Did unusually warm June temperatures compromise 2024 corn yields? It’s a big question for both farmers and grain market analysts. Is this just a weather scare, or has real damage occurred?
Bob Nielsen, retired Purdue Extension corn specialist, offers an answer, though it may be unsatisfying. “As usual, it depends,” Nielsen says. “We will lay out facts, and you decide.”
Yield potential prior to silking is determined by two components: plants per acre and potential kernels per plant. The final yield component, weight per kernel, isn’t determined until grain fill.
“Number of harvestable plants per acre is largely determined by V6,” Nielsen says. “There may be late-planted fields vulnerable to ‘floppy corn syndrome’ and stand loss from rapidly drying surface soils and small root systems.”
Number of potential kernels is determined from V5 to about V12 to V14, Nielsen explains. “You might imagine that the unusual hot spell in June could have impacted potential yield by influencing this ear size determination process,” he says.
He notes that harvestable ears initiate around V5. Maximum row number is set around V7. Determining maximum number of potential kernels per row likely happens at about V12 to V14. “Actual number of harvestable kernels is influenced by growing conditions during and after pollination,” Nielsen says.
What past seasons show
“My experience is that it requires extremely severe stress during V5 to V14 to decrease maximum potential kernel numbers,” Nielsen says. “Some fields were probably stressed enough during June to impact potential kernel numbers prior to pollination.
“Those include fields where early root development was severely compromised due to significant shallow soil compaction and where soils don’t allow deep root development. The combination of unusually warm temperatures for mid-June, moderately low humidities early, brilliant sunlight and suspect root systems can cause severe stress on young plants.”
Potential and actual kernels aren’t the same. “Actual kernels per plant is determined by pollination success and stress levels up to milk stage, or R3,” Nielsen explains. He adds that it’s not uncommon to find 50 to 55 ovules per row before silking. Yet common range at harvest is 30 to 40 kernels per row.
“Reduction in ovule number during ear size determination does not automatically translate to lower yield, especially if growing conditions during pollination and early grain fill stages are favorable,” Nielsen says.
What weather experts say
Trent Ford, Illinois state climatologist, was interviewed by Todd Gleason of the University of Illinois recently. Ford explained that high temperatures increase evapotranspiration. In fact, water loss per day in Illinois cornfields in late June was up to 0.25 to 0.3 inch per day.
“Humidity and wind speed are factors, but temperature is the driver,” Ford said. “Higher humidity helped hold it down, while higher wind velocity added to higher transpiration.”
Several days of high evapotranspiration rates dry out surface soils, resulting in a flash drought, he noted. However, in 2024 — unlike, say, 2012 — many areas went into June with replenished soil moisture reserves.
“The question is whether corn roots access that moisture,” Ford said. “In fields planted wet where roots haven’t developed well, that may be an issue. We could see lots of variability from field to field if warm temperatures continue.”