The drought and heat stress have taken their toll on kernel numbers in this year's corn crop in many Minnesota counties through unsuccessful fertilization, aborted kernels, and decreased kernel size and weight.
Unsuccessful fertilization results in varying degrees of incomplete kernel set. Drought and heat cause unsuccessful fertilization by delaying silk emergence until pollen shed is finished or by drying out exposed silks, making them non-receptive to germination.
Even if pollination occurs successfully, severe drought stress that continues into the early stages of kernel development (blister and milk stages) can easily abort kernels. Aborted kernels are distinguished from unfertilized ovules in that aborted kernels have actually begun development. Aborted kernels will be shrunken, mostly white, often with the yellow embryo visible.
Severe weather stress also causes decreased kernel size and weight, leading to decreased grain yield.
Corn growers in Minnesota may want to predict grain yields prior to harvest in order to help develop grain marketing and harvest plans.
One option to evaluate potential yield is to use the following yield component method developed by the University of Illinois. The principle advantage of this method is that it can be used as early as the milk stage of kernel development.
To get kernel counts multiply ears per section by average kernels per ear:
- Count the number of harvestable ears in 1/1000 of an acre (17 feet, 5 inches in 30-inch rows).
- Select three representative ears. If ear size is highly variable, select five or six ears. Count kernels per ear. Average these counts.
- Multiply the number from step 1 (number of harvestable ears in section) by the number from step 2 (average kernels per ear) to get number of kernels per 1/1000 of an acre.
To estimate yield, divide the number of kernels in 1/1000 of an acre by the number (in thousands) of kernels expected to be in a bushel at maturity. This number can range from less than 60 to more than 120, but 90,000 kernels in a bushel is a good starting point, so divide by 90 to get estimated bushels per acre.
This year we are expecting variation in corn growth and kernel set across a typical field, so it is desirable to sample multiple areas in the field for a better average.
Since weight per kernel will vary depending on hybrid and environment, the yield component method should be used only to estimate "ballpark" grain yields. In years like this when below normal rainfall occurs during grain fill (resulting in low kernel weights), the yield component method will likely overestimate yields.
To see the normal field corn development stages in August in Minnesota, visit www.extension.umn.edu/go/1120.