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Issue No. 2, Article 9/April 7, 2006

Time to Plant Corn?

After the early start to corn planting in several recent years, especially in 2005, we have come to think of early April as "the time" to plant corn, or at least to get started planting. Some of the earliest-planted corn in western and northwestern Illinois in 2005 came up quickly but then had to be replanted following the frost in early May. The weather in April 2005 was unusual, however, in that the first half of the month was warmer than the second half, followed by the early May frost. This month will not follow that pattern, since it remains cool, with soil temperatures only in the low to mid-40s throughout the central and northern parts of the state.

Unlike in 2005, soils are also wet in many areas now, and planting conditions are not nearly as good as those in early April a year ago. Soils were dry enough to apply some N fertilizer in March, but they now need to dry before they can be planted. Soils dry slowly when they are cool, so drying and warming will tend to happen together over the next weeks. Once soils are warm, then they will dry more quickly, so waits to plant following rainfall tend to shorten the later it gets in April.

Last year we initiated a new study to look again at the question of planting date and plant population on yield of newer hybrids. The earliest planting date resulted in damage by frost at most locations and was lost at Monmouth and Urbana. Following are yields at 30,000 plants per acre at different dates at several locations in 2005:

DeKalb: March 30--242 bu/acre; April 19--263; May 7--258; May 27--170

Monmouth: April 18--211; May 6--222; May 17--220; May 31--190

Urbana: April 19--115; May 9--114; May 26--107

Perry: March 30--152; April 18--167; May 9--153; May 31--155

The trial at Urbana seemed to suffer from a number of problems and should probably be discounted. But in general we see that under good to very good conditions, planting dates in late March to mid-April yielded a little less than those in late April to early May. While we know that late April was a cool and difficult environment for corn in 2005, we also can see that planting delays into early or even mid-May did not result in disastrously low yields compared to earlier planting. This is in line with results from our earlier work showing that the last week of April is the "ideal" time to plant in most years but that yield losses for the two weeks on either side of the ideal dates are small.

While we can't predict the planting date response for this year, it is reasonable to conclude that planting early into poor soil conditions is likely to be counterproductive. It is uncommon for soils to be as dry at planting as they were in the first half of April 2005, but there's no question that this helped the roots grow to depth as the surface soils dried in May and June, and that healthy root systems played a large part in the crop's ability to withstand dry periods during the season. Soils are unlikely to be that dry at planting in 2006 (or in any other year), so we can only try to minimize soil compaction during field operations; we will never eliminate it. Unfortunately, soils at field capacity (that is, with water content held against gravity) are quite "compactible," and driving heavy equipment over them, even if the surface has dried, will cause compaction that can restrict root growth. The only thing we can do is to make sure that the surface 6 inches or so is dry enough that tillage and planting operations aren't into soils that are too wet. Test this by squeezing a ball of soil. If it "squishes out" and doesn't break apart under pressure, it's very likely wetter than it should be. If you have to pull the tillage implement out of the ground to avoid pulling up wet lumps of soil or if the planter is placing seed into soil that won't "flow," then it's too wet.

Many people have used soil temperature as a way to suggest when planting should start. I don't, mostly because the soil temperature is usually increasing by the time the soil is dry enough to plant. This fact, coupled with better seed and seed treatments, means that corn seed is usually not at much risk from low temperature by itself. If it rains and turns cool or if the soil crusts after planting, then of course we can have stand problems. Once it's April, though, we expect that soil temperatures will generally move up, and if it's dry enough to plant, there is little reason not to start. Some producers in northern Illinois still wait to start until mid-April. Given the later soil warm-up and later last frost date there, this is reasonable. Above all, plant early only into soils that are in good shape to plant.--Emerson Nafziger

Emerson Nafziger

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