Corn typically requires 90 to 120 Growing Degree Days or GDDs from planting to emergence. Of course this range assumes adequate soil moisture and varies with planting depth, tillage system and crop residue cover. "As a rule of thumb, if 120 GDDs have accumulated since planting and seedlings haven't emerged, you should check the condition of planted seed ASAP," advises Clarke McGrath, Iowa State University Extension field agronomist at Harlan in western Iowa.

WATCH CAREFULLY: Keep an eye on emerging corn seedlings, and also check the corn you've planted that hasn't yet emerged. "It will be curious to see how stands turn out this spring, given our cold snaps," says ISU agronomist Clarke McGrath.

Some of the early planted corn this year, the corn that went in April 4 to 12 or so, is now spiking. It has had enough GDD's despite the cold temperatures, says McGrath, who authors the "Corn-Soybean Insight" column for ISU Extension each month in Wallaces Farmer magazine.

Another rule of thumb is corn in 50-degree F soils takes about 20 days to emerge, and this year that has held pretty true. "As soil temperatures get to around 60 degrees F, corn comes up in about 10 days, which is hopefully where the corn we plant in the next week or so will end up," he notes.

Track growing degree day accumulations for your fields
You can track GDD accumulations for the Corn Belt location of your choice by clicking on 'single site graphs' on ISU's Mesonet website. Your specific planting date information is easily selected from the drop-down windows. Choose the weather station near your farm from the list or select by clicking the "dot on the map" near your farm. Track the GDD accumulation at your location (a blue line is produced) and compare it to the normal GDD accumulation for your location (a red line is displayed). "It's helpful to also make a graph of last year to give you an idea of average GDD accumulation to help visualize the similarities and differences between this year and last year," he adds.

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You need to remember that GDD's are calculated based on air temperatures using the 86/50 method typical for corn production. Using that method, if air temperatures remain at or below 50 F, emergence will not occur. "We hope this isn't a continuing issue now that we are in early May," says McGrath.

Soil temperature may be the better predictor of emergence
Since GDD calculations are based on air temperatures, four-inch soil temperatures may actually do a better job of predicting seedling emergence than accumulated GDD's. The Mesonet provides a daily update of both the Iowa soil temperature and GDD. Lab studies have shown that for most corn hybrids grown in the Midwest, seedling emergence takes about three weeks if the soil temperature is 51 F and it takes about one week if the daily soil temperature holds near 70 F.

"Cold and wet soils like we currently have will slow down germination and emergence once corn is in the ground," McGrath noted on May 2. "These conditions can also give soil microbes that attack seedlings the advantage—so exercise patience and watch the weather forecast when getting ready to plant."

What about corn that's already in the cold ground?
"So far, I am optimistic," he says. "What I've dug up has looked pretty healthy, and the weather is improving. I remember there was a lot of excitement a year ago when we had a lot of corn in the ground when our May 1 blizzard came through. There were some cold days after that, but then it warmed up and the corn came out of that situation quite well in the end. I suspect unless things really get cold again for an extended period of time, we will have good stands this spring."

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McGrath describes some things farmers, agronomists and crop consultants have seen in prior years in similar weather conditions to what we've had this spring:

Imbibitional chilling—This is a common term for the chilling effect seeds may go through when they absorb water, especially when soil temperatures are less than the mid-50's for an extended time. "The last few days of April and the first two days of May this spring, soil temperatures have been from the mid-40's to low 50's at the 4-inch depth in our area, and I'd guess we'll gain ground on that over the next week based on the weather forecast," he says. "Keep in mind that with seed around 2-inches deep, temperatures can fluctuate a little more than at the 4-inch depth, so with some sun, soil temperatures often bounce back up this time of year."

On the other hand, it takes more BTU's of energy to raise the temperature of saturated soils vs. dry soils, slowing any warming. "So given the cold rains we've had this past week, we'll likely spend another 4 or 5 days with our planted corn suffering soil temperatures around 50-degrees or so," McGrath observed on May 2. Corn seed absorbs around a third of its weight in water early in the germination process. If this water is cold enough (exact temperatures vary by source, but upper 40's to low 50's are often mentioned), cell walls can become "brittle" and even rupture.

"When this happens, we've seen all sorts of impacts," he adds. "Seed that just swells and never continues growth, sometimes corkscrewed seedlings, ruptured coleoptiles, leafing out underground, seedling death and other interesting but not good phenomenon. The good news is that often this impacts a relatively small percentage of a field; only occasionally do we see enough problems to warrant any action. So far I haven't seen or heard of too much of this occurring this spring."

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Wide temperature swings— "We sometimes see 'corkscrewed' seedlings in conditions like we have now," says McGrath. "But more often we see these in drier soils and with wide temperature swings." Recall the discussion about water, soil and BTU's earlier in this article. Former ISU Extension agronomist Roger Elmore provided some research information that talked about soil temperature swings of around 27-degrees F or more being a primary culprit in causing this.

"Again, typically it is a small percentage of a field and growers may not even notice the corkscrewed seedlings in most years," adds McGrath. "Given our wet soils this spring, corn planted on the early-April planting dates is more likely to suffer the imbibitional chilling than the temperature swing 'corkscrewing'— but things can change quickly. I say once again, however, my early scouting of cornfields this spring hasn't shown many problems so far."

Insect injury—The longer a seed or seedling is small and growing slowly, the greater the odds of a pest finding it and attacking it.

Diseases—Cold, wet soils slow corn growth and leave it exposed to disease pathogens for a longer time. Some pathogens thrive in these conditions (pythium comes to mind) so while the corn struggles, pathogens have a better shot at infecting the corn plants.

Herbicide injury—This can also be more of an issue when seedlings are under a lot of stress and are growing slowly. "Experience tells me that usually plants grow through this with little, if any long term impact," says McGrath. "Also, while we sometimes point the finger to herbicide injury when we see slow or uneven corn emergence, the real culprit is simply poor conditions."

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He adds, "As a fertilizer/chemical dealer in my earlier career, I diagnosed tough-looking fields as having herbicide injury. In subsequent years as we moved away from using pre-emerge residual herbicides and moved to using total post programs, we'd see the same symptoms in the absence of any soil-applied herbicides. Lesson learned. While early season herbicide injury to seedlings does happen, it probably isn't as common as we think. Conditions like these do increase the odds of issues, though, so careful investigation is warranted for any field that exhibits problems that may appear to be herbicide related."

Today's corn hybrids are durable, can take a lot of stress
The good news: Today's corn hybrids are incredibly durable and can take a lot of stress based on the improved genetics alone. Advanced fungicide and insecticide seed treatments that seed companies offer will increase the odds of getting a healthy stand. "While these seed treatments have a limited window of protection, looking at the growing degree trends for early May—odds are we'll see the corn take off quickly, helping it fight any early season insects and diseases," says McGrath.

"The bottom line is, there are no guarantees that the earliest planted corn will be a perfect stand. But experience and the calendar tells us that if weather conditions improve this coming week, the odds are in our favor," he sums up. "The best thing farmers and crop consultants can do is keep an eye on the planted acres and monitor seedling development, and be sure to take some emergence and stand counts."