Only 1% of the Illinois corn crop was planted by April 14, compared to 38% on that date in 2012. With soils still cool, more rain falling this week, and a forecasted drop in temperatures later in the week, corn planting is unlikely to be early in 2013, according to University of Illinois crop sciences professor Emerson Nafziger.

 “Wet soils warm more slowly than dry soils, so with soils both wet and cool, we need to be ready for some ‘patience-requiring’ weather,” Nafziger said.

Cool soils alone are not much threat to corn seed, but soils that are both cool and wet, slow germination and emergence and provide an advantage to soil microbes that can attack corn seed. Under such conditions, producers may be inclined to set planting depth a little shallower. That sometimes helps, but seeds should never be placed less than about 1.25 in. deep, and planter settings should seldom be less than 1.5 in. deep.

“Remember, too, that soil close to the surface both warms faster during the day and cools down faster at night, so the overall effect of shallower placement on temperature experienced by the seed and seedling might not be very predictable,” Nafziger warned.

Unlike last year’s crop, which was planted in mid-March and suffered frost damage the second week of April, this year’s crop is in little danger of frost damage. As April progresses, chances of frost drop to very low levels by the end of the month in all but the northern edge of Illinois.

“That doesn’t mean zero chance,” Nafziger said. “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 most corn will not have emerged.”

The corn planted on April 5 in the planting date study here at Urbana germinated and sent out its radicle (root) by April 10, by which time it had accumulated about 70 growing degree days (GDDs), but the weather cooled after that and only about 30 more GDDs accumulated over the next week. Based on the crop’s needing about 110 GDDs from planting to emergence, it will be in the ground approximately three weeks by the time it emerges.

“Over the past 10 years, the GDD accumulation for April at the Champaign airport has ranged from 210 to 344, with an average of 263,” he said. “We’re on track to accumulate normal GDDs this April, but April temperatures don't always increase steadily, so later planting doesn’t always mean warmer temperatures after planting.”

The variability in daily accumulation of GGDs during April means that emergence can take anywhere from one to three weeks. Having corn take 20 days or more to emerge is not necessarily a problem if both emergence percentage and emergence uniformity – the time between first and last emergence within a field – are good. Under cool temperatures, it is normal for emergence to be spread over four or five days in a field.

People often worry that non-uniformity of emergence will reduce yield. Plants that lag in their development tend to stay behind and often lose the competition for light, water and nutrients. Although it is not true that such plants end up as “weeds” – that they reduce the yields of remaining plants -- the larger plants do not make up for the yield lost from the plants that get left behind.

Non-uniform emergence is a problem if it is related to factors such as seedling damage, soil crusting or low seed vigor. However, uniformity is also affected by temperature (GDD accumulation rates), as is the time between planting and emergence.

“I suggest that we consider emergence to be ‘uniform’ if it occurs over the time it takes to accumulate 20 or 25 GDD, whether that is one day or five days,” Nafziger advised. “As with other aspects of corn development, basing events on ‘thermal time,’ or GDDs, works better than using time measured in hours or days. 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. Once we get to June, 20 GDD is only one day’s accumulation.”

Although it takes patience to wait until wet soils dry out before planting, remember that it is still early, and yield potential is not yet at risk from delayed planting. “While planting date responses vary among years and sites in our research, we can consider the planting date response to be flat for the month of April, with losses starting to pick up slowly in early May,” Nafziger said. He cautioned that the damage related to working or planting into soils that are too wet can more than offset the gains related to early planting.

“While we hope to get much 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,” he concluded. “We need look back only one year to see that early planting does not guarantee high yields.”