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URBANA, IL -- While research shows that the last 10 days of April is, on average, the best time to plant corn in Illinois, expectations of below-normal temperatures in most of the state during the last week of April 2015 has some wondering if it makes sense to plant now or to wait until temperatures warm up, said a University of Illinois crop scientist.

"While planting before May 1 almost always yields more than planting later, we don't always see a good correlation between planting date and yield on a statewide basis," Nafziger said. "We didn't plant early in 2014 and had our best yields ever, while we planted very early in 2012 and had the lowest yields in the past 25 years. It is clear that what happens with weather during the season can override when the crop was planted, at least over a large area."

But for each individual field, Nafziger said growers should try to plant as early as conditions allow. "Even if planting a week or two later would have little effect on yield in that field that year, we need to 'start so we can finish.' Getting all fields planted by early May is a goal as we try to maximize yield potential," he said.

But could this year be an exception, with potential for harm from planting into cool soils in the last week of April, with the weather forecast indicating that temperatures may stay low for the next week?

Nafziger emphasizes the need to wait until soil is dry enough to allow planting into good seedbed and rooting (less compacted) conditions, especially when soils are cool. "We never want to work soils wet and plant under wet soil conditions if we can help it, but we certainly do not want to do that in April, especially when soil temperatures are less than normal," he said.

"So our first question should be whether or not the soil is dry enough; if the answer is no, then we wait," he said. "Cool soils dry slowly, and wet soils warm slowly, so waiting might take an extra measure of patience, especially if a neighbor brings out the planter."

Minimum temperatures 2 inches beneath bare soil in the April 21-23 period were in the low 40s or even upper 30s in the northern half of Illinois, and were only in the mid- to upper 40s in southern Illinois. This is more than 10 degrees cooler than soils were less than a week earlier, and the weather is not predicted to warm very quickly in the coming days.

"If we go by the old standard recommendation that corn should be planted only after the minimum soil temperature 2 inches deep exceeds 50 degrees, we would have planted for perhaps half the days in April through April 19, but none since then," Nafziger said.

It takes soil temperatures of 50 degrees or above to get the germination process underway, but does this mean avoiding planting corn into soils where temperature at seeding depth averages less than 50 degrees?

"Based on a lot of planting date work, we would say that the danger from planting into cool soils is minimal. It takes about 115 or so growing degree days (GDD, based on air temperature) after planting to get corn plants to emerge, and emergence has usually been good even when it has taken three weeks for this number of GDD to accumulate," Nafziger explained. "It's no more common to have stand problems after planting into cool soil than into warmer soil."

He added that the first planting in the planting date trial was made on April 1, and it emerged more or less on schedule, around April 16. "We can expect corn planted the last week of April to take at least this long to emerge, and longer if temperatures don't rebound going into May," he said.

"While we would prefer warmer and relatively dry soils, the next best scenario is having cool and dry soils. Most stand problems occur when soils are warmer, simply because that's when the plants are trying to grow faster. Still, warm soils help bring the crop up, and we hope that they start to warm soon," he said.

Heavy rainfall is not predicted for this coming week or so, which is a positive, Nafziger pointed out. "Taking the longer view, temperatures in May will inevitably start to rise at some point in time, and this will speed up emergence. Taking all the factors together, I would suggest that planting proceed as long as soil conditions are good, even if the germination process will be slow due to cool soils in the near term," he said.

One concern that some have mentioned is "imbibitional chilling injury," which has been reported when seeds and seedlings take up water that is colder than 40 degrees. Nafziger explained that this can stiffen plant cell membranes and lead to damage, in some cases distorting growth and reducing emergence.

"This has usually been linked with melting snow or very cold rainfall after planting. It's something to keep in mind, but it has been rare in Illinois. It should probably not keep us from planting in the last week of April. Higher, drier fields are less likely to suffer from this and should be planted first," Nafziger said.

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