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.