No. 7 Article 9/May 9, 2008

Corn and Soybean Planting: Issues Remain

The official (NASS) estimate for Illinois showed that only 28% of the corn crop and 1% of the soybean crop were planted by May 4. This compares with 5-year averages by this date of 76% for corn and 11% for soybean. Still, we made some progress, having planted 22% of the corn crop during the last week. And there has been considerable planting over much of the state in the past few days.

As we see the days going by with many fields still to plant, some may be wondering whether it might be too late for the "full-season" hybrid they chose to mature if it's planted only by mid-May. In all but rare cases, there is little cause for concern. Most hybrids sold as full-season for an area already have a reasonable cushion of growing degree days (GDD)--they need 200 to 300 fewer GDD (base 50F) than are available on average from normal planting time to the average date of first frost. For example, GDD accumulations for May 1 through September 30 range from about 2,700 in northern Illinois to 3,100 in central Illinois to 3,600 in southern Illinois. Hybrids considered to be full-season typically require about 2,600 GDD in northern Illinois to perhaps 2,900 GDD in southern Illinois. So if early frost occurs, hybrids in northern Illinois could be damaged before they are fully mature, but the chance of this happening in central and southern Illinois is low.

Several other factors affect hybrid maturity and the risk of having grain-filling end prematurely. One is that GDD accumulations in May are on average rather slow, so corn planted in mid-May experiences only 200 to 300 or so GDD less by some later date than corn planted in mid-April. Corn planted later also requires fewer GDD to reach maturity than the same hybrid planted early, which adds to the cushion. In general, then, hybrids on hand for planting should not be switched out for earlier ones unless planting goes into very late May. One exception to this would be in northern Illinois, where hybrids requiring 2,700 or more GDD might have been chosen in an attempt to push for maximum use of the growing season. In our northern Illinois hybrid trials, GDD requirement and yield are often not very well correlated, suggesting that such a strategy may not pay off very often. But it would be advisable to check into the availability of hybrids requiring 150 to 200 fewer GDD than any such full-season hybrid(s) that might be on hand for places in northern Illinois where planting is not yet underway.

Soybean responds considerably less to delayed planting than does corn, and in no part of Illinois should soybean planting take precedence over corn planting at this point in time. Our data relative to soybean planting delays are not as recent or as good as those for corn, but I agree with Dr. Palle Pedersen of Iowa State University that early planting of soybean is helpful in seasons (and fields) that foster high soybean yield, while in average or poor growing seasons, there is often little or no response to planting delays past mid-May. This is likely because an early start to flowering and pod setting tends to be related to higher soybean yields, but only when late-season conditions are favorable for filling seeds. The fact that we don't know how the season will turn out, however, means that we should try for earlier planting. With some seed of marginal quality this year and with soils continuing to show swings in temperature and moisture, we need to be careful to plant into reasonably good soil conditions and to not rush to plant before fields are ready to plant.

Soybean seeding rate continues to be an issue because of the increased cost of seed and, in 2008, the quality of available seed. Most seed is still sold by weight, though the seed size (measured by weight as number of seeds per pound) is usually specified. The standard recommendation that many agronomists now use 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 (154 bushels per acre) produced under irrigation in Missouri in 2007 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 a stand of 100,000 plants is really enough when conditions are right for very high yields. We haven't produced yields anywhere close to the 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 in our trials. While 100,000 may be the plant number that produces maximum yields under most conditions, it's usually best to try to establish plant stands of at least 120,000. This will help compensate for poor conditions that can reduce emergence and for water shortage later that can reduce plant growth and canopy development. It is definitely the goal to have full canopy cover by the start of seed filling; reduced vegetative growth, especially in wider rows, can make this difficult. Late planting often reduces the duration and amount of vegetative growth, so narrowing rows for late planting is often helpful.

The need to get a good stand is greater when soybeans are planted late, 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 how to make adjustments. Be sure to note the warm germination percentage provided with the seed, and adjust the seeding rate by dividing target stand by germination percentage (for example, divide target stand by 0.85 if the warm germ is listed at 85%) at the start of the rate-setting calculation. The calculator will help with this.

A note on the Illinois wheat crop: cool temperatures have delayed development but have increased tillering, and much of the crop in central and southern Illinois is in great shape as heading gets underway. Only 5% of the wheat crop was headed by May 4, compared to an average of 30% by that date. Watch for Fusarium head blight (scab) if it is wet at heading, for leaf diseases if it stays wet after heading, and for outbreaks of insects, especially armyworm and cereal leaf beetle, in heavy canopies. The relatively cool temperatures that have been good for wheat development also mean that an early harvest and start to doublecropping is not very likely. As a rule of thumb, it is at least 6 weeks from heading to harvest in southern Illinois, and it will likely be much less than this only if it gets very warm, which can hurt wheat yields. Otherwise, the main wheat problem so far this season was in some fields in northern Illinois that were badly damaged by water and ice over the winter. Some fields have been destroyed already in preparation for planting corn or soybean. The variety trial at our research center near DeKalb, which did not survive well enough to get yield data, will soon join them.--Emerson Nafziger

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