Since July 1, high humidity and temperatures have been heating up Iowa. June has led to dry conditions across much of central and southwest Iowa. Heat and dry weather are not desired in the two weeks before or after pollination.
“This four-week time period is finalizing the kernel number per ear,” explains Mark Licht, Iowa State University Extension cropping systems agronomist. “Water use by the plant is at its peak of between 0.25 to 0.45 inch per day. That water is transpired as a cooling mechanism, but it is also needed for silk elongation and filling fertilized kernels.”
What affect will heat and dry weather have? “It is very hard to separate the impact of heat from drought stress because most of the time they occur simultaneously,” he notes.
Licht, along with his ISU colleague Sotirios Archontoulis, provide the following observations and explanation of what’s happening in Iowa cornfields during the current pollination period. Archontoulis uses a computer program to interpret the situation and possible outcomes, depending on various weather scenarios.
How hot is too hot?
Heat stress alone typically requires a temperature of greater than 95 degrees F in the absence of moisture stress, Licht says. Temperatures greater than 95 degrees along with low relative humidity can cause silk desiccation. Pollen is not killed until temperatures reach above 100 degrees; however, the pollen is likely damaged with temperatures above the mid-90s. The good news is that pollination typically occurs in the midmorning hours. This summer, from June 25 to July 7, temperatures in Iowa were greater than 95 degrees.
Moisture (or drought) stress is indicated by continual leaf rolling and wilting of the corn plants. Because silks are very high in moisture content, moisture stress will slow down silk elongation. Silk elongation begins about seven days before silks emerge from the ear husks. Severe moisture stress will reduce the number of kernels. It has been estimated that nearly continual wilting in the two weeks prior to silk emergence can reduce yield by 3% to 4% per day; during silk emergence and pollen shed by up to 8% per day; and in the two weeks following by up to 6% per day.
In the ISU article What impact will the heat and dry weather have on pollination 2020?, Licht and Archontoulis show yield estimates using a computer model. These estimates are for corn at planting (May 1), and based on weather and growing conditions on June 1, July 1 and July 7 for seven locations across Iowa. These yield estimates are based on a crop model simulation that is used to generate the FACTS forecasts. The yield estimates use current weather up to July 7 and a 40-year weather history to finish out the growing season.
Estimates of lost yield potential are greatest in central and southwest Iowa. Minimal impact (and maybe even a benefit) in northern and eastern Iowa has occurred from the May to July weather patterns. However, there is a large uncertainty on these yield estimates as indicated by the worst- and best-case estimates.
“Despite the heat and drought conditions, crops maintain high yield potential. In some areas of Iowa, yields are trending down,” the ISU agronomists conclude. “The weather in the next two weeks is very critical. The remainder of July and August is still important in determining final yield.”
Other modeling insights as of July 7 include:
- water table depth has dropped to 6 feet on average from about 3 to 4 feet at planting
- zero to 2-foot soil layer is very dry
- 2- to 4-foot layer has moisture
- corn roots are about 4 to 4.5 feet deep