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Issue No. 6, Article 7/May 4, 2007

Getting Corn and Soybean Planted

The last official estimate has 36% of Illinois corn planted as of April 29. This compares with a 5-year average of 59%. More than 65% of the corn crop has been planted by this date on each of the past three years. This is reflected in the fact that only 47% of the crop has been planted by this date averaged over the last 10 years.

In the April 20 issue of the Bulletin I included statewide data from the last 14 years showing that the percentage of corn planted by April 30 in Illinois is correlated to average yield. This correlation is not great, with few years falling right on the trend line, but it still shows a trend. This means that using this as a prediction tool is not likely to be very accurate for a given year. If we apply the trend line calculated over the past 14 years to the percentage planted by April 30 this year (adding 4 percentage points for planting from April 29 to 30), the predicted statewide yield in 2007, with 40% planted by April 30, would be about 144 bushels per acre.

To look at this another way, six of the seven highest yields over the past 14 years have come when more than 50% of the corn was planted by the end of April, and the average yield over those seven years was 154. The average yield over the seven years when less than 50% of the corn was planted by the end of April was 136. The two "crossover" years were 1994, with somewhat slow planting progress but a yield of 156, and 1997, with early planting but a yield of only 129. It may be important that all but one of the yields above 150 have come since 2000, and so reflect at least in part the increasing genetic potential of newer hybrids. Six of the seven years in which more than 50% of the crop was planted by the end of April have also occurred since 2000, however, and so genetic potential can't be separated completely from conditions that allow earlier planting. Planting equipment plays a role in this as well, but as this year indicates, weather determines planting date more than anything that we can control.

As we see May days going by with a lot of fields still to plant, what decisions need to be made or changed? One that will be, and should be, slow to change is what crop to plant. A great deal of nitrogen fertilizer has been applied, and those fields will almost certainly be planted to corn, since planting soybean means losing the investment in N fertilizer. If N fertilizer remains to be applied, the corn price relative to soybean price continues to send a strong signal to plant corn. Also, of the major corn-producing states, Illinois has the highest percentage planted (Iowa, Nebraska, Indiana, and Ohio were all less than 20% planted as of April 29, and the national number was only 23%), meaning that we are better able to benefit from price increases that may come from further planting delays in the Corn Belt.

We can on average expect yield declines of about 1 bushel per acre for each day of delay for the first 10 days of May, 2 bushels per acre per day for the middle 10 days of May, and 3 bushels per acre per day for the last 10 days of this month.

Let's look at the nature of such yield declines. Most of the hybrids we grow in Illinois will, when the first frost does not come early, accumulate enough growing degree days (GDD) to mature even if planted in late May. This means that the corn crop doesn't "run out of time" when planting is delayed. In fact, modeling work at the University of Nebraska has shown that pushing back the grain-filling period into the cooler nighttime temperatures of September can increase yield. Pollination, which is the most stress-sensitive period in corn's life cycle, usually takes place in July whether corn is planted in mid-April or late May. Average temperatures are high during all of July, so temperature effects are not a big issue, at least on average.

Instead, we think that most yield loss from delayed planting comes from the increased risk of having inadequate water available to the plant during the critical pollination period. Because yield-limiting water deficit takes time to develop, such deficits are more likely in late July than in early July, so late-planted corn is more likely to be affected. Adding to this is the fact that late-planted corn, which develops during warmer weather, tends to favor top growth at the expense of root growth, and so is often less able to tap soil water during dry periods. This is made worse in some cases by late-planted corn's being planted into wet soils, with compaction further restricting roots.

Chances of developing water deficits that reduce corn yields are closely related to the ability of the soil to supply water when rainfall is low. Water use by an early-planted corn crop exceeds average rainfall amounts in Illinois from about mid-June to mid-August, with a total deficit of around 5 inches. This amount can easily be supplied by most soils in Illinois, which usually can hold 2 to 3 inches of plant-available water per foot of depth. Crops with shallower root systems cannot tap as much water as deeper-rooted crops, however. When rainfall is less than average, the need for soil-supplied water increases, and the cumulative loss of soil water means that the chance for yield-limiting deficits increases over time. Later-planted corn, with less effective rooting depth and later pollination, is thus much more likely to experience yield-limiting water deficits. Higher pressure from insects and diseases can also increase yield declines from late planting, in part by contributing to stress by reducing effective leaf area or normal plant function.

