As the Illinois corn  crop continues its rapid development with 8% of the crop in dent stage by July 22, its rating continues to decline. University of Illinois Crop Sciences Professor Emerson Nafziger says that on July 22, only 7% of the crop was rated as good, none was rated as excellent, and 66% was rated as poor or very poor.
“It’s of little comfort, but at least we’re getting close to the end of the slide in ratings because they can’t get much worse,” he says. “But such low ratings do raise questions about just how poor crop prospects are.”
In areas where there has been some rain, at least in the morning before leaves wilt in the afternoon heat, the crop has some ears and retains some green leaf area. What is the chance that such fields, many of which are rated “poor,” will produce a yield that is worth harvesting?
Some reports from recent visits to fields in central Illinois where good and poor fields are often close to one another suggest that the crop might be better than the ratings indicate, depending on whether there is rain. The poor rating may reflect uncertainty about whether conditions will improve.
In dry areas, however, canopy deterioration continues or is complete, with leaf area dead or dying as plants deplete the soil water supply. “We know that many such fields will produce no grain,” Nafziger says.
In fields where some kernels have been set and remain active, the stalks have not accumulated adequate levels of sugar or nutrients. Nitrate might have remained in stalks because there is not enough energy to convert it to ammonium and then to amino acids and proteins. Low sugar and nutrient levels or death of conductive tissue can cause kernels to stop growing.
Stalk quality is a concern when stalk nutrients are depleted to fill kernels, as often happens under dry conditions. “Stalks are likely to be weak this year, but with small ears, there won’t be much need for strong stalks in stressed fields,” says Nafziger. “Even so, lodging may start in fields that died prematurely, even before grain has dried down.”
With growing-degree days (GDDs) continuing to accumulate, the earliest-planted corn should now be in dough stage with some starting into dent. According to the Illinois Agronomy Handbook estimates, the milk stage (R3) is reached at approximately 1,925 GDDs, dough stage (R4) at 2,190 GDDs and dent stage (R5) at 2,450 GDDs.
The more stress the plant has endured, the lower its yield potential. At Urbana, corn planted on April 1 has accumulated about 2,140 GDDs. The corn planted on March 15 has accumulated 2,350 GDDs. Although leaf death, plant drydown, and the maturation process have accelerated in the dry soils, corn that is now at R4 in central Illinois is not far ahead of where GDD totals suggest it should be.
The grain harvest has started in southern Illinois; with early plant death and drying, yields are expected to be low. “In fields where ears are present, either kernel number or kernel size or both will be small,” Nafziger predicts.
In fields with some kernels set, where leaves are active (not rolled or wilted) for at least half of each day, it might be possible to get an idea now of how many kernels are likely to fill.
Estimating yield potential starts with estimating kernel number. However, with so much variability within stressed fields – for example, low areas might have ears and higher areas might have none – it is nearly impossible to estimate ear number. Data on previous yields along with topographic maps could be used to estimate how much of the field might have ears and sampling only in those areas.
Kernel count is obtained by counting the number of ears in 1/1000th of an acre (17 ft., 5 in. in 30-in. rows). If ear size is highly variable, select five or six ears (instead of the usual three) to represent the range of sizes from the row section. Count kernels per ear, average these counts, and multiply by the number of ears (with kernels) to get the number of kernels per 1/1000th of an acre. To estimate yield, divide the number of kernels in 1/1000th of an acre by the number (in thousands) of kernels expected to be in a bushel at maturity.
“In recent years, we have used 80, which is reasonable under good conditions,” Nafziger says. “This number can range from less than 60 to more than 120. As we do not know how long stress will last, it’s almost impossible to guess what kernel weights will be at maturity for a particular field.”
“If there is a fair amount of green leaf area and kernels have already reached dough stage, 90 might be a reasonable estimate,” he continues. “It doesn’t help much to try to estimate kernel depth at dough stage, especially if there are 16 or more kernel rows on the ear. Kernels tend to look shallow at this stage, but will lengthen later. If green leaf area is mostly gone and kernels look as if they are starting to shrink, kernels may end up very light. Using 120 or even 140 might be more accurate.”
In a field where the crop has dried up prematurely and has few ears, outcomes are predictable and a visit may not be needed. Some fields, however, might offer welcome surprises, producing grain even under stressful conditions.
“While we know there will be a corn crop in Illinois – and even a good crop in some areas – the only way to get an idea of potential yield in a given field is to visit it and assess it,” Nafziger says.