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Issue No. 4, Article 10/April 16, 2004

Planting: How Early Is "Early"?

There have been a few reports of soybean being planted already, probably reflecting a deliberate choice to plant soybean before corn. Our data from planting date work on soybean suggest that planting soybean before corn, at least when planting starts in mid-April, is probably not a good management choice. Both corn and soybean are expected to yield a little less when planted before April 15 than after April 25, but our data tell us to expect the percent yield loss from earlier planting to be greater with soybean than with corn.

More important, the planting date plateau--the range of dates over which we would expect little effect of planting date--is longer and starts later for soybean compared to corn. While we can't forecast how planting date responses might vary among years, our data show that the "average" ideal planting dates are about April 20 to May 5 for corn, and April 25 to May 20 for soybean. If we accept these, then it makes little sense to ever plant soybean before corn planting is finished, even if we don't believe that planting in mid-April causes lower yields. The risk of not getting corn planted on time is simply greater than this risk for soybean.

With the weather turning warmer this week and many fields in good shape to plant, we expect a fast start to planting by mid-April. I see little reason not to start planting in central and southern Illinois, as long as the soil is in good shape. The fact that the weather seems to be in a relatively dry pattern suggests that we needn't rush to get done by April 20 or 25, but if the weather continues to be dry, it will be useful to plant before the soil dries out to below planting depth.

Planting this early, with the possibility of a return to cooler and (especially) wetter conditions, means that we should not increase planting depth; seed lying in dry soil until it gets rain to emerge is usually better than seed planted too deep and trying to come up when it's cold and wet. It's impossible to predict emergence conditions when we start to plant early, but we do know that dry conditions are almost always better than wet conditions, regardless of temperature. We have known corn seed to lie in dry soil for a week or two with little ill effect, as long as it eventually gets rain to emerge. Of course, it's safer to have enough soil moisture at planting for the crop to emerge. This may mean skipping that last "leveling" tillage pass for some people and thus avoiding the exposure of more soil to drying that such tillage causes.

Crusting is reported to be serious in some of the fields that were worked, planted or not, in March. The degree of soil crusting always seems unpredictable--we normally associate the greatest crusting with warm, dry conditions after a beating rainfall. In this case, we had the rainfall, but drying conditions didn't develop very strongly. From March 20 to 31, we accumulated about 75 growing degree-days (GDDs) at Urbana but have had only about 60 GDDs since April 1. The 130 or so accumulated after the crop was planted in March (the heavy rains were March 25-26, and most planting was before that) should be enough for corn to emerge, though the upside-down pattern of warmer early followed by cooler weather may have thrown off the prediction that emergence should start within about 120 GDDs after planting. Breaking a crust to allow emergence after this kind of start is probably not going to work very well, given the strength of the crust. The cool temperatures the seedling has already experienced may also have decreased its ability to emerge, due to its having used up seed-stored materials and diseases that might have infected the seed.

We usually are pleased when planting can start early and end on time, as it appears may happen this year. Before we get too carried away, though, we should recall that early starts and finishes to planting in Illinois have not always meant high yields. It's good if it's dry in April, but only if it remembers to rain during the season.--Emerson D. Nafziger

Emerson Nafziger

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