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Issue No. 18, Article 6/July 27, 2007

Are Corn and Soybean Crops As Good As They Look?

The current soil water status in most of Illinois remains good, and sunshine has been abundant this past week, with relatively low humidity (and the cool nights that accompany low humidity.) This is all good news for the corn crop, and crop ratings have rebounded to high levels. Cool nights help to preserve sugars generated during the day, and in most fields the canopy remains healthy.

But we have also had some reports that might be cause for concern, and at the minimum would suggest that those who have until now enjoyed the look of the crop from the pickup or a highway overpass might want to walk in fields before long to take a closer look at plants.

One unusual phenomenon noted this year, found more commonly on certain hybrids, is the tendency to have a second ear that arises from a node on the shank of the main ear. Dr. Bob Nielsen has described this well, including photos. The following photo was taken at Monmouth on July 13, in a hybrid that showed this on more than half of the plants. These small ears have not pollinated, probably due to lack of pollen after their silks emerged. They likely do not represent a problem, but fresh silks like this might continue to attract corn rootworm adults.


A second ear developed from a node on the shank of the primary ear.

We are also seeing in some fields the unusual phenomenon of new silks emerging among the brown silks on the same ear. We had heard a few reports of this earlier but because pollination weather was so favorable did not give it much attention. These new silks cannot receive pollen so long after the main pollination is over, so this represents some loss in kernel number. To see this, peel back the husks and note how many kernel initials still have a fresh silk attached. These have not been fertilized, and without pollen they will remain unfertilized.


Fresh silks along with brown silks (left) and silks still attached to kernels at the end of the ear, indicating lack of pollination (right).

If conditions stay favorable for the next six weeks, corn plants should be able to fill a large number of kernels, and loss of kernels due to this "extended" silking may limit yields to some extent. The extent of such limitation depends on how many kernels are already pollinated and developing on these ears. If kernel number is high (500 or more per ear), then not having these tip kernels develop probably will not limit yields. My limited observation is that this occurred more frequently in later-maturing hybrids, but we do not know if certain hybrids show more of this than others. The husks and unpollinated silks tend to be quite long where we see this, and it is possible that the mass of early-emerging silks interfered with the later-emerging ones in some hybrids.

Finally, lower stalk diameter, especially in fields at high populations or where there is volunteer corn present, is not as large as we would like to see. This likely reflects the very rapid development after the rainfall in late June, followed by the heavy demand that development of ears is starting to place on the plants. Corn plants build strength and rigidity by producing lignin in the stalk rind. How much of this takes place depends on there being enough sugar in the plant to feed the ear and the lower stalk and to maintain roots. Lower stalk weakness has not become a problem yet, in part because the ear is not yet as heavy as it will be. But stalk strength should be monitored on each trip into the field. The easiest way to do this is to push on plants and see if the lower stalk bends easily or breaks over.

Soybeans. Soybean plants and canopies have developed very well, and podding is starting to get under way, especially in fields of early-maturing varieties. Because the canopy developed so quickly after water stress was relieved, there may be more shading of the lower canopy than is normal, and podding might suffer somewhat as a result. I looked at soybean plants at Urbana this week, and though they are taller than normal with good leaf color and a complete canopy, many of the racemes (clusters of flowers) were not yet forming pods.

Night temperatures in the upper 50s such as we have been experiencing might limit to some extent the rate at which soybean flowers turn into pods. There may not yet be evidence of flower abortion, but pod development depends on sugars from the leaf attached at the same node as the raceme, and if that leaf is shaded, pod numbers could be reduced on lower nodes. The crop will be favored physiologically if temperatures at night return to average, in the mid- to upper 60s.

People have recognized the critical importance of the flowering and podding processes in soybean, and there have been various attempts to find ways to increase the numbers of pods and seeds that form. Such attempts have included applying nitrogen in different forms and using growth regulator materials such as cytokinins, sold under a number of different names and formulations. Fungicides have also been promoted for this, through their potential to maximize daily photosynthesis, if not to produce some sort of growth response directly. While such materials might have some effect in irrigated soybean where shortage of water is preventable, they have never produced a consistent increase in yield.Emerson Nafziger

Author:
Emerson Nafziger

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