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Think 'thermal' with corn planting

No, not the thermal sweatshirts and coats you may still have to wear when you head out to the field in this spring that's been much cooler and wetter than normal in much of the nation's center.

Instead, it's all about "thermal time." Though the calendar -- a static measure of dates -- may tell you it's time to start planting corn right now, the living, breathing measure of thermal time is the real way to tell when it's time to get your crop in the ground. But, that's not always the easiest thing to consider in a year like this, says University of Illinois agronomist Emerson Nafziger.

"While it takes patience to wait until wet soils dry out before we can plant, remember that it’s still early, and we haven’t begun to lose yield potential from planting delays," he says. "While we hope to get most of our crop planted by the end of April, what happens after planting remains a lot more important to the corn crop than the exact date we are able to plant. We need look back only one year to see that early planting does not guarantee high yields."

So far this spring, soils in Nafziger's state -- like those in surrounding states -- are the model for where you don't want to plant corn yet. Soil temperatures are above the critical 50-degree mark in southern Illinois, but just barely, and with a cool forecast, those are likely to fall too.

Now add the rain that's been falling. That's when the trouble starts.

"Cool soils by themselves don’t represent much threat to corn seed, but soils that are both cool and wet slow germination and emergence, and provide an advantage to microbes in the soil that can attack corn seed. Some producers may be inclined to set planting depth a little shallower in an attempt to help with emergence under such conditions. That can help sometimes, but the shallowest-placed seed after planting should never be less than about 1.25 inches deep, and planter settings should seldom be less than 1.5 inches deep," Nafziger says. "Remember, too, that soil close to the surface both warms faster during the day and cools down faster at night than soil beneath the surface, so the overall effect of shallower placement on temperature experienced by the seed and seedling might not be very predictable."

If the potential for problems with microbes and other dangers because of planting into wet, cool soils isn't enough incentive to hold off, consider frost damage. Though temperatures -- at least in Illinois -- could be dipping south of the freezing point during this cold snap, the chances of that danger persist decline with each passing day.

"Compared to the corn that was planted in mid-March last year and suffered frost damage the second week of April, the crop this year should be in little danger of frost damage. Chances of frost drop quickly as April progresses, to very low levels by the end of the month in all but the northern edge of Illinois," Nafziger says. "That doesn’t mean zero chance -- we had frost through much of central and northern Illinois the first week of May in 2005 -- but frost through mid- or even late April is not likely to damage corn this year because there won’t be much corn emerged and growing if frost occurs."

These are all individual variables that help comprise "thermal time," the tracking of time as it relates to the compilation of growing degree days (GDDs). There are never 2 years that have the same thermal time; some years, for example, it may take just 1 day to accumulate 25 GDDs, while in others, it could take 5. But, the GDD scale is what should dictate when you plant.

"I suggest that we consider emergence as 'uniform' from the plant development standpoint if it occurs over the time it takes to accumulate 20 or 25 GDD. As with other aspects of corn development, basing events on 'thermal time' (GDDs) works better than using time measured in hours or days," Nafziger says. "The practical effect is that, once it warms up, plants that emerged in 110 GDD and those that took 130 GDD to emerge will differ very little in their stage of development.

"While planting date responses do vary among years and sites in our research, we can for all practical purposes consider the planting date response to be flat for the month of April, with losses starting to pick up, slowly at first, starting in early May," he adds. "It pays to remember during this time that it is easy to do more damage by working or planting into soils that are too wet than we could gain by planting a little earlier."

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