corn crop can make large differences in the response of the crop to dry soils. So far in 1999, weather for crop development and growth has been very good: lots of sunshine and adequate water supplies followed timely planting. In general, except in those fields that were replanted or were planted when soils were too wet, the root system has had very favorable soil conditions in which to develop. Soils had good moisture, but they have not been saturated for long periods of time, so soil air supply has been mostly favorable. Although we don't have a standard way to measure overall root status, we think that it is good. Soil water supply at the end of June was high in most areas, after normal or above-normal rainfall during June in most of the state.
Root systems reach maximum size at about the time of pollination, and when roots are healthy and deep, they provide an excellent system for the plant to extract water from the soil, even if soil moisture content decreases during dry weather. Productive soils can hold 10 inches or more of water available for plant use in the rooting zone. Water use during warm, dry weather in July can approach 1 1/2 inches per week, but even a month of dry weather will not totally deplete the water supply of a well-rooted plant in a productive soil.
Despite our estimates of root system size and general plant function, we cannot very accurately judge the timing or extent of stress on plants during dry weather. The plants can, however, give signals that help us judge how much stress they're going through. One positive sign we see in many fields is the normal to above-normal plant height. One of the first signs of water stress that we see in corn is shortening of the plants during vegetative growth; the internodes that are shortened are those that were developing when the stress took place. While it does appear that some tassels are not emerging quite to the extent they normally would, tall plants indicate that plants have not suffered greatly from lack of water. Because cell expansion is so sensitive to water status, silk emergence can also be decreased by continuing water stress. If fields have not yet pollinated, silks should be watched closely in dry areas to see if they are emerging normally at night. If the first silks do not appear within about 2 days of the start of pollen shed, then they are probably not elongating as rapidly as normal.
Other signs that the corn crop is weathering the shortage of rainfall reasonably well in many areas include the good leaf color that persists in most fields, good silk emergence in fields that have reached pollination, and the general lack of leaf curling by early afternoon. Once plants reach pollination, however, leaves are less able to curl tightly, so this symptom is less reliable. Still, the earlier in the day leaves show any curling, or begin to show the silvery-gray color that indicates loss of water from leaf surface cells, the more serious the shortage of water. Fields that have not yet reached pollination are most at risk at this point.
Suppose it remains dry after pollination is complete? The success of the pollination process can be assessed at brown silk: silks that detach easily from kernels indicate successful fertilization. Kernels can, however, stop developing (called kernel abortion), even after successful fertilization. The mechanism of abortion is not well understood, but there is little debate about the fact that more kernels abort when the plant is under water stress. Continuing lack of adequate soil water accelerates the abortion of kernels; in severe stress conditions, half or more of the developing kernels can abort.
Dry weather can also reduce photosynthesis, both by closing stomata (tiny holes in the leaf that allow carbon dioxide to enter the leaf) and by causing leaves to curl, reducing the photosynthetic area. Even kernels that continue to develop can be reduced in size if dry weather results in a decrease in photosynthesis, which is the source of sugars that fill kernels. The worst-case scenario would have the shortage of water continue, resulting in reductions in both kernel number and kernel size. Few if any fields in Illinois have suffered much of this so far, but it will take several inches of rain to remove the possibility of such effects.
A final note: The crop in Illinois has benefited from relatively moderate temperatures in July. Temperatures have seldom been above the low 90s so far, and wind speed has not been higher than normal. This has prevented excessive water loss and has kept to a minimum direct damage due to high temperatures. Temperatures in the mid- or upper 90s cause little problem for well-watered corn, and the ability of the crop to weather the dryness so far in July has been improved by the moderate temperatures.--Emerson D. Nafziger