With decent weather late last week and early this week, a fair amount of corn was planted. In typical Iowa spring fashion, the weather is now showing a turn for the worse. The 10-day forecast for much of the state shows cold and wet conditions predicted, with many nights having lows in the upper 30s to low 40s.

“In conditions like these, imbibitional chilling could be a concern for corn seeds that have been planted and are early in the germination process,” says Iowa State University agronomist Clarke McGrath. “Seed planted early this week could bear some extra attention as you scout your fields for germination and emergence over the next week or 10 days.”

McGrath is the On-Farm Research and Extension coordinator for the Iowa Soybean Research Center at Iowa State University. He’s had lots of experience scouting newly planted corn and soybean fields over the years as an ISU Extension field agronomist and working in private industry.

When corn seed absorbs cold water
Corn seed absorbs about a third of its weight in water early in the germination process, McGrath explains. If this water is cold enough, bad things can happen. The exact temperatures vary, but the upper 40s to low 50s are often mentioned by agronomists as the danger zone. Among the bad things that can happen, cell walls of the germinating seed or seedlings can become brittle and even rupture.

“When this imbibitional chilling happens, we see all sorts of impacts,” says McGrath. “For example, seed that just swells and never continues growth. Or sometimes we see corkscrewed corn seedlings, ruptured coleoptiles, corn leafing out underground, seedlings more prone to pathogens, seedling death and other interesting — but not good — phenomenon.”

Imbibition is the process by which seeds absorb water to begin germination. According to ISU Extension cropping systems agronomist Mark Licht, imbibition of cold water “disrupts the reorganization of cells during rehydration and can result in the loss of seed vigor and result in seed death.” The most critical time for imbibition is within the first day or two of planting, he says. Thus, the imbibitional chilling effects are more severe when seeds are planted into soils around 50 degrees or colder, compared to planting into warmer soils followed by a drop in temperature.

Beware of big swings in soil temperature
“Planting corn 24 to 48 hours ahead of a cold front that brings rain can be problematic for seeds that imbibe this cold water,” says McGrath. “The cold water itself may have little effect on time needed for imbibition to occur, but growth of the radicle root is directly tied to soil temperature.”

Licht says the radicle root of a germinating corn seedling will grow at soil temperatures as low as 46 degrees, but the mesocotyl and coleoptile require 60 degrees for growth. Thus, large soil temperature swings associated with early corn planting can hurt emergence or stunt the growth of affected plants.

The good news here is that for recently planted corn, soil temperatures were above 50 degrees for most of Iowa, notes McGrath. On the other hand, in the seed zone at around the 2-inch depth, soil temperatures can fluctuate throughout the day — easily 15 to 20 degrees or more. And cold and rainy weather that is in the current forecast will drop temperatures in the seed zone in a hurry.

“With this in mind, fields planted to corn over the last day or two — ahead of the forecasted change to colder weather — will warrant keeping an extra sharp eye on,” McGrath advises. Watch to see if the seed in the soil germinates and begins growing normally or if it shows signs of problems.

Scout emerging cornfields
Often these cold soil conditions and imbibitional chilling problems impact a relatively small percentage of a field, or perhaps only certain corn hybrids. “But occasionally we see enough problems to warrant action,” says McGrath. Sometimes a problem stand, if it is reduced enough, may need to be replanted.

“So far this week, I haven’t seen any issues yet, and I’ve only had a few calls about isolated potential problems,” he says. “But we are pretty early in the game with a limited amount of corn planted and only a little of it emerged as of April 26. So my message here today is more of a ‘heads up’ to let farmers know they should scout their fields and keep an eye on corn showing problems germinating and surviving.”

In addition to chilling issues, with stressful soil conditions you sometimes hear about or see “corkscrewed” seedlings. But more often you see these in drier soils and with even wider temperature swings. Roger Elmore, former ISU Extension corn agronomist now at the University of Nebraska, says some research showed soil temperature swings of around 27 degrees or more as being a primary culprit in causing this seedling condition.

“The good news,” notes McGrath, “is that today’s corn hybrids are incredibly durable and can take a lot of stress based on their improved genetics alone. And the advanced fungicide and insecticide seed treatments that seed companies offer can increase the odds of having a healthy stand of corn. While these treatments may provide a limited window of protection, looking at growing degree trends for early May, odds are we’ll see the corn germinate and take off quickly, with the treatments helping it fight off early-season insects and diseases.”

There are no guarantees on the earliest planted corn being a perfect stand. “But experience and the calendar tell us that if weather and soil temperature conditions improve sometime next week, the odds are in our favor,” says McGrath. “The best thing we can do is keep an eye on the planted acres and monitor the corn seedling development and emergence, and also take stand counts.”