“We remain optimistic that kernel numbers will be OK in many fields, but in the past two years we have had a great deal of kernel abortion, and there is every reason to believe that this will recur in 2012,” Nafziger said.
The factors that affect whether fertilized kernels abort are not entirely understood, but the supply of sugars available to the developing ear is known to be important for keeping productive kernels. Sugars are produced by photosynthesis. Plants that are rolling their leaves for most of the afternoon, as is happening in east central Illinois, are producing less sugar than normal.
“It’s highly likely that there will be fewer than normal numbers of kernels by the time kernel filling starts several weeks from now,” Nafziger predicted.
As stress conditions continue, a reduction in kernel size often starts to develop somewhere along the length of the cob, indicating the start of kernel loss from abortion, which is probably irreversible. The decreased number of kernels that fill is likely to be a primary yield barrier in fields under stress during pollination.
If conditions improve during grainfill, the reduced number of kernels may be able to get a little larger than they normally would. A healthy canopy increases the plants’ potential to make larger kernels. “However, we would not expect an increase in kernel size to make up much of the yield loss due to reduced kernel numbers,” Nafziger noted.
In areas that have been under stress for weeks, one of the first sign of rapid deterioration in yield potential is, or will be, loss of canopy color. Plants that are struggling to take up water are also not taking up much nitrogen, and leaves become less flexible with age. Thus, loss of canopy color after pollination often cannot be reversed completely, even after it rains.
Besides canopy color, light interception serves as an indicator of stress and photosynthetic capacity. As the leaves roll, light interception drops quickly. One of the best “drive-by” indicators of how a crop is doing at a given time is the amount of light that is hitting the ground. In fields where well over half the sunlight is reaching the ground, little photosynthesis is going on.
Based on this indicator, corn following corn seems to be struggling more than corn following soybean, perhaps due in part to incomplete recharge in some areas after last season. Fields that were in first-year corn in 2011 and second-year corn in 2012 may suffer more because both yields and water extraction in 2011 may have been higher