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Issue No. 19, Article 3/August 10, 2012

Notes on Corn and Soybean Yield Potential

The first "objective" crop yield estimates--those based on pod and kernel counts in the field--for U.S. corn and soybean crops will be coming out this Friday (August 10). Following serious water deficiencies over much of Illinois this year, these numbers will be closely examined. The rain in some areas over the past week will make more difference in some areas and crops than in others.

In years like this, it's normal for many people, especially those in areas with poor crops, to feel that the state or national yield estimates are too high. It's certainly the case that August 1 and final estimates are usually not the same, but there's no reasonable way to try to outguess the National Agricultural Statistics Service numbers. NASS carefully chooses sample fields in which to make counts, and these should be representative. In a year like this, with many fields having very low (or no) yield, it is possible for such random samples to miss the average. But it's not clear that poor fields would be underrepresented.

We would normally consider early planting and rapid corn crop development to increase the accuracy of the August yield estimate. Kernel numbers are more or less fixed, and there is at least some idea of how well the crop canopy is holding up, and so how well kernels will fill. By August 5, 38% of the corn crop was in dent and 6% was mature. So while getting accurate samples of ears and kernels within and across variable fields is difficult, yield prospects are becoming clearer.

The situation with the soybean crop is less clear. In traveling through some of the driest areas of the state on August 2, I found better growth than I had expected based on having seen the same area in mid-July. In western Cumberland County, I took photos of a field and of some plants in an area where the corn crop will yield little. Plants were about 2 feet tall, and while leaf area and canopy cover were less than ideal, plants did have pods that were beginning to fill. Each plant had about a dozen pods, ranging from flat to partly filled. Using our previous estimate of 3/4 of a bushel per seed filled per plant, the field could yield 25 bushels per acre if these pods all fill.

Early August photo of stressed soybeans in Cumberland County.

Plants from the Cumberland County field.

That area received some rainfall since then, and it's possible that more pods will develop, though how well they fill will depend on getting more rainfall in coming weeks. There are some much worse areas in Illinois, however, with soybean plants barely a foot tall, low pod numbers, and canopy color that is starting to fade. Such fields are rapidly running out of time to recover, and it's not clear how much help they'll get from the rain that fell or that might fall in coming weeks.

Standability is going to be an issue in many cornfields given the ongoing stress and the inability of the plants to produce enough sugars to both fill kernels and maintain stalks. This won't be an issue in fields without grain, since most have been harvested for forage or won't be harvested at all. For fields that have grain, this could be one of the worst years for lodging we have had recently. In those fields stressed "early and often," stalks have not developed much strength, since there was not enough sugar to produce the lignin that provides that strength. In fields under stress since kernel set, sugars will simply go to kernels and leave stalks depleted. Loss of leaf area will contribute to this.

In areas that have had more rainfall or where the crop was irrigated, even soybean plants may be subject to lodgin

g this year. The high temperatures have encouraged tall plants; plants are not tall in most areas only because lack of water has prevented their reaching the height they would have with adequate water. As an example, irrigated soybeans at Urbana are more than a foot taller than nonirrigated ones, and they are already leaning as pod-filling gets underway. Modest lodging may not reduce yields, as long as leaf area remains exposed and the canopy remains full.

The fact that most soybean plants were shortened by lack of water is probably more of an advantage than a disadvantage. As we have seen before, very tall plants often do not fill pods very well due to so much shading of lower leaves. Large plants and leaves also use water faster, and so can dry soils out more quickly. But plants that are only a foot tall often do not have much ability to set and fill pods, so the ideal is neither too tall nor too short.

Although flowering and pod formation in soybean are ahead of average--82% of the Illinois crop was setting pods on August 5, compared with the average of 52%--the difficulty in dry areas has been keeping the pods and starting to fill them. Once seeds begin to fill, pods tend to stay on plants, and seeds will often fill out to near normal size. Stressed plants may abort some of their flat pods to enable the others to fill. It's possible to have seeds stop filling when they're very small and end up like BBs, but this is relatively rare, and it typically results from something like frost or hail that stops photosynthesis quickly at some point after pods start to fill.--Emerson Nafziger

Emerson Nafziger

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