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Issue No. 10, Article 3/May 30, 2008

When Is the "Last Day" to Plant?

Heavy rains fell in parts of Illinois again this week, especially in the southern areas, which have received more than 20 inches of rainfall since March 1. Growing degree days continue to accumulate very slowly, and we will end May with monthly totals ranging from about 300 GDD in the north to 400 in the south, or about two-thirds of normal. As of May 25, corn was assessed at 87% planted by May 25 and soybean at 39% planted. These numbers include the corn acreage that will need to be replanted, as I discussed last week here. The 2008 spring planting season continues to worsen as we approach the end of May, though there are some places in Illinois where most of the fields are planted. Even there, crop growth is much below normal, and so most fields look late-planted, even if they were planted on time.

Soybean planting is much farther along in northern than in southern Illinois, but heavy rainfall following planting will pose problems for soybean just as it has for corn. Soybean seed quality is generally marginal this year, and getting seed for replanting might be difficult. Even top-quality soybean seed is inferior to corn seed in its ability to emerge, so we would expect very poor soybean emergence in fields where soils are very wet and cool after planting. Our replant data are not great for soybean, but we know that the 100,000 plants per acre that will normally produce a full yield when the crop is planted early will often not be enough for full yields when the crop is planted late. This is because late-planted soybean plants often are smaller when flowering begins, and they often are shorter and have less leaf area as they begin seed-filling. As an estimate, for each week that soybean is planted after May 31, the established population ought to be increased by 15 to 20 thousand plants per acre in order to maintain yield potential for the date on which the crop is planted. Any planting after June 1 should also be in rows no more than 15 inches apart.

A recurring question as we try to finish planting in 2008 is how late corn and soybean can be planted and still "produce a crop." I normally take this to be the planting date for which we can expect a crop to yield half its maximum expected yield. It's always a little dangerous to project yields beyond the last planting date in our studies, but using the response curves generated from our corn planting date data shows that we can expect 50% of the maximum yield when planting is done around June 15 to 20 in Illinois. We would expect actual yields of corn planted on June 15 to vary greatly depending on the weather the rest of the season. For example, one study planted at Urbana in early July 2007 yielded more than 170 bushels per acre. In another year, such a planting might well not be worth harvesting.

Our data from planting date studies on soybean aren't as good, but here the double-crop experience is helpful, at least in southern Illinois. I would estimate that soybean reaches its 50% of maximum yield expectation when planted around June 25 to 30 in southern Illinois, and perhaps a week earlier than this in the northern half of the state. Like with corn, the prospects for late-planted soybean are highly uncertain, with similar probabilities of having very good and very poor yields. It is clear that potential yields of both corn and soybean are decreased considerably by planting as late as mid-June, and that the expected yields of both crops are decelerating quickly by then, with losses approaching 3% or 4% per day of delay, as cumulative loss approaches 50%.

Given that expected yields of both crops drop rapidly as planting is delayed into June, the advantage that corn yield and income have over those of soybean is relatively slow to diminish. This means that switching from corn to soybean might not be highly profitable, even though our experience with double-cropping soybean after wheat harvest makes us more comfortable with late-planted soybeans. Should corn, or soybean, be switched to grain sorghum? This is a question for which we have little data. The last half of May is the typical planting date for grain sorghum--the average planted by May 25 over the past 5 years has been less than 50%, and soil temperatures in 2008 remain lower than ideal for planting this crop.

But just because planting later is the norm for grain sorghum does not mean that we should rush into planting a lot of it on acres that were intended for corn or soybean. If it remains cool through the early part of the summer or if temperatures drop earlier than normal in the fall, grain sorghum's sensitivity to low temperatures may mean low yields relative to those of late-planted corn or soybean. Grain sorghum does have more tolerance to periods of dry weather than corn or soybean, but this does not mean that it will produce high yields no matter what. In fact, the biggest problem with grain sorghum is that its yield is often much less than that of corn; the rainfall and warm temperatures that it takes to produce high grain sorghum yields will also improve corn yields, even if the corn crop is planted late.

Hence grain sorghum does not reduce weather-related risks by very much, and its lower yield potential means that while it may be a little "safer" than late-planted corn, the returns will likely be limited. In the outstanding year of 2004 when corn yields averaged 180 in Illinois, grain sorghum yields averaged 109 bushels per acre. Both were the highest on record. This is why we suggest that grain sorghum be considered first on soils where expected corn yield is less than 100 bushels per acre. That 100 bushels does not increase very fast even when planting is delayed.--Emerson Nafziger

Emerson Nafziger

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