It's now the last week of April, and soils have finally warmed and planters are rolling across Iowa, putting the 2015 corn crop in the ground. But some corn was planted earlier in April and those soils stayed cold and wet for quite a while after planting. What happens when the seed just sits there in such poor germinating conditions, waiting for soil temperatures to warm up enough for germination and emergence to take place?
Clarke McGrath and Aaron Saeugling, two Iowa State University Extension field agronomists in western and southwest Iowa, describe some of the possible problems to be looking out for with early planted corn in such situations. Here are some things they've seen in prior years in similar conditions:
Check your early planted corn for signs of imbibitional chilling
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 degree F range for an extended period of time. Over the last couple weeks, soil temperatures in Iowa have been in the mid-40s to low-50s at the 4-inch depth in many fields. "I would guess we'll gain ground on that over the next week based on the nice weather forecast," says McGrath.
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 the soil temperatures often bounce back up this time of year, he says. On the other hand, it takes more BTU's or energy to raise the temperature of saturated soils vs. dry soils, slowing any warming. So given the cold rains and saturated soils, corn spent a few more days than usual suffering temperatures around the low to mid 50s or so. On April 24, most of western Iowa was around 55 degrees, then it dropped to 50 to 52 over the weekend.
Is your early-planted corn leafing out underground?
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 40s to low 50s are often mentioned), cell walls can become "brittle" and even rupture.
"When this happens, we have seen all sorts of impacts," says McGrath. "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."
What causes those "corkscrew" corn seedlings?
"With wide temperature swings we sometimes see 'corkscrewed' seedlings in conditions like we have had this April," says McGrath. "But more often we see these in drier soils and wide temperature swings. That is, as we discussed previously in this article concerning water, soil temperature and heat units."
Some studies have shown that soil temperature swings of around 27 degrees or more are a primary culprit in causing this. "Again, typically it is a small percentage of a field and growers may not even notice most years," says Saeugling. "Given our wet soils in April, those early April planting dates are more likely to suffer the imbibitional chilling than this temperature swing 'corkscrewing', but things can change quickly. However, early scouting visits to farmers' fields haven't shown many problems so far."
Check emerging and yet-to-emerge corn for other problems, too
Look for signs of insect injury, diseases and herbicide injury on corn seedlings and young corn, advise the two ISU Extension agronomists.
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 the seed or seedling exposed to pathogens for a longer time. Some pathogens thrive in these conditions (pythium comes to mind). So while the corn struggles, disease pathogens have a better shot at infecting the young 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 us 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 emergence, the real culprit is simply poor conditions."
When scouting fields as a young agronomist, McGrath says he sometimes diagnosed tough looking fields of small corn seedlings as herbicide injury. "But in subsequent years as we drifted away from preemergence residual herbicides towards 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 bred to withstand more stress
The good news is today's hybrids are incredibly durable and can take a lot of stress based on the improved genetics alone, says McGrath. Advanced fungicide and insecticide seed treatments that seed companies offer increase the odds of a healthy stand. "While they have a limited window of protection, looking at growing degree trends for early May, the odds are that we'll see the corn take off quickly, helping it fight off early season insects and diseases," he adds.
The bottom line: There are no guarantees on the earliest planted corn being a perfect stand, "but experience and the calendar tells us that if conditions improve, the odds are in our favor," says Saeugling. "The best thing crop scouts and farmers can do is keep an eye on the planted acres and monitor seedling development and emergence and take stand counts."
Scout for seedling diseases in early planted corn
Besides keeping an eye on any chilling injury, the ISU agronomists say you also want to watch corn stands for seedling diseases. Refer to this article cropwatch.unl.edu/corn-seedling-diseases-2015.
Burndown herbicides still being sprayed, watch the temps
Some burndown herbicide applications are still being made. With overnight low temperatures sometimes hitting 40 degrees or below, and a lot of spraying still being done, here is an article from a couple weeks ago to refresh your memory about how to keep burndown herbicide applications effective in cool temperatures: Cold temperatures and applying burndown herbicides.