Yield potential at this point in the season is determined by two yield components: Plants per unit area (acre, hectare) and number of potential kernels (i.e., ovules) on each plant. The third yield component, weight per kernel, is determined later in the season during grain filling (Nielsen, 2021).
The number of harvestable plants per unit area is largely determined by the time the crop reaches the 6-leaf stage of development. The majority of corn around Indiana has surpassed this stage, but the minority of fields planted the last week of May or later remain vulnerable to "floppy corn syndrome" and stand loss caused by rapidly drying surface soils and small root systems (Nielsen, 2022).
The number of potential kernels (i.e., ovules) is determined early in the season from about V5 to about V12-14 (Nielsen, 2023). One might imagine that the current unusual hot spell of temperatures could influence potential yield by influencing this ear size determination process.
The harvestable ear on a plant initiates at about leaf stage V5 and MAXIMUM row number is set for the ear by about V7. Determining MAXIMUM number of kernels per row is a longer process and is thought to be set by about V12-V14. I emphasized MAXIMUM kernel number because ACTUAL or HARVESTABLE number of kernels is influenced by growing conditions during and following pollination (Nielsen, 2020).
My experience is that it requires EXTREMELY SEVERE stress during leaf stages V5 to V12-14 to decrease MAXIMUM kernel numbers. That being said, however, some fields in Indiana are probably stressed enough during the current hot spell to impact the potential for kernel numbers prior to pollination. Those fields would be ones where early root development has been severely compromised to begin with. Those would be primarily fields with significant shallow soil compaction (wet spring + tillage) or soils that simply do not allow for deep root development (e.g., eroded hillsides). The combination of unusually warm temperatures for mid-June, moderately low humidities until recent days (both of which lead to high rates of soil evaporation and plant transpiration), brilliant sunlight, and either restricted root development or simply small root systems due to young plants can cause SEVERE stress on young plants. I began to see crops rolling a week ago on eroded slopes BEFORE we had the onset of 90+ weather.
Furthermore, remember that the number of ACTUAL kernels per plant is determined by the success of pollination and levels of stress up to about the milk stage (R3) of kernel development (Nielsen 2018). The plant produces way more potential kernels (i.e., ovules) than are usually harvested. It is not uncommon to find 50-55 ovules per row on an ear leading up to silking but the common range in ear size at harvest is more like 30-40 kernels per ear. The point here is that a reduction in OVULE number during the ear size determination period does not automatically translate to lower yield, particularly if growing conditions during pollination and early grain fill stages are favorable for kernel set.
Nielsen, RL (Bob). 2018. Effects of Severe Stress During Grain Filling in Corn. Corny News Network, Purdue Agronomy Extension. http://www.kingcorn.org/news/timeless/GrainFillStress.html [accessed June 2024].
Nielsen, RL (Bob). 2020. Kernel Set Scuttlebutt. Corny News Network, Purdue Agronomy Extension. http://www.kingcorn.org/news/timeless/KernelSet.html [accessed June 2024].
Nielsen, RL (Bob). 2021. Grain Fill Stages in Corn. Corny News Network, Purdue Agronomy Extension. http://www.king.corn/news/timeless/GrainFill.html [accessed June 2024].
Nielsen, RL (Bob). 2022. "Rootless" or "Floppy" Corn Syndrome. Corny News Network, Purdue Agronomy Extension. http://www.kingcorn/news/timeless/FloppyCorn.html [accessed June 2024].
Nielsen, RL (Bob). 2023. Ear Size Determination in Corn. Corny News Network, Purdue Agronomy Extension. http://www.kingcorn.org/news/timeless/EarSize.html [accessed June 2024].