Soil water supply is closely tied to soil texture, with the maximum capacity to supply water to crops in soils that are high in silt (silt loams and silty clay loams) and decreasing capacity as either clay or sand content increases. Soil organic matter also helps store water available to crops. Soils in southern Illinois tend to be lower in organic matter, and some have high-clay layers that tend to restrict roots. Planting progress has been at least average in southern Illinois this year, and slower than average in northern Illinois, so the ability of the soil to supply water is somewhat greater in those areas with more planting delays. This might help mitigate the yield effects of delayed planting.

The unpredictable water supply explains why yield declines with delayed planting are so unpredictable. For example, if a major rain event on July 20 ends a very dry period, late-planted corn might yield more than early-planted corn. But for now, we can only use the "average" data we have, and they indicate that corn planted on May 10 will yield about 10 bushels less than corn planted on April 25. This penalty will increase to 20 bushels for corn planted on May 15 and to 30 bushels for corn planted on May 20. This will double to 60 bushels if planting is delayed until the end of May.

Soybean responds less to delayed planting than corn, so at some point the relative yield loss from corn exceeds that from soybean, and people not already committed to planting corn start to think about planting soybean instead.

When is that point reached? We would expect corn to lose about 15% of its yield potential if planting is delayed to May 20, and 30% by the end of May. In contrast, we would expect soybean to lose perhaps 5% by May 20 (some of our data show less loss than this) and maybe 10% by May 30. In relative terms, then, soybean would have an advantage over corn once planting is delayed past early May. From a net income standpoint, however, the picture is much different. Even when corn loses 50 to 60 bushels and soybean loses only 5 bushels when planting is delayed to late May, corn might still provide more return than soybean. This depends on soil productivity, as well as on costs and market opportunities for the two crops.

One decision that should stay in place as planting is delayed is choice of hybrid. Most hybrids have a GDD "cushion" (number of GDD remaining in the season at planting time minus the number needed to mature a hybrid) even if planted after mid-May. In addition, work at Purdue and Ohio State has shown that late-planted corn tends to require fewer GDD to mature than early-planted corn. That increases the cushion even more. The choice of hybrids has been somewhat limited this year already due to the increase in corn acres. So it will be hard to find good replacement hybrids with earlier maturity. This will (and should) help people make the decision to stick with the hybrids they have on hand if planting can be finished by late May.

Soybean seeding rate continues to be an issue given the increased cost of seed. Most seed is still sold by weight, though the seed size (really weight, measured in number of seeds per lb) is usually specified. The "standard" recommendation used by many agronomists is based on the fact that few studies have shown further yield increases as the number of plants (not seeds) increases above 100,000 per acre. On the other hand, the record-setting soybean yield (139 bushels per acre) produced under irrigation in Missouri in 2006 came from a seeding rate of around 300,000 seeds per acre and a reported stand of about 250,000 plants. This raises the question of whether 100,000 plants are really enough when conditions are right for very high yields. We haven't produced yields anywhere close to the new record yield, so we don't know the answer to this question. We do know that increasing seeding rates above 150,000 per acre, when establishment is 75% or higher, has seldom produced yield increases.

While 100,000 may be the plant number that produces maximum yields under most conditions, it's usually prudent to shoot for plant stands of 120,000 or so, given the possibility that conditions will reduce emergence. That's particularly true for late planting, since replanting late-planted soybean is more costly in terms of yield than replanting early-planted ones. Adjusting soybean seeding rate for planting conditions might involve some guesswork, but it is a reasonable approach. You might check out the seed drop calculator to see to how make adjustments.--Emerson Nafziger

Emerson Nafziger

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