Stu Ellis, FarmGate blog  |   July 20, 2012 resize text

The pollination period for corn is only a couple weeks long at best, and if it is under stress, the corn yield will decline.  But that is not the case for soybeans, and since they flower over many weeks, they can respond well to any stress by just putting on more flowers and pods.  So this droughty year that has hurt corn yields should not be hurting soybean yields,  Right?  Right?

Recently, conventional wisdom has shifted hope for drought salvation from the failed corn crop to the soybean crop, and its potential for a much better yield due to a longer reproductive period. Theoretically, soybeans should be blooming and setting pods in high gear because of the sunnier than normal conditions this year.  But in this case, more light does not always mean more yield because soybean yield prospects are not good in the current drought, says University of Illinois crop production specialist Emerson Nafziger.  In his weekly agronomic bulletin, Nafziger says soybeans are suffering more than many producers might realize, and would certainly want.

While the diminished canopy formation is one issue, a bigger issue is the lack of pods on soybean plants.  Nafziger says soybean plants may be blooming, but pods are not being formed from those blooms due to the moisture stress on the crop.  Showing his analysis with a number of photos, Nafziger says by this time in the growing season a soybean plant should have an abundance of pods, but they are not there.

1. Lower nodes on the soybean stem have few, if any, pods, with a slim chance of change.
2. The distance between the nodes is quite short, and stems are short overall contributing to the smaller plant size.
3. The racemes may hold dried up flowers, but if they have not produced a pod on the lower parts of the soybean plant, there is little chance they will still form a pod.

Nafziger says, “The yield potential of stressed soybean plants lies in their ability to respond to rainfall by producing more nodes at the top of the plant, and then by flowering and setting pods on these nodes. Racemes on upper nodes have good numbers of flowers, and pods are forming at some of these upper nodes. But we expect many small pods to abort under hot, dry conditions, and as long as conditions stay this way, even larger pods may begin to drop off the plant.

Basically, soybean plants have been flowering, and then either aborting flowers or failing to form pods where flowers have dried up.  As that situation continues, yield potential will decline.

But what if a good rain occurs, will the soybean plant recover and begin its reproductive process?  Nafziger says rain may result in new flowering at the higher parts of the soybean plant, but with the lack of a full canopy, the plant’s ability to produce sugar and fill any pods will be severely limited.  The photosynthesis process, which produces sugar, has been restricted by the shortage of moisture and soybean leaves that have turned over to conserve moisture, as well as the lack of a full canopy to capture the maximum amount of light.

Any double-soybeans that would have been planted later in the growing season have even a shorter plant height, and subsequently fewer nodes, flowers, and pods.  Nafziger says if better moisture conditions arrive, the later planted beans may have opportunities to recover with more flowering and pod-forming ability.  But he says the shorter height and lack of canopy will restrict the ability of double-crop beans to produce the sugars needed to fill pods.

But what if you can relieve any other stress on the crop, such as fungal or insect issues, would that help?  Nafziger says any other inputs, such as fungicides or insecticides will have little ability to help the soybean crop with its overwhelming lack of moisture.  He says it is still important to scout for pests, but any additives will have little ability to save the crop which is in such severe moisture stress.

Summary:
The soybean plant is suffering from severe moisture stress and subsequently internodes are short, blooms are not producing pods, and the lack of a canopy will restrict the ability of the plant to produce sufficient sugars to fill pods.  Any relief from the drought will allow the top of the plant to bloom and form pods, but the lack of a canopy will restrict yield as it will in shorter double-crop soybeans.

Source: FarmGate blog

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