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Soybean: crunch time to come

Emerson Nafziger
July 27, 2016
Recommended citation format: Nafziger, E.. "Soybean: crunch time to come." Department of Crop Sciences, University of Illinois at Urbana-Champaign, July 27, 2016. Permalink

The 2016 Illinois soybean story is similar to the corn story; current (July 24) crop ratings for both crops are similar to those we saw in 2014, when we produced the highest-ever yields for both crops. Illinois producers matched the 2014 soybean yield (56 bushels per acre) in 2015, despite the crop’s getting off to a very rocky start last year. Few surprises in crop production have been greater than that of seeing fields that looked marginal in July 2015 go on to produce 60, 70, or even 80 bushels per acre.

Compared to the 2015 soybean crop, the 2016 crop took off well and has looked good the whole season in most Illinois fields. Stands are good, growth is generally uniform, and unlike most seasons, there are few drowned-out spots in many areas if the state. In some places where June was dry then heavy rains came in July, Phytophthora has developed. But overall, the 2016 soybean crop is the most visually appealing one we’ve seen in a long time.

While we appreciate the outstanding appearance of this year’s soybean crop, we saw last year that soybean appearance even in late July is a poor predictor of yield. In fact, it’s possible that the soybean crop might even look (and be) “too good” for this time of year. We don’t say that about the corn crop, so why is soybean different?

Corn is a highly efficient crop that makes enough leaf area to form a complete canopy but not much more than that. Corn also has a single ear attached to the stalk, and all leaves feed sugars into that same stalk, so every leaf contributes to both forming and filling the ear. Pollination takes place over the course of a few days, and high yields depend on the number of kernels, so it’s “all hands on deck” for corn leaves, and having too much canopy or plants too tall is not an issue.

Soybeans differ in that flowering takes place over a period of several weeks, and the success of flowering (that is, number of seeds/pods formed) is closely tied to the ability of the leaf at each flowering node to photosynthesize at a rapid rate while the flowers are forming. When leaves are large and plants are tall, not all leaves can compete successfully for light, and they can be partially shaded at critical times, resulting in fewer pods formed and less ability to fill the pods that form.

Petioles (stems that support leaflets) in soybean can be as long as 18 inches to help leaves get up into sunlight. But when stems get to be more than 40 inches tall (to the tip of the stem, not the top of the canopy), lower leaves will be shaded or partly shaded much of the time.

Shading of leaves attached to the lower stem nodes is a problem not only in terms of pod numbers that form at those nodes, but also in these leaves’ getting enough sunlight later in the season to fill those pods. Tall plants with a lot of leaf area in mid- to late August may have few pods forming on the lower stem, and while this might be partly offset by having more pods at mid- to upper nodes, total pod numbers per plant is often decreased.

This “overgrowth” phenomenon in soybean has been known for a long time, and has given rise to various attempts aimed at reducing plant height or leaf area to “help” the soybean plant get over this problem. Dinging leaves with herbicide or growth regulator, “topping” plants by hand or mechanically, and generally undoing what (large) plants spent time and effort building has been the common theme. I’ve seen this work in the subtropics where intact soybean plants turn into vines, but in the Corn Belt, such treatments have almost always done more harm than good. It’s just difficult to get consistently positive results by beating up soybean plants.

By this time of the season soybean plants have not yet reached their maximum height; that will happen by about the second week of August, and plants could add 25 or 30% more to their height by then. Warm temperatures and good soil moisture will keep them growing. There’s not a lot we can do about that.

There is one thing we might not do that will provide some help: application of in-season nitrogen. Application of N (and fungicides) tends to keep leaves a little greener and to increase growth rates a little. Neither of these would be a good thing for a soybean crop that is already approaching stem height of 30 to 36 inches. For those who might want photos showing how tall your soybean crop is, have the person standing in them bend his or her knees a little. Or just acknowledge that really tall soybean plants are not usually the highest-yielding and that they are no indicator of best management.

Heavy canopies tend to keep humidity higher in the canopy, which can lead to more disease development. I haven’t heard too much about white mold so far, but these are the type of conditions that favor its development if it has already infected flowers. There are some other foliar fungal diseases such as frogeye leafspot that may be favored if it stays damp.

While we’ve found a yield increase of 2 bushels per acre or more from applying foliar fungicide in about half of the trials we’ve done, we’re not able to find many clues about when it might work and when it might not. There is no correlation between response and yield level, so “making high yields higher” doesn’t work as a principle. While we can’t rule out a physiological effect, it makes sense that using fungicides to help control fungal diseases should provide the most consistent return.

What’s our best-case for the 2016 soybean crop? Some cooler, drier weather will help slow growth down a little, but that may not be enough to bring back really high yield potential. Although we are concerned about heavy canopies, we have had years when this seemed to have less effect; only by looking at pod numbers per node in mid-August will we really know. If many nodes have 4 or 5 pods like we saw in 2015, we can consider this a false alarm, or at least a case where possible negatives were canceled by some positives, even if we don’t understand how.

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